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There's other ways to think about it. And so let's say that this first asteroid is twice the mass of either of these two smaller ones. And these two smaller ones have the same mass. Now we've attached the back of a rocket to each of these asteroids. In fact, this one over here has two rockets. And we're gonna assume th... | Force, mass and acceleration Movement and forces Middle school physics Khan Academy.mp3 |
Now we've attached the back of a rocket to each of these asteroids. In fact, this one over here has two rockets. And we're gonna assume that all of the rockets are equivalent and we ignite them all. And so they all exert the same force each on the asteroid. So for example, we have a net force acting leftward on this la... | Force, mass and acceleration Movement and forces Middle school physics Khan Academy.mp3 |
And so they all exert the same force each on the asteroid. So for example, we have a net force acting leftward on this large asteroid. We have the same net force acting on this smaller asteroid also going to the left. And on this other smaller asteroid, we have two times that net force acting to the left. So what I wan... | Force, mass and acceleration Movement and forces Middle school physics Khan Academy.mp3 |
And on this other smaller asteroid, we have two times that net force acting to the left. So what I want you to do is pause this video and think about which of these asteroids is going to be accelerated the most and which of these asteroids is going to be accelerated the least. All right, so you might have an intuition ... | Force, mass and acceleration Movement and forces Middle school physics Khan Academy.mp3 |
So let me write it like this. So you might get a sense that if you increase your force, that that's also going to increase your acceleration. And it does turn out that that is indeed the case. Now, the other notion that you might have is that the more of the stuff that there is, the more mass that you have, the harder ... | Force, mass and acceleration Movement and forces Middle school physics Khan Academy.mp3 |
Now, the other notion that you might have is that the more of the stuff that there is, the more mass that you have, the harder it is to accelerate it. So if your mass is larger, then your acceleration is lower. And it turns out that these things are all proportional. So for example, if we just compare these two masses ... | Force, mass and acceleration Movement and forces Middle school physics Khan Academy.mp3 |
So for example, if we just compare these two masses right over here, they have the same net force acting on it. And I keep saying net force, that means you just net out all of the forces acting in a certain dimension. For example, if I had another identical rocket acting in the opposite direction, they would net out. A... | Force, mass and acceleration Movement and forces Middle school physics Khan Academy.mp3 |
And this asteroid right over here wouldn't be accelerated at all. But going back to our example here, we have the same net force acting on each of these asteroids, but the first asteroid has twice the mass of the second asteroid. So how do you think the accelerations will relate? Well, as you might imagine, the acceler... | Force, mass and acceleration Movement and forces Middle school physics Khan Academy.mp3 |
Well, as you might imagine, the acceleration on the larger asteroid is going to be half the acceleration on this asteroid. Or another way to think about it, this asteroid is going to have twice the acceleration as this first asteroid. And that's because it has half the mass. And one way you can relate force, mass, and ... | Force, mass and acceleration Movement and forces Middle school physics Khan Academy.mp3 |
And one way you can relate force, mass, and acceleration, and this is one of the most important equations in all of physics, is that force is going to be equal to mass times acceleration. Or I could say the magnitude of the force is equal to the mass times the magnitude of the acceleration. So notice, in this example r... | Force, mass and acceleration Movement and forces Middle school physics Khan Academy.mp3 |
Hi everyone, Sal Khan here, and welcome to Middle School Physics. I have Iman Howard, who manages all of our STEM content. Iman, why should folks be excited about middle school physics? So, middle school physics is like the only science out there that explains how things happen. And so basically, everything's made of m... | Introduction to Middle school physics Khan Academy.mp3 |
So, middle school physics is like the only science out there that explains how things happen. And so basically, everything's made of matter. Me, you, the chair that I'm sitting on, and this course is gonna explore how we exist in the natural world. So for example, we talk a little bit about movement and forces, and we ... | Introduction to Middle school physics Khan Academy.mp3 |
So for example, we talk a little bit about movement and forces, and we learned that everything, everything that we have a collision with has this equal but opposite force that's applied when the collision happens. So that's why when you give those high fives and then your hand starts stinging, it's because the same for... | Introduction to Middle school physics Khan Academy.mp3 |
So like, I'm thinking like Star Wars, there's like this force energy, like gravitational, there's magnetic energy, there's electric energy. And then finally, we get into waves. And we talk a little bit about how waves exist, whether it's sound waves or even the waves in the ocean. What do you think's exciting about it?... | Introduction to Middle school physics Khan Academy.mp3 |
What do you think's exciting about it? Oh, well, that's a dangerous question to ask me. I wanted to be a physicist and I still aspire to be it because we kind of wake up in this cosmos and we're just trying to understand where we fit in. And physics asks the most fundamental questions about how the universe works. And ... | Introduction to Middle school physics Khan Academy.mp3 |
One type of temporary magnet is called an electromagnet. So what is electromagnetism? Well, the hint is in the name itself. Electro for electrical and magnet for, well, magnet. Let's take a moment to look at the definition of what an electromagnet is. Electromagnets are materials that become magnets in the presence of ... | Electromagnetism Forces at a distance Middle school physics Khan Academy.mp3 |
Electro for electrical and magnet for, well, magnet. Let's take a moment to look at the definition of what an electromagnet is. Electromagnets are materials that become magnets in the presence of electricity. But how does that even happen? Well, it turns out that electrically charged particles in motion actually have s... | Electromagnetism Forces at a distance Middle school physics Khan Academy.mp3 |
But how does that even happen? Well, it turns out that electrically charged particles in motion actually have small magnetic fields around them. So if we run electricity through a wire, a magnetic field will be created around the wire. Now we can control the strength of this magnetic field in a couple of ways. We can m... | Electromagnetism Forces at a distance Middle school physics Khan Academy.mp3 |
Now we can control the strength of this magnetic field in a couple of ways. We can move more electric charges through the wire at a faster rate, and we do this by increasing the electrical current. The second way is to increase the density of the charged particles, and we can do this by looping the wire into a coil. Th... | Electromagnetism Forces at a distance Middle school physics Khan Academy.mp3 |
This gives us more charged particles with magnetic fields in a small space, strengthening the magnetic force. The other way we can control electromagnets is the direction of the magnetic field, and we can do this by changing the direction of the electricity. So if we go back to this wire example from earlier and change... | Electromagnetism Forces at a distance Middle school physics Khan Academy.mp3 |
This makes electromagnets quite different from permanent magnets. So let's take a look at that and compare permanent magnets to electromagnets. Electromagnets are typically made of loops of wire in a coil. The wire is typically made of metal like copper and wrapped around pieces of metal like iron, nickel, or cobalt. T... | Electromagnetism Forces at a distance Middle school physics Khan Academy.mp3 |
The wire is typically made of metal like copper and wrapped around pieces of metal like iron, nickel, or cobalt. This is different from a permanent magnet because permanent magnets don't need this wire. Permanent magnets also have fixed poles. You can't change the north and south poles on these magnets, but as we now k... | Electromagnetism Forces at a distance Middle school physics Khan Academy.mp3 |
You can't change the north and south poles on these magnets, but as we now know, for electromagnets we can change these poles by changing the direction of the electrical current. So if we have an electromagnet with a north and south pole that looks like this and a current flowing in this direction, well, we can change ... | Electromagnetism Forces at a distance Middle school physics Khan Academy.mp3 |
Electromagnets have a fixed strength, but we just talked about how we can change the strength of electromagnets, so electromagnets have adjustable strength. And finally, electromagnets need a power source in order to generate the electricity required to produce magnetic fields. Permanent magnets do not need a power sou... | Electromagnetism Forces at a distance Middle school physics Khan Academy.mp3 |
On the other hand, electromagnets can also be turned on and off. So if we have a magnet that's spinning, we can turn it on and off. Now you might be thinking, if electrical charge can affect magnetism, can magnetism affect electrical charge? Absolutely! Let's look at how we can do that. The only way to do this is by ch... | Electromagnetism Forces at a distance Middle school physics Khan Academy.mp3 |
Absolutely! Let's look at how we can do that. The only way to do this is by changing the magnetic field around the charged particles. In fact, spinning magnets is how most of the electricity we use in cities and homes is generated. A turbine spins a magnet inside a coil to produce electricity, and since electromagnets ... | Electromagnetism Forces at a distance Middle school physics Khan Academy.mp3 |
Nothing is touching it once you let it go. How can there be a force on it? Well, this is because Earth's gravitational force is pulling the ball, and gravity is a non-contact force. Non-contact forces don't have to touch an object to exert a force on it. Instead, these forces act over a region. So, if an object is in t... | Fields Forces at a distance Middle school physics Khan Academy.mp3 |
Non-contact forces don't have to touch an object to exert a force on it. Instead, these forces act over a region. So, if an object is in that region, it will be affected by the force. In this case, the ball is in Earth's gravitational field, and so it feels an attractive force towards the Earth, and the ball falls to t... | Fields Forces at a distance Middle school physics Khan Academy.mp3 |
In this case, the ball is in Earth's gravitational field, and so it feels an attractive force towards the Earth, and the ball falls to the ground. Field forces include non-contact forces such as electric, magnetic, and of course, gravitational forces. So, since these forces are non-contact, they can exert a force on ob... | Fields Forces at a distance Middle school physics Khan Academy.mp3 |
To explain these non-contact forces, scientists eventually developed the idea that these objects were surrounded by something called a field. So what is a field? A field extends through space from an object with certain physical properties. What are those? Well, for gravitational forces, these affect objects with mass.... | Fields Forces at a distance Middle school physics Khan Academy.mp3 |
What are those? Well, for gravitational forces, these affect objects with mass. So, any object with mass has a gravitational field surrounding it that points towards the object's center. The further you move away from the object, the less dense the field, and weaker the field becomes. Electric forces affect charged obj... | Fields Forces at a distance Middle school physics Khan Academy.mp3 |
The further you move away from the object, the less dense the field, and weaker the field becomes. Electric forces affect charged objects. So, an electric field surrounds any object with a net charge, and the direction of this field will depend on the charge. Magnetic fields will affect magnets and any other material w... | Fields Forces at a distance Middle school physics Khan Academy.mp3 |
Magnetic fields will affect magnets and any other material with magnetic properties. Each spot on a field has two things associated with it. Magnitude and direction. And these help us predict what forces objects will experience when they're in the field. So, let's look at an example to help understand this. Say we have... | Fields Forces at a distance Middle school physics Khan Academy.mp3 |
And these help us predict what forces objects will experience when they're in the field. So, let's look at an example to help understand this. Say we have a planet. Now, the planet has a lot of mass, so we know it's going to be surrounded by a gravitational field that points towards the center of the planet. I can draw... | Fields Forces at a distance Middle school physics Khan Academy.mp3 |
Now, the planet has a lot of mass, so we know it's going to be surrounded by a gravitational field that points towards the center of the planet. I can draw these little field lines that show the direction of the field and its strength. As we move away from the planet, the field will start to weaken, and I'm going to re... | Fields Forces at a distance Middle school physics Khan Academy.mp3 |
Now, let's say there's an asteroid moving near the planet in this direction. I know that the asteroid, as it's shown here, is in the outskirts of this planet's gravitational field, so it is going to feel some gravitational attraction towards the planet, which we can draw with this vector FG, which is force of gravity. ... | Fields Forces at a distance Middle school physics Khan Academy.mp3 |
And the closer the asteroid gets to the planet, the stronger the field and the stronger the force of attraction it will feel. And so, in this way, scientists can use fields to help predict behavior of objects experiencing non-contact forces. And all of this may sound kind of odd, but you probably already think about fo... | Fields Forces at a distance Middle school physics Khan Academy.mp3 |
Magnets are these neat objects that are able to attract metals like iron. Magnets are used in all sorts of things from holding paper on your refrigerator to computers to compasses. So magnets can be used to stick things together, point us in the right direction, and even lift things. And they do this through magnetic f... | Magnetic forces Forces at a distance Middle school physics Khan Academy.mp3 |
And they do this through magnetic forces. If you've handled two magnets, you've felt magnetic forces, even when the magnets weren't touching each other. That's because magnetic forces are non-contact forces, which just means they can affect other objects they aren't even touching. Magnets will attract or repel each oth... | Magnetic forces Forces at a distance Middle school physics Khan Academy.mp3 |
Magnets will attract or repel each other and this attraction or repulsion is a magnetic force. But magnetic forces don't affect everything the same way. Otherwise a magnet would stick to you, not just a refrigerator. In this video, we're going to talk about the magnetic forces between two magnets. So why do magnets som... | Magnetic forces Forces at a distance Middle school physics Khan Academy.mp3 |
In this video, we're going to talk about the magnetic forces between two magnets. So why do magnets sometimes attract each other and other times repel each other? Well, this has to do with the orientation of the magnets. Orientation is really just a fancy word for how the magnets are positioned compared to one another.... | Magnetic forces Forces at a distance Middle school physics Khan Academy.mp3 |
Orientation is really just a fancy word for how the magnets are positioned compared to one another. You see, it turns out that each magnet has a north and a south pole. But what does this have to do with attraction or repulsion? Well, as you may have heard, opposites attract. So if you face the north pole of one magnet... | Magnetic forces Forces at a distance Middle school physics Khan Academy.mp3 |
Well, as you may have heard, opposites attract. So if you face the north pole of one magnet to the south pole of another magnet, guess what? They will be attracted to each other. But if you turn one of those magnets around so that you have two north poles facing each other, they will repel. And the same thing would hap... | Magnetic forces Forces at a distance Middle school physics Khan Academy.mp3 |
But if you turn one of those magnets around so that you have two north poles facing each other, they will repel. And the same thing would happen if it was two south poles facing each other. So the direction of the magnetic force completely depends on the orientation of the magnets. Orientation though is just one thing ... | Magnetic forces Forces at a distance Middle school physics Khan Academy.mp3 |
Orientation though is just one thing that affects magnetic forces. The strength of magnetic forces depends on a couple of things. For one, distance. If you've ever held two magnets, you may have noticed that when you move them closer, they seem to almost jump together. Or if you try to push two like poles together, the... | Magnetic forces Forces at a distance Middle school physics Khan Academy.mp3 |
If you've ever held two magnets, you may have noticed that when you move them closer, they seem to almost jump together. Or if you try to push two like poles together, they get harder and harder to hold together the closer you get. This is because magnetic forces depend on distance. The closer the two magnets are toget... | Magnetic forces Forces at a distance Middle school physics Khan Academy.mp3 |
The closer the two magnets are together, the stronger the force between them. So as the distance decreases, the force increases. But the farther away they are, the weaker the magnetic force is. So distance increases, force decreases. The other big factor that affects how strong a magnetic force is, well, the magnets th... | Magnetic forces Forces at a distance Middle school physics Khan Academy.mp3 |
So distance increases, force decreases. The other big factor that affects how strong a magnetic force is, well, the magnets themselves. Some magnets are really weak, like a lot of refrigerator magnets. Others are so strong that even tiny ones can be almost impossible to pull apart. Some of these stronger magnets are ev... | Magnetic forces Forces at a distance Middle school physics Khan Academy.mp3 |
Any massive object that is in motion then has kinetic energy, but how much? First, let's consider some comparisons. This nice rat family, papa, mama, brother, and sister are sitting down to dinner at a long table passing blocks of cheese back and forth. Papa rat asks for the cheddar cheese and there are two identical b... | Kinetic energy Energy Middle school physics Khan Academy.mp3 |
Papa rat asks for the cheddar cheese and there are two identical blocks. Brother rat pushes one and sister rat pushes the other so that the second cheese is traveling twice as fast as the first cheese. Which piece of cheddar cheese do you think has more kinetic energy? Yes, it's the one going faster. Now papa rat doesn... | Kinetic energy Energy Middle school physics Khan Academy.mp3 |
Yes, it's the one going faster. Now papa rat doesn't need both pieces of cheddar so he eats one and sends one back along with a small piece of swiss that weighs half as much as the piece of cheddar. Papa rat has better manners than his children so he sends them both back at the same speed. Which piece of cheese would y... | Kinetic energy Energy Middle school physics Khan Academy.mp3 |
Which piece of cheese would you think has more kinetic energy now? Yes, the heavier or more massive object, in this case the cheddar, will have more kinetic energy. Let's make it a little more complicated. Brother and sister rat are full so they send the cheeses back for mama rat. Brother rat pushes the larger piece of... | Kinetic energy Energy Middle school physics Khan Academy.mp3 |
Brother and sister rat are full so they send the cheeses back for mama rat. Brother rat pushes the larger piece of cheddar and sister rat pushes the smaller piece of swiss so that the swiss is going twice as fast as the cheddar. Now which cheese has more kinetic energy? In fact, it turns out that it's the swiss in this... | Kinetic energy Energy Middle school physics Khan Academy.mp3 |
In fact, it turns out that it's the swiss in this scenario. Kinetic energy depends on both mass and speed but the dependence on speed is stronger. This estimation of kinetic energy can be quantified in an equation that lets us calculate kinetic energy exactly. We said kinetic energy depends on the mass and the speed wh... | Kinetic energy Energy Middle school physics Khan Academy.mp3 |
We said kinetic energy depends on the mass and the speed which we'll write as v for velocity so we can start with ke equals m times v. But we said that it depends more on the speed so the velocity here is actually squared. This means that if an object's mass doubles its kinetic energy also doubles but if its speed doub... | Kinetic energy Energy Middle school physics Khan Academy.mp3 |
So this is the equation for kinetic energy one half m v squared. Let's apply this equation to our cheesy example. Say the swiss has a mass of 0.05 kilograms which makes the cheddar's mass 0.1 kilograms. When both cheeses have the same speed say two meters per second the cheddar's kinetic energy is one half times 0.1 ki... | Kinetic energy Energy Middle school physics Khan Academy.mp3 |
When both cheeses have the same speed say two meters per second the cheddar's kinetic energy is one half times 0.1 kilograms times two meters per second squared which is 0.2 joules. The swiss's kinetic energy is one half times 0.05 kilograms times two meters per second squared which is 0.1 joules or half the kinetic en... | Kinetic energy Energy Middle school physics Khan Academy.mp3 |
For example, here we have the sun, 93 million miles away on average. And imagine the different materials that the light has to travel through from the sun to say hit one of these sand particles right over here. Think about what it needs to be transmitted through. Well, it's going to travel through 93 million miles of t... | Wave transmission Waves Middle school physics Khan Academy.mp3 |
Well, it's going to travel through 93 million miles of the vacuum of space. And that's one of the amazing things about light waves is that they don't need a medium. They can travel through vacuum, through emptiness. But then it's going to travel through several miles of Earth's atmosphere. So it's going to travel throu... | Wave transmission Waves Middle school physics Khan Academy.mp3 |
But then it's going to travel through several miles of Earth's atmosphere. So it's going to travel through several miles of Earth's atmosphere. It'll hit the lenses of these sunglasses. It'll actually travel through the lenses of the sunglass. The sunglass has some width or some depth to it. And then it'll go out onto ... | Wave transmission Waves Middle school physics Khan Academy.mp3 |
It'll actually travel through the lenses of the sunglass. The sunglass has some width or some depth to it. And then it'll go out onto the other side and it will hit the sand right over here. Now, one thing you might realize is the amount of transmission and what gets transmitted is dependent on the wavelengths of the w... | Wave transmission Waves Middle school physics Khan Academy.mp3 |
Now, one thing you might realize is the amount of transmission and what gets transmitted is dependent on the wavelengths of the wave, in this case, the wavelengths of light, and also about the material that they are going through. So for example, these sunglasses right over here, many sunglasses try to keep out UV ligh... | Wave transmission Waves Middle school physics Khan Academy.mp3 |
And we could also see that this sunglass right over here, it kind of has an orangish color, which means that things that are closer to that end of the spectrum, closer to the red, the oranges, and the yellows are getting through, which means that it's filtering out blue light. So the blue light isn't getting transmitte... | Wave transmission Waves Middle school physics Khan Academy.mp3 |
And then of course, the light will get to that sand particle. Now, transmission, as I mentioned, it isn't just about light waves. We could talk about one of our other favorite types of waves, for example, sound waves. If you are in a room, you have probably experienced the fact that even if you were to close the door, ... | Wave transmission Waves Middle school physics Khan Academy.mp3 |
If you are in a room, you have probably experienced the fact that even if you were to close the door, and I do this a lot, because I record a lot of videos, this is me in my little closet recording a video, this is a top view for what I'm doing right now. A lot of times my kids are in other parts of the house and they'... | Wave transmission Waves Middle school physics Khan Academy.mp3 |
Those air particles are knocking one into another. But in order to make it to me, they need to get through that wall. And the way they do that is they get transmitted to through that wall. So those air particles make the particles or make the atoms or the molecules in the wall start vibrating. They bump into each other... | Wave transmission Waves Middle school physics Khan Academy.mp3 |
So those air particles make the particles or make the atoms or the molecules in the wall start vibrating. They bump into each other. And then the particles on the other side of the wall will bump into the air in my little closet. And then we will have once again, the sound waves make it to me. Now the overall magnitude... | Wave transmission Waves Middle school physics Khan Academy.mp3 |
If you have, you might have noticed that once you pulled the sock off, it was still attracted to the shirt, even when they weren't touching. What is even happening here? Well, it turns out there's an electric force between the shirt and the sock. Electric forces are a type of non-contact force, which means they can act... | Electric forces Forces at a distance Middle school physics Khan Academy.mp3 |
Electric forces are a type of non-contact force, which means they can act on objects that aren't even touching. If you've ever noticed two balloons repelling each other, or if you've ever noticed your hair sticking to something like a balloon or a sweater, that's what I'm talking about. But why is there an electric for... | Electric forces Forces at a distance Middle school physics Khan Academy.mp3 |
Well, it turns out that electric forces are caused by a property of matter called electric charge. Matter is made up of tiny particles that can have positive, negative, or neutral charge. Neutral just means that the electric charge is zero, not positive or negative. When you add up all these charges, most objects tend ... | Electric forces Forces at a distance Middle school physics Khan Academy.mp3 |
When you add up all these charges, most objects tend to have a net charge that is about neutral. Otherwise, we'd be attracted to all sorts of things, just like that sock. However, an object's charge can change. So in the dryer, all that heat and movement allowed some negative charges from the shirt to move to the sock.... | Electric forces Forces at a distance Middle school physics Khan Academy.mp3 |
So in the dryer, all that heat and movement allowed some negative charges from the shirt to move to the sock. Now when you try to separate the two, they are both electrically charged, and there is an electric force between them. So now that we know what causes an electric force, let's look at what affects its direction... | Electric forces Forces at a distance Middle school physics Khan Academy.mp3 |
An electric force can attract or repel an object. But how do you know if an electric force will be attractive or repulsive? Well, as the saying goes, opposites attract. An object with a negative electric charge will be attracted to a positively charged object. If the second object is also negatively charged, well, the ... | Electric forces Forces at a distance Middle school physics Khan Academy.mp3 |
An object with a negative electric charge will be attracted to a positively charged object. If the second object is also negatively charged, well, the two objects will experience a repulsive force and be repelled from each other. So in order to have the sock and the shirt attracted to each other, they must have opposit... | Electric forces Forces at a distance Middle school physics Khan Academy.mp3 |
What about the strength of that electric force? Strength will depend on a couple of factors. First, the charge of each object is proportional to the force. The stronger the charges, the stronger the electric force. So the greater the electric charge, the greater the electric force. Another factor, how far apart the obj... | Electric forces Forces at a distance Middle school physics Khan Academy.mp3 |
The stronger the charges, the stronger the electric force. So the greater the electric charge, the greater the electric force. Another factor, how far apart the objects are from each other. The electric force will weaken as the distance increases. So the force decreases as our distance increases. As you move the sock a... | Electric forces Forces at a distance Middle school physics Khan Academy.mp3 |
But focusing on light for a second, we've already talked about how if we go from one medium to another, if we're transmitting from one medium to another, that the light can go through a medium, and as it goes to that boundary from one medium to another, its direction can change. And we talk about that in another video,... | Absorption and reflection Waves Middle school physics Khan Academy.mp3 |
Now there's other things that light can also do, and that's the focus of this video, reflection and absorption. Now you probably have a sense of what happens with reflection. We can see a reflection of the mountains in the lake right over here. And the reason why we can see the reflection of the mountains in the lake h... | Absorption and reflection Waves Middle school physics Khan Academy.mp3 |
And the reason why we can see the reflection of the mountains in the lake here is because light that is coming from this mountain is hitting the lake, and then it is bouncing off of it, and then coming to an observer's eyeball right over here. And so they see the light that's coming from here, they see it as coming fro... | Absorption and reflection Waves Middle school physics Khan Academy.mp3 |
Light is coming in, it hits that other material, and then if it bounces off, that's reflection. Now the other thing that light can do is get absorbed. Absorbed is when the material doesn't reflect any and doesn't transmit any through it, if it's completely absorbing. And so that would be a situation like this. Now the ... | Absorption and reflection Waves Middle school physics Khan Academy.mp3 |
And so that would be a situation like this. Now the reality in the real world is we have oftentimes a little bit of all of the above happening. For example, when we look at this white snow up here, this is reflecting pretty well. So there's light that's coming from the sun, or maybe it's being reflected off the clouds,... | Absorption and reflection Waves Middle school physics Khan Academy.mp3 |
So there's light that's coming from the sun, or maybe it's being reflected off the clouds, or maybe it's getting through the clouds, and when it hits that snow, it gets reflected. Now the reason why we don't see a reflection the way that we see in this lake right over here is the snow reflects it in all different direc... | Absorption and reflection Waves Middle school physics Khan Academy.mp3 |
But if you go down here where we see the trees, the same light from the sky is hitting it, but not as much light is coming back to our eye. And that's because this part of the mountain, it might be trees, it might be rock, it might be dirt, is absorbing more of the light. But it's still reflecting some. We can still se... | Absorption and reflection Waves Middle school physics Khan Academy.mp3 |
We can still see it a little bit. So this might be where most of it is getting absorbed, but a little bit of the light gets reflected, which we can see right over there. And if we think about the water right over here, some of the light is probably making it through and probably refracting as it does so. Some of it is ... | Absorption and reflection Waves Middle school physics Khan Academy.mp3 |
Some of it is getting absorbed as it makes its way through the water. If this was a really deep lake, when you get to the bottom, it could be very, very dark. And then as we talked about, it looks like a good bit is reflecting. When we look at the image that we see in the lake, it looks almost as bright as the real thi... | Absorption and reflection Waves Middle school physics Khan Academy.mp3 |
When you hear the word gravity, you probably just think of things falling like an apple from a tree. But did you know it's also the reason why your lamp is staying on the floor? That's because gravity is so much more than things falling down. Gravitational forces are these invisible forces that pull objects together. S... | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
Gravitational forces are these invisible forces that pull objects together. So gravitational force is actually attracting the lamp to the floor. And these forces exist between all objects with mass. So let's write these key points out about gravitational forces, which I'm going to use GF to represent. We said they are ... | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
So let's write these key points out about gravitational forces, which I'm going to use GF to represent. We said they are attractive forces and that they exist between all objects with mass. Objects with mass. To explain this, we first need to remember a couple of things. Mass is how much matter objects have, and matter... | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
To explain this, we first need to remember a couple of things. Mass is how much matter objects have, and matter is the stuff an object is made of. Any object with mass generates a gravitational pull, so there is a gravitational force of attraction between every object. The amount of gravitational force between two obje... | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
The amount of gravitational force between two objects will depend on two things. The masses of the two objects and the distance between them. The mass of each object is proportional to the gravitational force. This means that the more mass an object has, the stronger its gravitational force. And now we can understand w... | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
This means that the more mass an object has, the stronger its gravitational force. And now we can understand why gravity makes things fall. The Earth is massive, literally. It's almost six septillion kilograms. That's a six with 24 zeros after it. So it generates a huge attractive force. For comparison, my lamp is only... | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
It's almost six septillion kilograms. That's a six with 24 zeros after it. So it generates a huge attractive force. For comparison, my lamp is only one kilogram, which is why if I jump, I fall towards the Earth and not towards my lamp. But we said the mass of the object is just one factor affecting the strength of its ... | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
For comparison, my lamp is only one kilogram, which is why if I jump, I fall towards the Earth and not towards my lamp. But we said the mass of the object is just one factor affecting the strength of its gravitational force. The other is the distance between objects. The more distance between the objects, the weaker th... | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
The more distance between the objects, the weaker the gravitational pull between them. For small objects without much mass, it doesn't take much distance for their gravitational forces between each other to be so weak that we don't notice them. For something like the Earth, you have to go really far away to not be affe... | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
I mean, look at the Moon. It's almost 240,000 miles away. That's almost 400,000 kilometers. And it still feels effects from Earth's gravity. That's why it's orbiting us. But since the Moon is also a pretty massive object, we do experience the effects of its gravitational pull on the Earth. This is why we have tides. | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
And it still feels effects from Earth's gravity. That's why it's orbiting us. But since the Moon is also a pretty massive object, we do experience the effects of its gravitational pull on the Earth. This is why we have tides. The Moon's gravitational force will pull on Earth's water, which results in us having high and... | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
This is why we have tides. The Moon's gravitational force will pull on Earth's water, which results in us having high and low tides. Now you might be wondering if gravity can affect the Moon's gravitational pull. Or cause tides. How can we even move around? Why don't we just face plant it on the ground because Earth's ... | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
Or cause tides. How can we even move around? Why don't we just face plant it on the ground because Earth's gravity is pulling us towards it? It turns out that actually gravity is a pretty weak force. We only even notice its effects when an object is massive like planets or stars. And the gravitational force on you is w... | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
It turns out that actually gravity is a pretty weak force. We only even notice its effects when an object is massive like planets or stars. And the gravitational force on you is way weaker than most forces you exert every day. In fact, every time you pick up a glass of water, you feel it. In fact, every time you pick u... | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
In fact, every time you pick up a glass of water, you feel it. In fact, every time you pick up a glass of water, you're overpowering the entire mass of Earth. How cool is that? | Gravitational forces Forces at a distance Middle school physics Khan Academy.mp3 |
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