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Kinetic energy/ net work problem

  1. Apr 17, 2007 #1
    1. The problem statement, all variables and given/known data

    Suppose you throw a stone having a mass of 0.50 kg vertically upward. Let us assume that your hand exerts an average force of 110 N over an arm displacement (upward) of 0.60 m. Through the following sequence of questions, we shall explore, in terms of the energy and momentum concepts, what happens to the stone. Follow the sequence carefully for the exercise that it provides in using and interpreting the energy and momentum concepts, setting up numerical expressions, and indicating your line of reasoning. Do not resort to calculating accelerations or using the kinematical relations except to check your results for internal consistency.


    b.) Calculate the work done on the stone by your hand in the act of throwing.


    c.) Calculate the net work done on the stone during the act of throwing. Be sure to be able to explain why this number differs from the one obtained in part (b).


    d.) Calculate the change in kinetic energy of the stone during the act of throwing.

    e.) Calculate how high the stone will rise (making use of kinetic and potential energy concepts.)

    f.) Using the result obtained in part (d), calculate the velocity of the stone at the instant it parts contact with your hand.


    g.) Calculate the change of momentum that was imparted to the stone in the act of throwing.


    h.) What magnitude of net impulse, in what direction, must have been imparted to the stone by the act of throwing?


    i.) What total magnitude of impulse was imparted to the stone by your hand?





    2. Relevant equations



    3. The attempt at a solution

    For part c and d, i read in my textbook that net work is the same as the kinetic energy. the equation for kinetic energy is 1/2mv^2. there is no velocity given in the problem. i am really stuck with this one and dont know what to do.
     
  2. jcsd
  3. Apr 17, 2007 #2

    Doc Al

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    Staff: Mentor

    Hint: Is the force of your hand the only force acting on the stone?
     
  4. Apr 17, 2007 #3
    by factoring in gravity, i was able to get all of them except for e. and i. what is the difference between the total magnitude of impulse of the stone imparted by the hand, and the magnitude of net impulse imparted by the act of throwing?

    also, i dont understand how i will be able to calculate how high the stone will rise with this information
     
  5. Apr 17, 2007 #4

    Doc Al

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    Staff: Mentor

    One includes only the force imparted by the hand; the other includes all forces.

    Hint: Once the stone leaves the hand, mechanical energy is conserved.
     
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