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Impulse - I am suspecting a wrong answer on the key.

  1. Nov 16, 2011 #1
    Impulse -- I am suspecting a wrong answer on the key.

    A 2.4-kg ball falling vertically hits the floor with a speed of 2.5 m/s and rebounds with a speed of 1.5 m/s. What is the magnitude of the impulse exerted on the ball by the floor?

    * 9.6N/s
    * 2.4N/s
    * 6.4N/s
    * 1.6N/s
    * 1.0N/s

    Answer key says 9.6N/s


    My calculations say 2.4N/s


    Impulse = ∆Momentum

    ∆Momentum = mv(Initial) - mv(final)


    ∆Momentum = (2.4kg)(2.5m/s) - (2.4kg)(1.5m/s)

    ∆Momentum = 6 - 3.6 = 2.4 N/s


    What is the right way of doing this?
     
  2. jcsd
  3. Nov 16, 2011 #2

    lurflurf

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    Homework Helper

    Re: Impulse -- I am suspecting a wrong answer on the key.

    That is a cute question.

    I=Δp=mΔv

    You have Δv wrong (sign error).

    Δv=(-2.5-1.5)m/s

    Do this for practice
    A 40 000 Kg sled is sliding towards you at 50 m/s what impulse do you need to apply to change its velocity to
    a)49.999 m/s towards you
    b)49.999 m/s away from you
     
    Last edited: Nov 16, 2011
  4. Nov 16, 2011 #3
    Re: Impulse -- I am suspecting a wrong answer on the key.

    So then


    Impulse = ∆Momentum

    ∆Momentum = mv(Initial) - mv(final)

    So the v(final) should be negative (my coordinate plane being down as the positive direction)
    then

    ∆Momentum = (2.4kg)(2.5m/s) - (2.4kg)( - 1.5m/s)

    which gives me
    ∆Momentum = 6 + 3.6 = 9.6


    Thanks!
     
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