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Homework Help: Impulse problem

  1. Jul 10, 2010 #1
    1. The problem statement, all variables and given/known data
    Which of the following could be accomplished with the least average force?
    a: accelerating a 2 kg ball from 0 m/s to 9 m/s in half a second.
    b: accelerating a 6 kg ball from 0 m/s to 6 m/s in one second.
    c: accelerating a 9 kg ball from 0 m/s to 8 m/s in two seconds.
    d: accelerating a 10 kg ball from 0 m/s to 7m/s in two seconds

    2. Relevant equations
    ma Δt= Δp

    3. The attempt at a solutionAnswer given as correct is D. I used proportions to solve this problem since Δp is proportional to average force. Since initial p is zero for all the problems than Δp is lowest with answer A; pf- pi= 18 kg m/s. If it had asked for force alone than the seconds matter and then you solve for force gives f= Δp/ Δt. If you do it this way a,b, and c all give 36N while d give 35N. But the question does not ask for lowest force it ask for lowest average force. Am I right or did I make a mistake somewhere?
  2. jcsd
  3. Jul 10, 2010 #2
    Ya unfortunately the average force is defined as the Impulse ( or change in momentum) divided by the change in time. I think you were mistaking net force for average force.

  4. Jul 11, 2010 #3


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    Staff Emeritus
    Science Advisor
    Homework Helper

    ... and it's proportional to the time as well. From the impulse equation:
    Δp = Faverage·Δt​
    Δt must be taken into account.
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