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B Force required to move an object upwards

  1. Oct 12, 2016 #1
    Standing within a field, should the force required to move an object in the opposite direction of the field without acceleration be equal to the force exerted by the field on it or a little more than that?
     
  2. jcsd
  3. Oct 12, 2016 #2

    DrClaude

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    If the object is not accelerating, then there is no net force on the object, therefore all forces must balance out exactly.
     
  4. Oct 12, 2016 #3
    I know that but by intuition it seems to me that if an equal force is applied then the object will not move.
     
  5. Oct 12, 2016 #4

    DrClaude

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    Then your intuition is wrong :smile:.

    If the object is initially at rest, then yes, to get it to move, you need an unbalanced force. But as soon it has reached the desired speed, balanced forces will make it continue at constant speed.

    Note that you had asked about
    if there is no acceleration, then an object that was not moving cannot start moving.
     
  6. Oct 12, 2016 #5
    Is this true for the Earth's gravitational field also?
     
  7. Oct 12, 2016 #6

    DrClaude

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    Yes.
     
  8. Oct 12, 2016 #7

    CWatters

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    Yes. When a helicopter is hovering the lift force created by the rotor is exactly equal to the weight of the aircraft. In order to accelerate up or down one of the forces (lift or weight) must be greater than the other so that there is a net force. See also Newtons first and second laws in Wikipedia.
     
  9. Oct 12, 2016 #8

    Dale

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    The word "move" usually refers to velocity. Force is proportional to acceleration. You can have velocity without acceleration (movement without force) and acceleration without velocity (force without velocity, at least momentarily). So they are fairly separate concepts that you should not try to mix inappropriately.
     
  10. Oct 12, 2016 #9

    russ_watters

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    Right: I think it is common colloquially for people to consider "accelerate" to be a subset of "move". But in physics, they have precise definitions that are separate.
     
  11. Oct 12, 2016 #10
    @CWatters, in your example the helicopter rose up to a certain height due to unbalanced forces and then the forces which act on it get balanced. So should not the helicopter rise up with constant velocity according to newton's first law?
     
  12. Oct 12, 2016 #11

    jbriggs444

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    Please review what @russ_waters just said in #9. "move" and "accelerate" are different things.
     
  13. Oct 12, 2016 #12

    A.T.

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    Yes, if the net vertical force is zero, the helicopter will hover or climb with constant velocity.
     
  14. Oct 12, 2016 #13

    russ_watters

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    CWatters did not say anything at all about what happens when the helicopter is moving up. But yes, if it is moving up (or in any direction) at constant velocity, the net force is zero.
     
  15. Oct 12, 2016 #14
    @Dale, please clarify what you mean by "acceleration without velocity (force without velocity, at least momentarily)"
     
  16. Oct 12, 2016 #15
    @jbriggs444, i know that "move and accelerate are different things".
    To move means to change position with time while to accelerate mean to change the velocity with time.

    To move without acceleration means to move with constant velocity which require a zero net force and to move with acceleration means to move with a changing velocity which requires a non-zero net force.
     
  17. Oct 12, 2016 #16
    @A.T. ,Why should the helicopter hover? Shouldn't moving up with constant velocity be the only option?
     
  18. Oct 12, 2016 #17

    Dale

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    When a ball is thrown vertically it will go up and then fall back down. It accelerates at -g the entire time, but at the top it is momentarily at rest. So it has a non zero acceleration and a zero velocity at that point.
     
  19. Oct 12, 2016 #18
    @russ_watters, i said that the helicopter must have moved up with acceleration caused due unbalanced forces and then when balanced forces act on it shouldn't it move up without acceleration instead of hovering?
     
  20. Oct 12, 2016 #19
    @A.T., but initially the helicopter was moving up with acceleration and was not at rest.
     
  21. Oct 12, 2016 #20

    A.T.

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    What matters is the velocity it had, when the net force became zero.
     
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