Hey there! Sign in to join this conversationNew here? Join for free
    • Community Assistant
    • Thread Starter
    Offline

    20
    ReputationRep:
    Community Assistant
    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
    Offline

    0
    ReputationRep:
    (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
    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:
    Offline

    10
    (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?
    Offline

    10
    (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
    Yes. But I suspect there is a lot more to this problem than you have written here.
    • Community Assistant
    • Thread Starter
    Offline

    20
    ReputationRep:
    Community Assistant
    (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

    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...
    Offline

    0
    ReputationRep:
    (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?
    Offline

    10
    (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.
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Would you rather give up salt or pepper?
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

    Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

    Write a reply...
    Reply
    Hide
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.