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Force to lift an object

  1. Nov 26, 2012 #1
    1. A block weighs 2N. What is the force required to move the block up by 2m?
    a) 0N
    b) 1N
    c) 2N
    d) 4N



    Wanna hear from experienced teachers and personnel on this question.
     
  2. jcsd
  3. Nov 26, 2012 #2

    PhanthomJay

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    Re: Force

    You first need to give us your thoughts on what and why your answer might be.
     
  4. Nov 26, 2012 #3
    Re: Force

    I would think that the answer is 4N. As F must be greater than the weight before a resultant force can cause a movement in the desired direction. As to 2N, it only overcome the weight which implies that it is in equilibrium, hence no movement will be made.

    Anyone has a different opinion or correction to make?
     
  5. Nov 26, 2012 #4

    PhanthomJay

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    Re: Force

    How about 2.0000000000000000000000000000001 N...would that be enough to move it? If so, should there be a choice
    e) none if the above ?
    Why or why not?
     
  6. Nov 26, 2012 #5
    If force must be greater than the weight to lift an object up, then 2.00000000000000000000000000000001 N will be possible.

    As this is a multiple choice question, there is a correct ans. The choice none of the above would be redundant i guess.
     
  7. Nov 27, 2012 #6

    haruspex

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    It depends whether it's already moving. If it happens to be moving upwards already - ever so slightly - 2N will be adequate.
     
  8. Nov 27, 2012 #7
    Supposed that the object is stationary on the floor. In order to lift it up, does it necessary mean that the force to lift the object upwards must be greater than the object's weight?
     
  9. Nov 27, 2012 #8

    CWatters

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    Yes.

    The question is badly worded.

    If it's stationary at the start then the "required" force must be slightly greater than 2N otherwise it can't accelerate and will retain it's initial velocity of zero. That means 4N is the only valid answer.

    If it's already moving upwards then only 2N is "required" to maintain the initial velocity, 4N would accelerate it upwards.

    If it's moving fast enough at the start even 0N or 1N might be sufficient to reach 2m, although it will be decelerating as it goes up.

    I would answer 2N and complain the question is badly worded because it doesn't state if the object is stationary or already moving.
     
    Last edited: Nov 27, 2012
  10. Nov 27, 2012 #9

    PhanthomJay

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    Since the weight is accurate to just one significant figure, then so must be the accuracy of the force required to lift it. Let us suppose the question was worded not as a multiple choice question, but rather,
    If you answer '2.01 N , or 2.0001 N, or 2.0000000001 N', all your answers would be incorrect.

    Now let us suppose that the problem is worded again as a multiple choice question, as such:
    Now I know what my answer would be; how would you answer?
     
  11. Nov 27, 2012 #10

    haruspex

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    You're overlooking the real objective in an examination, which is to figure out the answer the examiner wants. 4N is clearly the 'trap' answer, being arrived at by multiplying the weight by the distance (and ignoring units). So this cannot be the required answer, leaving 2N as the only candidate.
     
  12. Nov 27, 2012 #11

    PhanthomJay

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    Indeed.
     
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