# Kinetic and Potential energy

1. Sep 28, 2009

### fawk3s

Hi

I was just wondering if a body can have potential and kinetic energy at the same time?
Say there was a knife on a table. So the knife would have potential energy over Earth. And I would move that knife abit left, giving it kinetic energy.
The potential energy which the knife has over Earth would remain, right? Or would it somehow transfer into kinetic energy?

My friend said that if a body has kinetic energy, it cant have potential energy. I dont want to agree, so I thought I'd ask here for confirmation.
But if my friend is right, whats the point of the equation E=Ek+Ep, if either Ek=0 or Ep=0?

I mean, I can be wrong, and I usually am. And if I am, dont mock me lolz.

fawk3s

2. Sep 28, 2009

### XanziBar

You're right, an object can have potential energy and kinetic energy at the same time.

Let's say you knock your knife over the table so that you give it an initial kinetic energy as it heads to the ground in addition to its gravitational potential energy that it had from being on the table. In fact, if it were a charged particle in an electric field it could have electric potential energy also.

So, an object can have any number of different kinds of energies it wants, it just depends on the physical situation.

3. Sep 28, 2009

### Gerenuk

True.
For example for four bodies A, B, C and D there are
the kinetic energies A, B, C and D
and the potential energies AB, AC, AD, BC, BD, CD
All these exist at the same time.

4. Sep 28, 2009

### Staff: Mentor

In airplanes, the concept of trading between the two is very important. Ie, when decending into a landing, you need to dissipate potential energy without gaining kinetic energy (and actually losing some eventually). After takeoff, you gain potential energy while maintaining a certain minimum kinetic energy.