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Homework Help: Work and acceleration of a touchdown

  1. Nov 26, 2011 #1
    1. The problem statement, all variables and given/known data
    After scoring a touchdown, an
    84.0-kg wide receiver celebrates by leaping
    1.20 m off the ground. How much work
    was done by the wide receiver in the

    2. Relevant equations
    W= FD

    3. The attempt at a solution
    I know that the correct answer should be:
    (84.0 kg)(9.80 m/s2)(1.20 m)=988 J
    I'm just wondering why the acceleration for the wide-receiver is 9.8m/s^2.
    If his acceleration is 9.8 and the acceleration of gravity is 9.8, wouldn't the total acceleration be 0? Also, when a person jumps, he starts off with a positive velocity which decreases, so shouldn't his acceleration be negative?
  2. jcsd
  3. Nov 26, 2011 #2
    Once he is in the air, the only force acting on him is his weight i.e. the pull of gravity on him. But weight W = mg.

    Hence if we use F = ma (downwards) for the man

    we get

    W = ma
    mg = ma
  4. Nov 26, 2011 #3
    OK I get that the acceleration should be 9.8, because there's only one force, but why is it positive instead of negative?
    Last edited: Nov 26, 2011
  5. Nov 27, 2011 #4
    The acceleration due to gravity is always towards the centre of the Earth. In our case we can say that the acc due to gravity is always downwards.

    Hence if an object is going upwards this DOWNWARD acceleration due to gravity will decrease this velocity and so it acts as a retardation while if the object is going downwards this DOWNWARDS acceleration due to gravity will increase the velocity.
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