# Kinetic energy of Earth and ball system

1. Apr 13, 2015

### lkijmj

1. The problem statement, all variables and given/known data
There is a system consists of a ball and the earth. Explain why you can consider the kinetic energy of the system equals to the kinetic energy of the ball.

2. Relevant equations
$F=-\frac{m_{1}m_{2}}{{r}^{2}}$

3. The attempt at a solution
The magnitude of the force acting on the earth is same with it of the ball. Then the kinetic energy of the earth should be larger than it of the ball. (??)

Last edited by a moderator: Apr 13, 2015
2. Apr 13, 2015

### Staff: Mentor

3. Apr 13, 2015

### paisiello2

Show explicitly that this is (or isn't) the case.

4. Apr 14, 2015

### haruspex

The question is somewhat flawed. The KE of a system depends on the reference frame. In a reference frame centred on the sun, the Earth's KE is much the greater. You need to interpret it as being in a frame centred on the common mass centre. Given that, what can you say about the relative speeds?