# Momentum has to be conserved

## Main Question or Discussion Point

I suppose this sounds a little silly, but:

Momentum has to be conserved, but often at the cost of some kinetic energy. For example, if someone throws a 1kg mass at 50kg person at 10 m/s, he catches it:
mv=mv
(1)10=v(51)
v=10/51
KE1=.5(1)(10)^2=50j
KE2=.5(51)(10/51)^2=.98j

Heat is thus created, but my question is: where? I suppose in this situation it would be in the glove... but how exactly does that heat come about? Heat is, I guess, kinetic energy on a smaller scale (moving particles). How do these particles begin to move?

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Think about friction, sound all other waves that have some energy

Meir Achuz
Homework Helper
Gold Member
HiPPiE said:
Momentum has to be conserved, but often at the cost of some kinetic energy. For example, if someone throws a 1kg mass at 50kg person at 10 m/s, he catches it:
mv=mv
(1)10=v(51)
v=10/51
KE1=.5(1)(10)^2=50j
KE2=.5(51)(10/51)^2=.98j

Heat is thus created, but my question is: where? I suppose in this situation it would be in the glove... but how exactly does that heat come about? Heat is, I guess, kinetic energy on a smaller scale (moving particles). How do these particles begin to move?
There is not much friction here. The ball striking the glove causes it to vibrate. This vibration quickly degrades into a random vibration of the molecules in the glove, which raises their internal energy
(U in thermodynamics). This increase in U raises the temperature of the glove. Heat is the transfer of this internal energy from the glove to your hand, which gives you a warm feeling because the batter is out.

Danger
Gold Member
Meir Achuz said:
which gives you a warm feeling because the batter is out.
:rofl:
I would propose further that some of the muscular effort used to maintain the glove in position when the ball hits it produces heat in the catcher and could be considered part of the system.

Friction - i meant air resistance, no distances were given

Ok, I see. Makes sense...

Oh and thanks :)