Main content

### Course: Middle school physics > Unit 3

Lesson 1: Kinetic energy# Kinetic energy

Any object in motion has the property of kinetic energy. Simply put, kinetic energy is motion energy and depends on an object's mass and speed. As mass or speed increases, so does kinetic energy. Created by Khan Academy.

## Want to join the conversation?

- My brain is not braining. . . HELP ME!(47 votes)
- the kinetic energy get faster based on how hard you push it, also swis went fastest :)(16 votes)

- i wonder if this is based off of her dinner experience...

maybe she's trying to tell us that her daughter has bad politeness manners...(26 votes) - Which is the best and simplest definition way to remember Kinetic and Potential Energy? Not too long, but a very short definition.(12 votes)
- Potential energy is energy that is capable of being exerted so potential energy is energy not released and kinetic energy is energy caused by an object moving. Hope this helps.(13 votes)

- Why do poeple dream, is there a place in the brain that makes people dream? If someone loses that place in the brain can they not dream or sleep or not dream.(13 votes)
- Man Idk but thats a good question(5 votes)

- Does this video show an example of how math and science go hand it hand(7 votes)
- Yes, because math is what is used to explain science in more precise ways.(10 votes)

- For the 1/2 part of KE=1/2mv^2 does the 1/2 only apply to the m part or is it applied for m and v^2?(5 votes)
- The kinetic energy formula means: 1/2 multiplied by the mass multiplied by velocity squared. You don't multiply half of the mass with half of the velocity squared because that would give you 1/4mv^2 instead.

Does that help?(11 votes)

- If the lighter cheese is lighter, then why is the heavier cheese going faster? They contradicted this in the acceleration section. Thanks!(6 votes)
- Acceleration and speed are different things. Acceleration is a vector, while speed is not. Acceleration measures how much an object speeds up over a period of time, and speed measures how far an object travels over a period of time.(9 votes)

- why do we do math in science?!?!?!?!?!(4 votes)
- Math is the language of the universe. Since science aims to explore and explain the universe around us, science must speak in math.(8 votes)

- this lady gets it. she explained it beautifully(7 votes)
- My brain hasen´t turned on........HELP(6 votes)
- Try taking a short break and then coming back to it. It should help, as it did for me.(2 votes)

## 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.