1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    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

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    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

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    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

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

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

    arildno

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    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

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    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

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Oh, I see, your baseball weigh 100g, not 100kg as you used..
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Momentum Questions
  1. Momentum Question (Replies: 4)

  2. Momentum questions (Replies: 4)

  3. Momentum questions (Replies: 9)

  4. Momentum Question (Replies: 5)

  5. Momentum question (Replies: 5)

Loading...