If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Kinetic energy

# Kinetic energy

Any object in motion has the property of kinetic energy. Simply put, kinetic energy is motion energy. Learn about how kinetic energy is quantified and which characteristics of an object affect how much kinetic energy it has. Created by Khan Academy.

## Want to join the conversation?

• Which is the best and simplest definition way to remember Kinetic and Potential Energy? Not too long, but a very short definition. • What is a short definition for kinetic and potential? • When we make new discoveries we need to be able to share them with others. And the first thing we have to do is make sure everyone is on the same page. We do this by using units and frames of reference, which are also called reference frames. We talk about units in another video, so let's look at what a frame of reference is. Let's say this blue box thing is a car, and it's going 45 miles per hour. Someone standing on the side of the road would see it pass at 45 miles per hour. Now, if this yellow truck is going 40 miles per hour someone sitting in the yellow truck would observe the blue traveling at five miles per hour. How could the person on the side of the road see the blue car traveling at 45 miles per hour and a person in the yellow truck see the blue car moving at five miles per hour? This is because both observers are using different frames of reference. So let's go ahead and take a look at that, starting with the speed of the blue car. The person on the side of the road is using their frame of reference of being at rest. So relative to them, the blue car is moving at 45 miles per hour. To the person in this yellow truck, which remember • • Hello, everyone. m going to explain this for your minds. Let's talk about kinetic energy. Now, kinetic might be an unfamiliar word, but it just comes from a Greek word that means of motion, so kinetic energy is energy from motion. 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 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't need both pieces of cheddar, so he eats one and sends one back, along with this 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 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 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 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, which we'll write as V for velocity, so we can start with K E 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 doubles, the kinetic energy actually quadruples. And there's also a constant factor of 1/2 at the beginning of the equation, but we won't go into the details of the math of deriving this today. So, this is the equation for kinetic energy, 1/2 M V squared. Let's apply this equation to our cheesy example. Say the Swiss has a mass of .05 kilograms, which makes the cheddar's mass .1 kilograms. When both cheeses have the same speed, say two meters per second, the cheddar's kinetic energy is 1/2 times .1 kilograms, times two meters per second squared, which is .2 Joules. The Swiss's kinetic energy is 1/2 times .05 kilograms times two meters per second squared, which is .1 Joules, or half the kinetic energy of the cheddar. So we can see that at the same speed, the cheddar has more kinetic energy because it has more mass. But when the Swiss has a speed of four meters per second and the cheddar still has a speed of two meters per second, the Swiss's kinetic energy is now 1/2 times .05 kilograms times four meters per second squared, which is .4 Joules. So now, the kinetic energy of the Swiss is twice the kinetic energy of the cheddar. So we can see that even though the cheddar has more mass the Swiss has more kinetic energy because it's going faster. In summary, kinetic energy is the motion energy of an object. The equation for kinetic energy is 1/2 M V squared. So as mass increases, kinetic energy increases, like the more massive cheddar versus the Swiss, and as velocity increases, kinetic energy increases even more, like the speedy Swiss versus the slower cheddar. Thanks for watching, and I hope you learned a little bit of something.
(1 vote) • • • • • ## Video transcript

- [Instructor] Hello, everyone. Let's talk about kinetic energy. Now, kinetic might be an unfamiliar word, but it just comes from a Greek word that means of motion, so kinetic energy is energy from motion. 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 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't need both pieces of cheddar, so he eats one and sends one back, along with this 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 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 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 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, which we'll write as V for velocity, so we can start with K E 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 doubles, the kinetic energy actually quadruples. And there's also a constant factor of 1/2 at the beginning of the equation, but we won't go into the details of the math of deriving this today. So, this is the equation for kinetic energy, 1/2 M V squared. Let's apply this equation to our cheesy example. Say the Swiss has a mass of .05 kilograms, which makes the cheddar's mass .1 kilograms. When both cheeses have the same speed, say two meters per second, the cheddar's kinetic energy is 1/2 times .1 kilograms, times two meters per second squared, which is .2 Joules. The Swiss's kinetic energy is 1/2 times .05 kilograms times two meters per second squared, which is .1 Joules, or half the kinetic energy of the cheddar. So we can see that at the same speed, the cheddar has more kinetic energy because it has more mass. But when the Swiss has a speed of four meters per second and the cheddar still has a speed of two meters per second, the Swiss's kinetic energy is now 1/2 times .05 kilograms times four meters per second squared, which is .4 Joules. So now, the kinetic energy of the Swiss is twice the kinetic energy of the cheddar. So we can see that even though the cheddar has more mass the Swiss has more kinetic energy because it's going faster. In summary, kinetic energy is the motion energy of an object. The equation for kinetic energy is 1/2 M V squared. So as mass increases, kinetic energy increases, like the more massive cheddar versus the Swiss, and as velocity increases, kinetic energy increases even more, like the speedy Swiss versus the slower cheddar. Thanks for watching, and I hope you learned a little bit of something.