Momentum Q: What Happens When an Object Stops?

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In summary: The ball is gaining momentum because it's moving in a downward direction. The Earth is also moving downwards, but because the ball is already on the ground, it doesn't add to the Earth's momentum.
  • #1
adamg
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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).
 
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  • #2
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 ? :wink:)

Live long and prosper.
 
  • #3
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
Yeah, because the principle of conservation of momentum states that it must be a 'closed system' i.e. no external forces. Momentum can't 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
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 can't 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?
 

Q: What is momentum?

Momentum is a measurement of an object's motion and is calculated by multiplying its mass by its velocity.

Q: How is momentum conserved?

Momentum is conserved in a closed system, meaning that the total momentum of all objects in the system remains constant before and after a collision or interaction.

Q: What happens when an object stops?

When an object stops, its momentum decreases to zero. This can happen due to external forces, such as friction, or internal forces, such as collisions with other objects.

Q: Can momentum be transferred between objects?

Yes, momentum can be transferred between objects through collisions or interactions. The total momentum of the system remains the same, but the individual momentum of each object may change.

Q: How does mass affect momentum?

Mass has a direct impact on an object's momentum. An object with a greater mass will have a greater momentum for the same velocity as an object with a smaller mass.

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