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Force and Momentum problems

  1. Mar 15, 2015 #1
    1. The problem statement, all variables and given/known data
    A catcher stops a .250kg ball traveling at 100km/hr in .19s.
    a. Calculate the change in momentum of the baseball [-6.9ns]
    b. Calculate the force the catcher applies to the baseball [-36.5n]

    2. Relevant equations
    △p=m△v
    f△t=m△v

    3. The attempt at a solution
    a. △p=m△v
    △p=m(vi-vf)
    △p=mvi-mvf
    △p=(.25kg*100m/s)-(.25kg*0m/s)
    △p=25ns

    b. F△t=m△v
    F=m△v-△t
    F=((.25kg*100m/s)-(.25kg*0m/s))-(.19s)
    F=24.81n

    Both answers are wrong. Correct me please.
     
  2. jcsd
  3. Mar 15, 2015 #2

    RaulTheUCSCSlug

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    Gold Member

    In your attempt you have 100m/s but in your question, you have 100km/hr, you see the difference? Looks like you need to do a conversion.
     
  4. Mar 15, 2015 #3

    phyzguy

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    Science Advisor

    You need to pay more attention to the units. Getting the units right in these problems is usually the most difficult part. 100 km/hr is not the same as 100 m/s.
     
  5. Mar 15, 2015 #4
    Yes! Thank you so much. I will be careful with the units.
    And attempt to solution B, I was using an Impulse formula and I made a mistake.
    When you try to solve for the Force. You must divide time from the force.
    So It would be,
    F(.19s)/.19s=(.25kg*27.78m/s)/.19s
    F=36.55 N
     
  6. Mar 15, 2015 #5

    phyzguy

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    Science Advisor

    I think this is now correct.
     
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