# Kinetic energy

1. Jan 17, 2010

### yaseen shah

would any one deeply tell me what is kinetic energy.

2. Jan 17, 2010

### Gerenuk

I think kinetic energy is something you invent to have a number - instead of the vectorial momentum - which together with the invention potential energy is constant over time.

At classically speaking it seems to be so. More answers could come from special relativity.

3. Jan 17, 2010

### Staff: Mentor

Energy is the capacity to do work, and work is force times distance. Kinetic energy is the capacity that something has to apply a force over a distance by slowing down. For example, a bullet is going very fast and can hit a target and decelerate rapidly thereby applying a large force over some distance.

4. Jan 17, 2010

### Naty1

You can get some insight into KE from an introductory physics text which derives it from
W = Fd = max....a change in velocity of a mass times a distance traveled.

the KE of a body in motion is equal to the work it can do as it is brought to rest....so it's based on a useful, convenient definition.

For a deeper understanding, check the relationship with potential energy (PE) because in some instances they are equal in magnitude and opposite in sign....as in throwing a ball vertically in the air for example...when the decreasing velocity (decreasing KE) is replaced by an equal increase in magnitude of PE as the ball rises. Things reverse as it falls.

In relativity, things get more complex because each inertial (constant velocity) observer measures the apparent velocity of an object differently...so each thinks (correctly) the object has a different KE.

Parts of the Wikipedia write up on KINETIC ENERGY are easy to understand; some are opaque.
http://en.wikipedia.org/wiki/Kinetic_energy

from a post above: "....which together with the invention potential energy is constant over time... not sure exactly what this means, but in general it is not accurate.; see my example where this is true....with regard to a ball rolling on a flat surface, it is NOT.

Last edited: Jan 17, 2010
5. Jan 17, 2010

### Cryxic

It's the energy associated with motion, as opposed to potential energy, which is associated with position. For example, if you're standing still holding a soccer ball in your hands, the ball has potential energy. In other words, it's got the "potential" to do something (to fall to the ground). Once you release the ball, the ball's potential energy is converted into kinetic energy since the ball is moving under the influence of gravity.