Kinetic energy of Earth and ball system

[lkijmj]

Homework Statement

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.

Homework Equations

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

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. (??)

 

[jedishrfu]

I fixed your post, you need to bracket your equation with ## on the front and on the back.

https://www.physicsforums.com/help/latexhelp/

 

[paisiello2]

...the kinetic energy of the Earth should be larger than it of the ball. (??)

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

 

[haruspex]

Homework Statement

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.

Homework Equations

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

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. (??)

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?