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Homework Help: Question about acceleration.

  1. Dec 7, 2014 #1
    1. The problem statement, all variables and given/known data
    if there are is a person standing in front of wall with 19 inches between the wall and the person, and the person slams their hand against the wall in a matter of .29 seconds. Also, prior to the impact, the hand was moving 32 inches/sec with a resiliency of .5 inches. How do you solve for acceleration?

    2. Relevant equations
    a = (vf-vi)/t (not sure about this equation)

    3. The attempt at a solution
  2. jcsd
  3. Dec 7, 2014 #2


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    That will give you the acceleration in the first part.
    For the second part, you'll need a different equation (or this plus another one) as you don't know the deceleration time yet.
  4. Dec 7, 2014 #3


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    I think you have better post the question exactly as posed. Solve for acceleration when? Before impact, during impact? Both phases?

    Before impact:

    If it takes 0.29 seconds to travel the 19 inches then you might expect it to be moving at roughly 19/0.29 = 65 inches/second. However the problem says just prior to the impact, the hand was moving at only 32 inches/sec. That suggests the hand isn't moving at constant velocity before it hits the wall. So you perhaps you need to work out what's going on in that phase first. I suspect a "pulled punch" ??

    During the impact:

    If I have understood the problem correctly.... the hand impacts at 32 inches/sec but is compressed 0.5 inches during the impact. You can treat the 0.5 inches as the stopping distance to work out the average deceleration.
  5. Dec 7, 2014 #4

    sorry for my wording. The question is asking for the entire acceleration from start to finish within those 19 inches, and "prior to the impact", that was referring to my initial velocity. I'm just confused as to how to set up my problem and equation in order to find the acceleration.
  6. Dec 8, 2014 #5


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    It's still not 100% clear what the problem is.

    Are you saying the hand has an initial velocity of 32 inches/sec and then travels 19 inches in 0.29 seconds? If so I believe you can use one of the other SUVAT equations..

    S = Ut + 0.5at2
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