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How does an object move up with the same force as weight?

  1. Nov 19, 2013 #1
    I really don't understand this concept:

    If an object is at rest initially. The object has mass 10 kg. Then if we apply 100 N (g=10m/s/s) in upward direction. How does it move in a constant velocity?

    Because the way I understand it, the object should be at motion initially to go in a constant velocity if net force is 0.
     
  2. jcsd
  3. Nov 19, 2013 #2

    Nugatory

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    If the object is at rest, the ground underneath it is already applying an upward force of 100N - otherwise the object would be falling towards the center of the earth at 10 m/s/s.

    So when you apply your 100N of force as well, that's plenty to start the object moving upwards. But as soon as it leaves the ground, the ground is no longer applying a force to the object, so we just have your 100N up and gravity's 100N down, for a net force of zero. Net force of zero means no acceleration, no change of speed... But we've already started the object moving up, so no change of speed means it keeps moving up.
     
    Last edited: Nov 19, 2013
  4. Nov 19, 2013 #3
    So, if the object was in my hand my hand would apply an upward force of 100N. Then what is the work done in moving the object 2m high. And what force is used in moving the object up and if the 100N is used isn't it cancelled by weight already?
     
  5. Nov 19, 2013 #4

    Nugatory

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    If the object starts in your hand and at rest, you're applying 100N to just hold it there. You'll have to increase that force just the least little bit to start it moving, but once you've started it moving you can go back to 100N to exactly cancel out gravity and keep it moving at a constant speed.

    The work done is the 100N you're applying, over a distance of 2m. You're doing that work no matter what gravity is doing; if there were no gravity you'd get a lot more acceleration and speed out of your 200J (100N times 2m) but it would still be 200J of work you did.
     
  6. Nov 20, 2013 #5
    If we start to apply a force increasing from 0 to 100N, the object will not move upwards. The application of the applied force decreases the normal force the earth has on the object from 100N to 0N. The Gravity and the 100N apllied force still cancel out to zero.
    Different words - same theme.
     
  7. Nov 20, 2013 #6

    sophiecentaur

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    I think the answer must hinge on the strict description of the experiment - how near the experiment is to the simple, ideal, theoretical model. You're always on potentially dodgy ground when you try to apply the simple theory to a practical case and then assume you can depart from the ideal without some paradoxical behaviour.
    If the upward force is exactly the same as its weight, the object will neither move (accelerate) up or down. Any excess force upwards will result in a small but finite upwards acceleration (assuming a uniform gravitational field, of course).
     
  8. Nov 20, 2013 #7

    adjacent

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    Newton'f first law:Every body tends to stay at rest or move at a constant velocity in a straight line,unless an external force acts on it.(or no net force is there)
    When the object is at rest,there are no net forces.When it is moving at a constant velocity,there are no net forces,but the body should be moving before the net force becomes zero.



    In case of gravity,If it is not moving,there is a 100N force applied upwards beforehand.
    You have to make it move upwards by applying a net force directed upwards to start to move it upwards.Then if net force becomes zero(Upward force equals downward force),it will move with the speed it had just before net force becomes zero.
     
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