# Homework Help: Do you consider both regular and rotational kinetic energy?

1. Apr 11, 2015

### fangrz

1. The problem statement, all variables and given/known data
If I have a ball moving in a circular path (ball is connected to a string), as shown in this picture:
http://w3.shorecrest.org/~Lisa_Peck.../circularmotion/Images/cent_force_on_ball.gif

should I say that the energy of the ball is both its kinetic energy (1/2)mv^2 and its rotational kinetic energy = (1/2)Iomega^2?

2. Relevant equations
kinetic energy (1/2)mv^2
rotational kinetic energy = (1/2)Iomega^2

3. The attempt at a solution
I was just thinking about this. I mean, what if you have a car moving in a circle--do you consider both kinetic energy and rotational kinetic energy?

2. Apr 11, 2015

### TSny

Hello fangrz.

In principle, the total kinetic energy would include rotational KE as well as translational KE. However, if the radius R of the ball is much less than the radius r of the circle, then the rotational KE is very small compared to the translational KE. You can easily check this. So, the rotational KE is often neglected in this type of situation. Then the ball is effectively treated as a "point particle".