# What information does kinetic energy and work give us about a body?

When I studied momentum I learned that momentum says about a body how hard is it to stop it. In addition, I learned that impulse tells us in some way how much time would I need to stop the object.

If you can understand what I mean by the paragraph above can you answer my question in the same way?

What does kinetic energy and work says us about a body?

## 1 Answer

Kinetic energy tells us how much work is required to stop a body.

The total work done on a body equals its change in kinetic energy. That's what the kinetic energy theorem says.

The "work of a body" makes no sense. Work is something done ON a body BY an agent. It can be seen as an exchange of energy from that agent to the body. For instance, if you make a work of 10 J on a baseball to throw it, that energy is taken from molecules in your muscles and given to the ball.

EDIT : to stop a body, you need to make a negative work. If you throw a ball up on a shelf, it will leave your hand with much kinetic energy, but the gravity will do a negative work on the ball (ie. slow it down).

• The first line was the one that answered my question. Could you explain the same thing about energy, what does it tell about a body? – Pichi Wuana Feb 10 '16 at 19:48
• You mean potential energy ? Total energy ? – Teacher77 Feb 10 '16 at 19:54
• Kinetic energy. – Pichi Wuana Feb 10 '16 at 19:55
• ??? It's already what my first sentence is saying : the kinetic energy of a body represents how much work would be required to stop it. For instance, if a ball has 20 J of kinetic energy, one would need to make a work of -20 J on it in order to stop it. – Teacher77 Feb 10 '16 at 20:00
• Right.. right. I got confused. I actually meant by work. What does work tell about a body? Sorry! I know the kinetic energy theorem but didn't help me to understand. – Pichi Wuana Feb 10 '16 at 20:02