Homework Help: Collision of two masses

1. Apr 14, 2014

utkarshakash

1. The problem statement, all variables and given/known data
Two small bodies initially at rest and free to move from a distance of 1m from each other are subjected to only their gravitational force of attraction. They approach each other and collide and do not separate. In respect to this collision which of the following statement is true?
a)The total gravitational potential energy(GPE) of 2 masses has increased on collision.
b)The total gravitational potential energy(GPE) of 2 masses has decreased on collision.

3. The attempt at a solution

I think option a) is correct as when the two bodies approach each other, the GPE increases as it is inversely proportional to distance. But option b) is the correct answer.

2. Apr 14, 2014

rude man

The GPE of one mass due to the other mass is always negative. It takes negative energy to move one mass from infinity to near the other mass. So the closer they get, the more negative the GPE.

3. Apr 14, 2014

utkarshakash

OK. But what happens after they stick to each other? How does one define GPE of the system after that instant?

4. Apr 14, 2014

rude man

GPE can't change after the collision. It's now a theoretical number based on the fact that they can't get any closer to each other.

For point masses the GPE approaches -infinity.

EDIT: I shouldn't have said the GPE can't change after the collision. If you then separate the masses the GPE will increase again.

Last edited: Apr 14, 2014