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Homework Help: Newton's law?

  1. Aug 9, 2006 #1
    Q: if the thread(see attach) is pulled in the direction shown, which way will the spool roll?

    i know that the spool will roll to the right according to newton's law (for every action there's an equal and opposite direction). but there's also a moment apply to the spool (the moment arm is from center to the bottom of the thread). could you say that there's is an equal and opposite moment(with moment arm from center to the top of the thread)? What law states this?
    -what else is this question trying to get you to understand, if any?

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  2. jcsd
  3. Aug 9, 2006 #2

    Doc Al

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    Why do you think Newton's 3rd law is relevant here? All that tells you is that the force that the thread exerts on the spool is equal and opposite to the force that the spool exerts on the thread. So what?

    If you want to understand how the spool moves, first identify all the forces acting on the spool. Then apply Newton's 2nd law to both translational and rotational motion and see what you can deduce.
  4. Aug 10, 2006 #3
    Sorry Doc Al for my poor understanding, but I don't quite understand what you said. The force applied on the roll will produce a torque (which cause rotational motion to the right) and traslational motion to the left. But i don't know how to deduce from Newton's Second Law that the roll must rotate to the right instead of left.
  5. Aug 10, 2006 #4
    thank you for clarifying for my bad word choices(moment and moment arm). Torque is the correct term for it.

    I have to agree with Harmony. I don't see how 2nd law fits into this. According to Newton's 2nd Law:
    The rate of change of the momentum of a body is directly proportional to the net force acting on it, and the direction of the change in momentum takes place in the direction of the net force.
  6. Aug 10, 2006 #5


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    As Doc Al says, you need to consider only the forces acting on the spool.
  7. Aug 10, 2006 #6

    Doc Al

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    If ever you want to determine how forces affect the motion of a body, Newton's 2nd law is key.

    Start by identifying the forces acting on the spool. Hint: There are two horizontal forces acting on it. In additional to their translational effects, these force also exert torques on the spool.

    Once you have properly identified the forces (please draw yourself a clear diagram indicating where the forces act), then set up Newton's 2nd law for (1) Translation and (2) Rotation. You'll get two equations. Solve for the resulting acceleration.

    (Another hint: Assume that the spool rolls without slipping.)
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