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Homework Help: Impulse of a firefighter's net on boy

  1. Mar 20, 2010 #1
    1. The problem statement, all variables and given/known data
    A boy of mass 60.0 kg is rescued from a hotel fire by leaping into a firefighter's net. The window from which he leapt was 8.0 m above the net. The firefighters lower their arms as he lands in the net so that he is brought to a complete stop in a time of 0.40 s.

    1.What is his change in momentum during the 0.40 s interval?
    Answer: 751.2 kg·m/s

    2.What is the impulse on the net due to the boy during the interval? [Hint: Don't ignore gravity.]
    Answer: N·s

    3.What is the average force on the net due to the boy during the interval?
    Answer: kN i got 1878 N and converting to kN is 1.88 kN but for some reason it indicates it is wrong

    2. Relevant equations

    2.i don't know


    3. The attempt at a solution

    on 2. I don't know what to do since it mentions not to ignore gravity does that means i just have to multiply answer1xgravity?

    What is wrong with number 3? I thought i got it right since i just did 751.2/.40s=1878 N
    Last edited: Mar 20, 2010
  2. jcsd
  3. Mar 20, 2010 #2


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    The net force exerted an impulse of 751.2 kg·m/s, bringing the boy to rest. However, the net force is composed of the force on the net and gravity.
  4. Mar 22, 2010 #3
    Is this mean for problem 2, we:

    751.2 * 9.8 = 7361.76 correct?
  5. Mar 22, 2010 #4


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    No. What does impulse times acceleration equal? (Hint: not anything meaningful for this question)
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