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Homework Help: Finding force from momentum

  1. Apr 10, 2013 #1
    1. The problem statement, all variables and given/known data
    The question is in the first and second image. The question I am going to ask is c(ii).

    I know one way of finding the force is to divide the momentum by the time, and my answer was correct.
    But the marking scheme(third image) shows another way of doing it which includes an extra value of 0.08, and I don't know where does it come from. What exactly is it?

    2. Relevant equations
    1. F= p/t

    2. F= mv/t

    3. The attempt at a solution
    Since the solution in the marking scheme used F= mv/t, and v/t = acceleration, so I tried calculating the acceleration. To no avail.

    Positive velocity part: 9/2.4 = 3.75
    3.75 x 0.78 = 2.9N, correct answer but no 0.08.

    Negative velocity part: 6/1.6 = 3.75
    Same as above.

    I know this may seem a bit lame but I cannot feel comfortable if I don't find out where that 0.08 come from. Please help me, thanks.

    Attached Files:

  2. jcsd
  3. Apr 10, 2013 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    The fault is the teacher's.

    He/she miscomputed F = m delta v / delta t. The 0.08 looks like maybe a mistaken roundoff of 0.78 kg. But plainly and simply, that number does not belong there at all. The answer is 10.3/3.5 = 2.9N.
  4. Apr 11, 2013 #3
    Thank you.
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