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Three questions.

  1. Mar 30, 2005 #1
    1. If you apply a force to an object only for just and instant, for examples, if you hit a ball with a force of 10 N and it moves 100 meters, are you making work?

    2. How can a constant force that is applied to an object during a period of time can make it move with constant speed? According to Newtons second law, F=ma, so the object should accelerate.

    3. My teacher says that if we push a pendulum with the exact force it requires to reach its maximum high, the time it will take to reach that point will be infinite. How can this be possible?

    Thanks in advance.
     
  2. jcsd
  3. Mar 30, 2005 #2

    brewnog

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    2. If the force you apply is balanced by another force, the object can move at constant speed. Think about pushing a block with constant force; if the force you apply is opposed by friction in the opposite direction then a constant speed can be achieved. Newton's second law refers to the net (overall) force acting on the object.
     
  4. Mar 30, 2005 #3
    Work only happens while the force is being applied. After the force is done, no work is being done.

    2. It can't. If a single force is being applied to an object, that object will accelerate according to his second law. Only in the presence of more than one force will variation of this law occur, in which case the principles of superposition will give you the net force.

    3. This is not true. If its the exact force required to reach that point, it will reach that point for an infinitely short amount of time and leave. If you were JUST shy of the exact force, it would not reach the point at all. What he said is like saying if you tried to jump up to the moon it will take you an infinite amount of time to get there.
     
  5. Mar 30, 2005 #4
    1. If you apply a force to an object only for just and instant, for examples, if you hit a ball with a force of 10 N and it moves 100 meters, are you making work?

    i was. noone can hit a ball instantly - it always takes some certain time, during which i apply a force and the ball accellerates. The work one makes can be approximated as the kinetic energy of a ball just after the hit.
     
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