# Momentum question

1. Feb 16, 2004

### adamg

If a child of 50kg is riding a bike at 10ms-1, and suddenly stops pedaling, obviously they will eventually come to a halt due to friction and air resistance. I was just wondering what happened to their momentum. It decreases from 500kgms-1 to zero, where does it go? (Similarily for any object that gradually comes to a halt due to friction/resistance).

2. Feb 16, 2004

### drag

It "goes" into the medium with which the slowing body
is interacting (the source of the friction). It can
also turn to thermal energy of the slowing body
and/or the medium or into sound waves in the body and/or
the medium.
(It's not a HW Q, is it ? )

Live long and prosper.

3. Feb 16, 2004

### HallsofIvy

Staff Emeritus
Momentum remains constant as long as there is no external force.
"friction and air resistance" are external forces. I wouldn't say that momentum "goes" anywhere. There is no "conservation of momentum" in the presence of external forces.

There is "conservation of energy" in very general terms. Here the kinetic energy of the bicycle goes into the heat energy produced in the brakes, the pavement, and, in a very small measure, the body of the rider and the bicycle, as well as the air.

4. Feb 16, 2004

### adamg

Yeah, because the principle of conservation of momentum states that it must be a 'closed system' i.e. no external forces. Momentum cant really change into anything else though can it, because nothing else has the units kgms-1.

The reason I thought up this question was that a saw an example in a book as follows: a car crashes into a wall and comes to a halt. The momenum is transferred to the Earth, which makes it spin ever so slightly faster. Hence momentum is still conserved.

This just got me thinking that momentum must always 'go' somewhere. But I guess momentum can just 'disappear' in the presence of an external froce then can it?

5. Feb 16, 2004

### adamg

Just another question (and not hwk, purely curiosity). When a ball falls to Earth, the Earth is also moving upwards towards the ball, hence total momentum remains zero.
But what is going on as a ball rolls down a hill? The ball is gaining momentum, yet the Earth cant be gaining momentum can it, since the ball is already on the surface?
I thought it was another case of gravity being an external force. But how can gravity be an external force in one situation, yet not in another?

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