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Homework Help: Average force and a balance []

  1. Oct 14, 2011 #1
    I have to deliver this problem resolved in 4 hours, I didn't think it was going to be so hard...

    1. The problem statement, all variables and given/known data
    A man of mass M is on the top of a balance with a very high damping coefficient. In a given instant he throws a ball of mass m vertically, catching it again later.

    a) Explain what are the forces that act on the balance during the vertical movement of the ball, knowing that it hits a maximum height h, from the starting point.

    b) Calculate the average force measured by the balance during the ball's movement.

    2. Relevant equations

    [tex]Favg = \frac{\Delta p}{\Delta t}[/tex]
    [tex]\Delta U = mgh[/tex]
    [tex]-\Delta U = \Delta K[/tex]

    3. The attempt at a solution

    First, I didn't even understand the question very well... During the movement of the ball, is it when the man is throwing it or when it's on the air? If it is when it's on the air this is a 7th grade physics question, because the only forces on the man are the gravitational force and the normal reaction, so on the balance the only force is a force of the same magnitude as the normal reaction...

    And in case is during the throwing of the ball, I don't know the [tex]\Delta t[/tex] to calculate the average force...
    Last edited: Oct 14, 2011
  2. jcsd
  3. Oct 14, 2011 #2


    User Avatar

    Staff: Mentor

    While he is accelerating the ball upwards, he is exerting extra force on it. The result being that he imparts a momentum p to the ball. While he is exerting a force on the ball, it is exerting an equal but opposite force on him.
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