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Physics in vertical jump and basball hit

  1. Jan 22, 2015 #1
    1. The problem statement, all variables and given/known data
    Okay so these are from movies. I am trying to prove them right or wrong using physics. how can I figure out the pounds of force of a vertical jump when I have the:

    height of the jump = 6 feet
    Time to reach top = .797 sec
    Mass of person = 100lbs



    2. How can I figure out this problem.....

    If a baseball is thrown with a velocity of 90mph, and the ball exits the bat at 140mph, what eas the bat speed? Consider the ball was hit with a wood bat. On its most dense part with the barell diameter of 2.5 in, the bat is 30 oz

    Friction- how far would a runner slide if he fell on ice after running 15 mph



    2. Relevant equations


    3. The attempt at a solution
    For the first one I messed up somehow using the fact that initial velocity was 0m/s^2 and I got that the force would be only around 50lbs. The number should be in te thousands I'm suspecting. Please help
     
  2. jcsd
  3. Jan 23, 2015 #2

    haruspex

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    You have to make some assumption about how the force varies over the period before leaving the ground. Constant is a bit unrealistic as it means power increases linearly with speed. Constant power might be nearer the mark.
    Be careful how you define that. It's the movement of the mass centre that's important for the forces etc. Most vertebrates can only get their mass centres up about three feet from the point of leaving the ground.
    From starting the jump (from crouching? How low?) or from lift off?
    You will also need to know the mass of the ball; and the coefficient of restitution... or you could just suppose it to be perfectly elastic.
    I don't see how the diameter matters. I guess you mean the bat is struck at its mass centre.
    Again, you need more data. What do you think that might be?
     
  4. Jan 23, 2015 #3
    For the friction what other data do I need?

    The mass of the ball is 5 oz

    The height of the jump is 6 feet. It is from a movie and I'm trying to prove it is not realistic

    I don't know what restitution is.
     
  5. Jan 23, 2015 #4
    The jump time is from lift off and using the mass center
     
  6. Jan 23, 2015 #5

    haruspex

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    You are trying to calculate a distance given an initial speed and a final speed. You don't know the time, so what else will help?
    What equations do you know related to collisions? What is or may be conserved?
    It's a measure of how well bodies rebound from collisions. Take two bodies, initially moving at speeds u1, v1. Positive is to the right, say. Take it all to be in one straight line. After collision, they move off with speeds u2, v2. It turns out that if you were to repeat the experiment with the same bodies at different speeds you would find there is a constant (usually written as e, for elasticity, or R for restitution) such that v2-u2 = R(u1-v1). (Note the sign reversal.) For more info G**gle for coefficient of restitution.
    In a perfectly elastic collision, the coefficient is 1. The fraction of energy retained is the square of the coefficient.
    Mass centre up 6 feet - definitely unrealistic.
    What take-off speed would be needed? What SUVAT equations do you know? (You should have listed these in the OP.)
     
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