Momentum Q: What Happens When an Object Stops?

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When an object, like a child on a bike, stops due to friction and air resistance, its momentum decreases from a positive value to zero. This momentum does not "go" anywhere but is transformed into other forms of energy, primarily thermal energy and sound, due to the interaction with the medium causing the resistance. Momentum conservation applies only in closed systems without external forces; thus, when external forces are present, momentum can appear to "disappear." The discussion also touches on how momentum is transferred in collisions, such as a car hitting a wall, where it can affect the Earth’s motion. The conversation raises questions about the nature of gravity as an external force, illustrating the complexities of momentum in different scenarios.
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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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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.
 
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.
 
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?
 
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?
 
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Thread 'Variable mass system : water sprayed into a moving container'

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Thread 'Struggling to understand the concept of this question (ice cube in water optics question)'

TL;DR Summary: I came across this question from a Sri Lankan A-level textbook. Question - An ice cube with a length of 10 cm is immersed in water at 0 °C. An observer observes the ice cube from the water, and it seems to be 7.75 cm long. If the refractive index of water is 4/3, find the height of the ice cube immersed in the water. I could not understand how the apparent height of the ice cube in the water depends on the height of the ice cube immersed in the water. Does anyone have an...
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