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Some Simple Physics Questions

  1. Nov 16, 2009 #1
    I have 3 simple (for most people on this forum) multiple choice questions that I need help with.

    1.The centre of mass of an object...
    A. Must always lie outside the object, at a point where there is no mass.
    B. May lie outside the object, at a point where no mass is.
    C. Must always lie within the mass of the object



    2. Consider a ball on the end of a string being whirled in a horizontal circle. The horizontal force acting on the ball points...
    A. outwards, away from the centre of the circle
    B. inwards, towards the centre of the circle
    C. ... nonsense. There is no force at all on the ball



    A man is standing on a turntable holding a dumbell in each of his outstretched hands. He is rotating at 1 rev/s. If he now brings his hands in close to his chest,
    A. his rotational speed decreases
    B. his rotational speed is unchanged
    C. his rotational speed increases

    If you could please explain the answer to each question I would be grateful, I'm having issues with rotational motion. Thanks!!
     
  2. jcsd
  3. Nov 16, 2009 #2

    Hey there;

    I suggest you to think about question 1.There is a lot of examples from our daily lifes about the center of the mass.

    Imagine a metal sphere, as you know the center of its mass is also the center of the sphere (ofcouse we take it as an "ideal sphere").So the center of the mass for a sphere is inside of the object.

    If we consider another example, take a "V" shaped wire now.Do you think the center of its mass is going to be on one of the wires? or is it going to be on where the lines intersect each other? or is it on somewhere else?.In other words from which point should i hang this V shaped wire with a rope to make it stand still?

    About your question of rotational motion, i can suggest you to do a little experiment.Find a disk that is turning (maybe try to turn your cd player on when its hatch is not closed so you can see the disk turning).If we put masses to different locations on the disk, lets say one close to the center and one that is far from the center.Which one is going to fall first?.Do you think the radius has an effect on a "force" that is acting on the masses?
     
  4. Nov 16, 2009 #3
    So based on the different experiments I did, I'm inclined to think that the answer for
    question #1 is C and the answer for question #2 is B and the answer for Question 3 is A.

    Are my assumptions correct?
     
  5. Nov 16, 2009 #4
    Unfortunately it is not correct, its not always inside of the object.Remember the example i mentioned, "V" shaped wire.A V shaped wires center of mass is not located inside the object it actually is located at some point that is close to the point where 2 lines intersect, but definitly its not on one of those wires.So its somewhere outside of the object...
     
  6. Nov 16, 2009 #5
    I understand now, so the answer would thus be "B" because it sometimes can be outside of the objects mass?

    Are my other assumptions correct?

    Thank you very much for all your help Cryphonus :smile:
     
  7. Nov 16, 2009 #6

    ideasrule

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    Homework Helper

    #3 is not A. The classic example for the phenomenon described here is a rotating figure skater who pulls her arms in. What happens to rotational speed?
     
  8. Nov 16, 2009 #7
    Ohh, it increases right?

    So for question 1, The centre of mass of an object may lie outside the object, at a point where no mass is.

    Question 2: Consider a ball on the end of a string being whirled in a horizontal circle. The horizontal force acting on the ball points inwards, towards the centre of the circle.

    and Question 3:A man is standing on a turntable holding a dumbell in each of his outstretched hands. He is rotating at 1 rev/s. If he now brings his hands in close to his chest,
    his rotational speed increases.
     
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