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Homework Help: Calculating work done to catch a ball

  1. Aug 12, 2011 #1
    A boy catches a ball of mass 150g, which is moving at 20ms^-1

    A) calculate the force which he must exert to stop it in (i) 0.10s (ii)0.50s

    Im resonably sure i use the formula W=F.D

    what confuses me is that to find the force i have to use F=M.A but 20ms^-1 is not accelleration. if not this formula then which one do i use?

  2. jcsd
  3. Aug 12, 2011 #2


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    The ball comes to a stop in the given time. What is the speed of the ball when it stops? Use one of the kinematic motion equations to find the acceleration.
  4. Aug 12, 2011 #3
    am i allowed to use the understanding that impulse equals the change in momentum.

    so F*T=M*V

    im thinking this becuase i have been given mass and velocity? so i would get 3000 jolues
  5. Aug 12, 2011 #4
    wait never mind, sorry that was kinda stuuupid, i havnt used the time at all.
  6. Aug 12, 2011 #5
    im looking at the kinematic equations but i dont know what one i should be using.

    i've rearranged

    Velocity=initial velocity + accelleration * time

    i got (velocity/time)-initial velocity = accelleration

    is this correct. ALSO how can i use this i only have velocity, not initial velocity. Or are they the same?
  7. Aug 12, 2011 #6


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    This is the closest you have got. Your only error to that point is to call the 3000, 3000 Joules.

    The units of momentum are not Joules.

    Work & Energy interact Force, distance, mass and velocity

    Momentum & impulse interact Force, time, mass and velocity.

    What information were you given? Which approach should you use?
  8. Aug 12, 2011 #7
    i think i've worked it out on my 1's, for anyone else who might need this information you use the formula

  9. Aug 12, 2011 #8
    so am i right to say i need to use the formula F=M*(V/T)?

    it has given me (i)30,000 and (ii)6,000. are the units joules for these? I'm trying to find the work done to stop the ball.
  10. Aug 12, 2011 #9


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    You seem to have lost track of the question - I will quote it here - with emphasis.

    "calculate the force which he must exert to stop it in (i) 0.10s (ii)0.50s"

    Numerically your answers are correct, and being forces, the unit will be Newton.

    Notice that you were not trying to find the Work done to stop the ball. Indeed, at no time during the question was the word Work - or energy for that matter - used.
  11. Aug 12, 2011 #10


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    yes, that's final velocity = initial velocity + acceleration*time , or

    vf = vi + at

    where vf = velocity when ball stops and
    vi = velocity just before the ball makes contact with the glove.
    Your algebra is not so good. If you solve for acceleration, then

    a = (vf -vi)/t

    Now using F = ma, you get

    F = m(vf -vi)/t

    Which is exactly the same as the impulse= change in momentum equation,

    F(t) = m(vf -vi)

    As PeterO has noted, correct your units for force.
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