Kinetic energy of Earth and ball system

Click For Summary

Homework Help Overview

The discussion revolves around the kinetic energy of a system consisting of a ball and the Earth, specifically addressing why the kinetic energy of the system can be considered equal to that of the ball. Participants are exploring the implications of reference frames on kinetic energy calculations.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Some participants attempt to reason that the kinetic energy of the Earth must be larger than that of the ball, questioning the validity of this assumption. Others suggest that the interpretation of kinetic energy is dependent on the chosen reference frame, particularly emphasizing the need to consider the center of mass.

Discussion Status

The discussion is ongoing, with participants providing insights into the complexities of kinetic energy in different reference frames. There is an acknowledgment of the flawed nature of the original question, and some guidance is offered regarding the interpretation of relative speeds in the context of the center of mass.

Contextual Notes

Participants note that the question may be flawed due to the dependence of kinetic energy on the reference frame, which is a critical aspect under discussion.

lkijmj
Messages
4
Reaction score
0

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. (??)
 
Last edited by a moderator:
Physics news on Phys.org
lkijmj said:
...the kinetic energy of the Earth should be larger than it of the ball. (??)
Show explicitly that this is (or isn't) the case.
 
lkijmj said:

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?
 

Similar threads

  • · Replies 6 ·
Replies
6
Views
3K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 32 ·
2
Replies
32
Views
3K
  • · Replies 9 ·
Replies
9
Views
2K
  • · Replies 4 ·
Replies
4
Views
6K
  • · Replies 9 ·
Replies
9
Views
1K
  • · Replies 8 ·
Replies
8
Views
1K
  • · Replies 15 ·
Replies
15
Views
4K
  • · Replies 5 ·
Replies
5
Views
2K