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Angular momentum

Hi,

Just a quick check -if I have two gears starting with one spinning and then they come together and both spin I take the angular momentum of one of them as negative as it is spinning in the opposite direction?

Thanks :smile:
Original post by natninja
Hi,

Just a quick check -if I have two gears starting with one spinning and then they come together and both spin I take the angular momentum of one of them as negative as it is spinning in the opposite direction?

Thanks :smile:


That would mean the final momentum is 0. Because they must both spin at the same rate. But the initial momentum will not be 0 because only one is spinning... :eek:
Original post by hello calum
That would mean the final momentum is 0. Because they must both spin at the same rate. But the initial momentum will not be 0 because only one is spinning... :eek:


You need to consider the wider system, to include what the gears are attached to, if you want to apply conservation of momentum.

It's similar to the problem of throwing mud at a wall and the mud sticks.
What happened to the momentum of the mud? Is momentum conserved?
Original post by natninja
Hi,

Just a quick check -if I have two gears starting with one spinning and then they come together and both spin I take the angular momentum of one of them as negative as it is spinning in the opposite direction?

Thanks :smile:


Yes. But I suspect there is a lot more to this problem than you have written here.
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natninja
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Original post by hello calum
That would mean the final momentum is 0. Because they must both spin at the same rate. But the initial momentum will not be 0 because only one is spinning... :eek:


Actually not for this problem as they have different masses, so they don't spin at the same rate...

Original post by Stonebridge
Yes. But I suspect there is a lot more to this problem than you have written here.


Actually not much more - just that they have different masses and radii

Anyway I figured it had to be positive still by checking with the case where the gears are the same and if I made it negative then if the gears are the same physics breaks XD

Anyway thanks guys :smile:

P.S. this was for a homework question which was from a past paper for the first year physics at my uni - no candidate answered it correctly...
(edited 10 years ago)
Original post by Stonebridge
You need to consider the wider system, to include what the gears are attached to, if you want to apply conservation of momentum.


For simplicity, do we just ignore the fact that momentum is negative? Or do we not use conservation of momentum?
Original post by hello calum
For simplicity, do we just ignore the fact that momentum is negative? Or do we not use conservation of momentum?


Ignore nothing.
(angular) momentum will always be conserved but you just have to make sure you include the whole system and not just part of it. The system includes all objects that experience a force as a result of the interaction. In the case of the gears you would need to consider the forces on the axles and mountings.

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