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Homework Help: Momentum Questions

  1. Oct 10, 2005 #1
    Hello,

    Due to a teacher strike where I live, I've been trying to teach myself the momentum unit . Could anyone go through these questions with me? Or tell me what formulas I should use to solve these problems?

    1. A mass of 6.3 kg traveling at 6m/s is acted on by a force giving an impulse of -31.5 N/s. What is the velocity of the mass after the impulse?

    I know that I have to do something with the final momentum and the intial momentum, but I don't know what formula to use.

    2. What force must be imparted to a 100g baseball to change the velocity from 40.0 m/s to -50.0 m/s in 1.2 ms?
    b) what is the impulse of this force?

    thanks
     
  2. jcsd
  3. Oct 10, 2005 #2

    arildno

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    Derive it then, from Newton's second law and the definition of impulse.
     
  4. Oct 10, 2005 #3
    When I tried that I got:

    delta p = pf-pi (change in momentum= momentum final - momentum initial)
    deltap=m(vf)-m(vi)
    37.8=6.3(vf)-6.3(6)

    The answer for number 1 is supposed to be 1.0 m/s. Where am I going wrong?
     
  5. Oct 10, 2005 #4

    arildno

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    Well, putting in the wrong number, for startes.
    Where did you get 37.8 from, it is supposed to be -31.5
     
  6. Oct 10, 2005 #5
    I thought I was supposed to calculate the momentum and put that in...

    -31.5= 6.3(Vf) - 6.3(6)
    -31.5/6.3= Vf - 37.8
    -42.8= Vf

    Is that better?
     
  7. Oct 10, 2005 #6

    arildno

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    Impulse equals change in momentum, okay?
    So, you have been given the impulse as -31.5, agreed?
    You also know that initial momentum is 6.3*6=37.8, agreed?
    Thus, you have:
    -31.5=6.3*Vf-37.8
    That is, adding 37.8 to both sides:
    6.3=6.3*Vf

    Understood?
     
  8. Oct 10, 2005 #7
    I see, I did it in the wrong order. Thank you so much! :smile:
     
  9. Oct 10, 2005 #8

    arildno

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    No problem.
     
  10. Oct 10, 2005 #9
    Would I use the same formula for the next problem too?
     
  11. Oct 10, 2005 #10

    arildno

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    It is simplest to answer b) first, and then determine the force afterwards.
     
  12. Oct 10, 2005 #11
    2. What force must be imparted to a 100g baseball to change the velocity from 40.0 m/s to -50.0 m/s in 1.2 ms?
    b) What is the impulse of this force?

    delta p= pf-pi
    delta p= m(vf)-m(vi)
    delta p= 100(-50)-100(40)
    delta p= -5000 -4000
    = -9000

    is that how I set that up?

    If i divide the impulse by the time I get the force: -9000/ 1.2 = -7500. Which gives me the right answer for a.
     
    Last edited: Oct 10, 2005
  13. Oct 10, 2005 #12

    arildno

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    Rather, you should say:
    a) By DEFINITION of the symbol delta(p), delta(p) = mvf-mvi.
    b)By Newton's 2.law, we have
    I=delta(p), where I is the impulse.

    Thus, you have:
    I=mVf-mvi

    You have derived the right answer for I.
     
  14. Oct 10, 2005 #13
    Thats an easier way of thinking of it :), thank you.

    How do I get the correct impulse? In the answer key, it says that the answer for b. is -9 N/s

    edit: I think the answer key might be wrong:( thank you for your help, I'm starting to understand this now :D.
     
    Last edited: Oct 10, 2005
  15. Oct 10, 2005 #14

    arildno

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    Oh, I see, your baseball weigh 100g, not 100kg as you used..
     
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