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Homework Help: Net force on ball from ground to top

  1. Sep 14, 2007 #1
    1. The problem statement, all variables and given/known data
    A ball is tossed up into the air. It reaches maximum height and comes back down.

    What is the net force on the ball at the following situations:

    a. The ball is moving upward as it is released.

    b. The ball is at it's highest point. (when it comes to a stop i suppose)

    c. The ball is moving downward.

    2. Relevant equations

    F = ma ?

    3. The attempt at a solution

    a. Force is up and decreasing.

    b. Force is 0

    c. Force is down and increasing.

    Please let me know what you guys think. I have other problems also.
  2. jcsd
  3. Sep 14, 2007 #2


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    Gold Member

    Welcome to the Forum.a is correct before release. b is wrong. Think again.
    What is making the ball slow down and then come back down ?

    C is formally correct but in practical terms, not so.
    Last edited: Sep 14, 2007
  4. Sep 14, 2007 #3
    At all points the net force on the ball is the force of gravity i.e. its weight acting downwards (ignoring air friction). Because of this force only, the ball first decelerates coming to rest at the top most point and then accelerates downwards.
  5. Sep 14, 2007 #4
    Vijay just gave you the answer, so I'll see if I can help you understand it:

    Let's suppose that at the very peak of it's flight, the force was equal to zero, as you initially thought. You also said the relevant equation is F=ma. Well, if F is zero, then the acceleration would also be zero. We already know that the velocity at that peak is zero. If the acceleration was also zero, then it'd just sort of stay there.
  6. Sep 16, 2007 #5
    thank you for all your help! Mentz was wrong @ first but thank you for clearing up the rest.
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