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Homework Help: Calculating momentum of soccer ball.

  1. May 7, 2015 #1
    1. The problem statement, all variables and given/known data
    A soccer player, kicking a soccer ball, gives the ball a velocity of +26.8 m/s. The mass of the ball is 0.425 kg, and the duration of the impact is 1.05  10–3 s.

    a) What is the momentum of the soccer ball after it has been hit?

    b) What is the change in momentum of the ball?

    c) What is the impulse imparted to the ball?

    2. Relevant equations
    P = mv
    (F*t)=P = mvf = mvi

    3. The attempt at a solution

    a) p = mv
    p = (0.425kg)(26.8m/s)
    p = 11.4 kg(m/s)

    b) (F*t)=P = mvf = mvi
    = (0.425)(26.8) - (0.425)(0m/s)

    c) this questions makes me unpleasant, I thought the change in momentum is same as impulse imparted to the ball. What's the difference between b) and c) ?
    Last edited: May 7, 2015
  2. jcsd
  3. May 7, 2015 #2

    Simon Bridge

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    ... so far so good: you should check your assumptions for the next bit...
    ... this answer is not finished.
    You know the initial momentum is 0 and the final momentum is p (your label above) so the change in momentum must be p ##\Delta p = 11.4##kgm/s

    That is correct.
    What makes you think there should a difference?
    Sometimes there is more than one way of saying the same thing, and the teacher want's to test that you understand this.

    You can finesse the answer by working out the average force imparted to the ball too.
  4. May 7, 2015 #3
    Oops for b) it is same as the answer from a) but in different units 11.4 N/S

    I will find more info about the difference between those two.
  5. May 7, 2015 #4

    Simon Bridge

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    N/S is not the correct SI units for change in momentum or for specific impulse - you want N.s
    Impuse is force times time ... so Newtons times seconds. "Seconds" is lowers case "s".
    Upper case "S" is the SI symbol for the "siemen" - the unit of electrical conductance.

    N.s are the same units as kg.m/s - the "N" stands for "kg.m/s2".
    This should make sense because change in momentum should really be in momentum units just like change in time is in time units and change in length is in length units.

    See also: http://www.periodni.com/international_system_of_units.html
  6. May 8, 2015 #5
    I usually write down units correctly, but I often make an error when I'm typing into computer :P However, I will definitely check that website as it contains a lot of useful physics.
  7. May 8, 2015 #6

    Simon Bridge

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    There's often a mark (in exams) associated with getting the right units so it's worth taking some time over.
    The answers for all three are basically the same number with the same units ... only the context has changed.
  8. May 8, 2015 #7
    Yup, I
    Thanks you for help, advice sir
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