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Torque and Friction

  1. Jun 18, 2015 #1

    I'm new here. I have an interest in physics and have spent the day educating myself on a few concepts. I've stumbled across the concept of angular momentum and I am struggling to understand a particular aspect...

    I understand in principle the idea that angular momentum is conserved, however I do not understand the part that says "unless acted upon by an external torque".

    What would that external torque be? If I put a bicycle upside down and start spinning the wheel with my hand I understand that I am applying a torque to the wheel and creating angular momentum. If I left the wheel to spin it would eventually slow down to a stop. So my questions are:

    If the wheel has slowed down to a stop, where did the angular momentum go if it is conserved?
    What external torque is being applied to the wheel when it slows down?
    Does friction count as a torque? i.e. air resistance etc acting on the wheel - are these torques? And if that does count as torque, please could you explain how?

    Please go easy on me as the last time I studied physics was over 10 years ago in school. I am just an interested amateur keen to expand my knowledge.

    Thanks so much.
  2. jcsd
  3. Jun 18, 2015 #2


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    I think you need to understand the concept of a system boundary. Conservation of angular momentum holds as long as there are no torques that are external to the system. If you find a system where angular momentum appears not to be conserved that invariably means you have "forgotten" to include something in your system (eg within your system boundary).....

    When you spun up the wheel you actually changed the angular momentum of the planet by the same amount. You pushed on the wheel, the wheel pushed back on you and you braced yourself against the planet causing the angular momentum of the planet to change.

    The reverse happens when you stop the wheel. Friction tries to make the bike frame rotate. If you stop it by bracing yourself against the ground then the angular momentum of the planet changes back.

    Yes friction can cause a torque. A torque is a force acting at a distance (radius) from a point of rotation..

    Torque (Nm) = force(N) * distance(m)

    Air resistance acting on the tyre creates a torque equal to the drag force * radius of the tyre
    Rim brakes create a torque equal to the frictional force * radius of the rims
  4. Jun 18, 2015 #3


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    Might also be worth you looking at how they point the Hubble Telescope in different directions. It has reaction wheels inside. To rotate the telescope in one direction they spin the reaction wheel in the other direction. From the outside it looks like the telescope just rotated (which would be a breach of conservation of angular momentum) but if you take into account the reaction wheel inside then there has been no net change of angular momentum.

    You might also be interested to find out how falling cats manage to turn over so they always land on their feet. Ditto high divers. At first glance these appear to be examples of a breach of conservation of angular momentum but they aren't.
  5. Jun 18, 2015 #4
    Thanks for your response. I understand what you're saying about the system boundary - I hadn't considered at all the wider system i.e. the planet.

    I'm going to read up on Hubble now.

    The conservation of angular momentum is absolutely fascinating.
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