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Did i do this problem correctly

  1. Sep 17, 2007 #1
    idk if i did this correctly so far so here goes.

    The question states

    A large rubber ball (8.80 kg) is fired straight down from the roof of a building from a spring loaded mechanism that stored 987 J of elastic potential energy. The building roof is 14.0 meters above ground. The rubber bounces straight back up (noiselessly--don't ask how) back to a height of 9.5 meters. Assuming no heat is lost to the air around the ball, calculate the increase in temperature of the ball (Specific Heat of rubber is 1250 J/kgK

    What i did was:
    M= 8.80kg
    Spring Potential Energy= 987 J
    Building roof= 14.0 m
    The rubber bounces back up= 9.5 m
    Specific Heat of rubber is 1250 J/kgK

    1) I found the Change in Height= 14.0-9.5= 4.5m
    2) Then i used the gravitational potential energy formula which is
    (mass)(g)(the change in height)
    3)g=10
    4)Gravitational Potential Energy= (8.80)(10)(4.5)
    gravitational potential energy= 396 J
     
  2. jcsd
  3. Sep 17, 2007 #2

    dynamicsolo

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    Don't forget that the ball wasn't dropped from rest from the top of the building; it was *launched* by a spring gun. What was the ball's initial kinetic energy when it first started down to the ground? What was the total mechanical energy it had when it hit the ground? How much total mechanical energy did it have at the end of its rebound? Did all of the mechanical energy loss go into heating the ball?
     
  4. Sep 17, 2007 #3
    well i don't have the velocity to use the kinetic energy formula u got any advice for that.
     
  5. Sep 17, 2007 #4

    dynamicsolo

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    (Hmm, the edit function doesn't work with this browser...) I also wanted to say that what you have calculated so far is the *change* in the gravitational potential energy of the ball. Your value looks correct. (Is it OK to use g=10? I'm just asking whether your grader accepts this level of precision.) What *else* do you need?
     
  6. Sep 17, 2007 #5

    dynamicsolo

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    You don't need to know the velocity of the ball at any point. You only need to consider the amounts of each type of energy (kinetic; gravitational potential; heat or internal). It's the change in internal energy of the ball that is going to cause the temperature change.
     
  7. Sep 17, 2007 #6
    i got that but i need the formula though
     
  8. Sep 17, 2007 #7

    dynamicsolo

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    The formula for kinetic energy is KE = (1/2)M(v^2). What do you need it for?
     
  9. Sep 17, 2007 #8

    Chi Meson

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    Calculating KE is not necessary. You need to know how much total mechanical energy you started with, and how much you have after the first bounce. It is more than 396 J.
     
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