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Homework Help: Circular motion

  1. Mar 2, 2004 #1
    I have got a prolem here: a particle is moving along a circular path at constant speed, use the definition of the term work to explain why the resultant force acting on the particle must be acting at right angles to its path.

    Also why must the direction of the acceleration be towards the centre of the circle?

    Thank you very much.

  2. jcsd
  3. Mar 2, 2004 #2


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    If it wasn't, the object would move in a straight line.

  4. Mar 2, 2004 #3


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    Does the speed or kinetic energy of the object change?

    How much work is the "centripetal" force doing on the object? (Centripetal is in quotes because, for the purposes of the excercise, you cannot assume that the force is towards the center of the circle.)

    What does that tell you about the force acting on the particle.

    There is a differece between having a component that is perpendicular to the velocity, and being entirely perpendicular to the velocity. For example, in parabolic motion where the initial velocity is perpendicular to the force, the particle is obviously not traveling in a straight line, but the force is only initially perpendicular to the velocity.
  5. Mar 2, 2004 #4


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    W = Fd.

    If the force was inline with the path, it would mean infinite work is being done, and the rotation speed would increase.

    Draw a picture of a guy holding a rope connected to something like a rock. If the guy swings the rock around at a constant speed, where is the force coming from? The only force I see is the rope connected to the rock; which is perpendicular to the motion of the rock.
    Last edited: Mar 4, 2004
  6. Mar 4, 2004 #5
    thanks a lot.
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