About this video
The conservation of energy says that if no energy enters or leaves a system, its total energy stays the same. Using a bouncing ball, the video shows gravitational potential energy turning into kinetic energy and back, then explains why real balls stop bouncing: energy leaves the earth-ball system as sound, heating the air and vibrations. Dribbling works because you keep adding energy back in.
You'll be able to
- state the principle of conservation of energy for a system.
- track how potential and kinetic energy change as a ball falls and bounces.
- explain why a dropped ball eventually stops bouncing without energy being destroyed.
- explain how adding energy to a system keeps a ball bouncing to the same height.
Key words
- Conservation of energy
- If no energy enters or leaves a system, the total energy in that system stays the same.
- System
- The group of objects we choose to track, such as the ball and the earth.
- Kinetic energy
- Energy an object has because it is moving.
- Gravitational potential energy
- Energy stored in the earth-ball system that depends on how high the ball is.
- Thermal energy
- Energy of the tiny jiggling of particles, which we notice as warmth.
Transcript
- [Instructor] Have you ever dropped a ball, watched it bounce a few times, and then eventually come to rest on the ground? Nothing fancy, right? But this simple, everyday phenomena actually opens the door to one of the most powerful concepts in physics, the conservation of energy.
So let's dive in and explore what it really means. So what is conservation of energy? Well, the principle says that if you take any system, for example, the system made of this basketball and the earth and no energies entering or exiting that system, then the total energy within it stays the same.
That's it, that's the idea. Let's take an example. Imagine you just dropped the ball now, okay? At the moment before it starts moving, what's the kinetic energy of the ball? Well, it's zero because the ball isn't moving yet, and the ground isn't moving either so it's kinetic energy is also zero.
But we know the ball is high above the ground so the system of the ball and the earth has some potential, some gravitational potential energy. Let's say for the sake of taking some numbers, that potential energy is 6 joules, and we're gonna consider the potential energy of the system when the ball is touching the ground to be zero.
Okay, so right now, the total energy of this ball-earth system is 6 joules, right? What happens when we let the ball go? Well, the ball falls down, but we're not touching it, which means we're not adding any energy into the system and there's no energy leaving it either.
This means, based on energy consideration, the total energy of the earth-ball system should still be 6 joules, even though the ball is moving. But wait a second, we know as the ball gets closer to the earth, it's gravitational potential energy decreases. Maybe at this point, the gravitational potential energy is just 3 joules.
Where did the other 3 go? (gasps) It didn't disappear, it got converted to kinetic energy. The ball is now moving so it has kinetic energy, and that's what's beautiful about this concept. As the ball falls, the potential energy is converted into kinetic energy.
And if you know how much the potential energy is lost, we can figure out how much the kinetic energy was gained, and from that, we can even calculate how fast the ball was moving. That is why I love conservation of energy, it lets us do all kinds of cool calculations.
And by the time the ball reaches the ground just before it hits the ground, again, the total energy still stays the same, but now the potential energy, the gravitational potential energy of the system is almost zero, which means all of that energy is now converted to kinetic energy.
The ball will now have maximum kinetic energy. But what happens when the ball hits the ground and bounces back up? Let's look at that. At the moment of impact, the ball compresses and then rebounds, and again, no energy is entering or leaving the system. So now everything starts reversing.
Right after the bounce, all the energy is kinetic. Then as the ball rises, it starts slowing down. The kinetic energy is being converted back into potential energy, and eventually it reaches the same height as it started, comes to a stop, and at that moment, all of that kinetic energy is converted back to potential energy.
But wait a second. If this is what's happening, then the process should keep repeating forever, isn't it? The ball should just keep bouncing back to the same height and just keep going on over and over. I mean, if the energy conservation is true, shouldn't the ball keep bouncing back to the same height over and over again forever?
Clearly, that doesn't happen, so what's going on over here? Well, here's the key. Remember how I said that as the ball is falling down, no energy is leaving the system? Let's see if that's really true. First of all, when the ball hits the ground, you hear some sound, right?
Sound is a form of energy, and that energy has to come from somewhere. It's coming from the earth-ball system. That means the energy is actually escaping the system. So our assumption was actually wrong, the energy does exit the system, and as a result, the total energy reduces.
It doesn't disappear though, it's now a part of the air, and that energy might end up in your eardrums and makes your eardrums vibrate. It's there, but it's no longer part of the earth-ball system. Also, as the ball falls, it also collides with air molecules, transferring some of its energy to them.
This heat up the air slightly, and that's another form of energy loss to the surroundings. Also, when the ball hits the ground, it vibrates, and so does the ground, some of the particles vibrate. That's thermal energy, the tiny jiggling of particles. Technically, this thermal energy is still within the earth-ball system, but it's no longer useful in the sense it's no longer a part of the kinetic and the potential energy that we are tracking.
So long story short, because of all of this energy exiting the system, as time goes on, less and less energy is available in the form of kinetic and potential, and that's why this number reduces, total energy reduces, and so the height to which the ball rises keeps reducing until eventually there's not enough energy left to bounce at all.
That's when the ball comes to rest. All of that energy would've been transferred to the surrounding. This is the reason why we see what we see. Isn't that beautiful? Isn't that amazing? So the final question we could be having now is how do we keep this ball bouncing back at the same height?
Well, since the energy of the earth-ball system keeps escaping, we need to add some energy back in if we want to keep that ball bouncing back to the same height. And that's exactly what happens when you're dribbling a basketball. Every time you dribble, you are pushing the ball, adding energy to the earth-ball system from your own body.
That energy lets the ball bounce back to the same height again. And what's amazing is that while the ball-earth system keeps losing energy to its surrounding, you are constantly pumping energy back into it. So over time, your body's energy gets transferred into the earth-ball system, which then gets transferred into the surrounding through sound, heat and motion.
That's how energy flows. It's always there, it's always conserved, but it changes form and moves around. That is amazing, right? Finally, everything we've talked about doesn't just apply to gravitational potential energy. Remember, there are other kinds of potential energy.
The principle of conservation of energy applies to all forms of energy: electric, magnetic, even light waves for that matter. For example, we can apply it to electric circuits to predict how bright a bulb will glow, or we can use it to understand how the energy flows through an entire ecosystem.
That's how powerful the conservation of energy really is.
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