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Gravity and upward motion

  1. Feb 1, 2013 #1
    Hello everyone, I am have some confusion regarding gravity and a ball travelling upwards.

    Suppose you throw a ball upwards in to the air. At the beginning, the ball is at rest atop your hand, Freaction=mg. When your hand moves upwards to throw the ball, Freaction>mg and the ball accelerates upwards with acceleration a,while in contact with your hand. Upon release, the ball will travel upwards with the same acceleration.

    But with the increase in height, a will decrease. I know that this is due to the pull of gravity but I can't seem to understand fully as when the ball soars, the upward thrust is greater than that of the weight of the ball, mg.

    Thus if the thrust is greater than the pull of gravity on the ball, there is a net force acting upwards, propelling the ball skywards perpetually, which is illogical.

    Could somebody please correct my reasoning and end my confusion? Many thanks in advance :)
  2. jcsd
  3. Feb 1, 2013 #2


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    Science Advisor

    No. Upon release, the acceleration of the ball drops to -g.

    No, the velocity v will decrease. The acceleration during flight is constant -g (ignoring drag)

    There is no thrust once the ball leaves the hand. Just gravity.
  4. Feb 1, 2013 #3
    I am starting to see your point here, but allow me to ask: when the ball leaves the hand, it will still travel upwards, right? Shouldn't the be a upward force compelling it to move?
  5. Feb 1, 2013 #4
    No. In the absence of force the ball will continue to move at the same speed, in the same direction, forever. This principal is called momentum. Once the ball leaves your hand the only force acting on it is gravity, which is why it comes down. If not for gravity it would continue upward into the universe forever

    All this assumes no air resistance.
  6. Feb 1, 2013 #5

    Doc Al

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    Staff: Mentor

    Sure, since it has an initial upward velocity.
    No. In the absence of force, things keep moving at the same velocity. (Newton's 1st law.) Force is needed to change velocity, not maintain it. Once it leaves your hand, the only force acting on the ball is gravity, which accelerates the ball downward.
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