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Homework Help: The momentum principle

  1. Sep 8, 2013 #1
    1. The problem statement, all variables and given/known data
    At a certain instant a particle (mass=0.5 kg) is moving in the +x direction with velocity 14 m/s. During the next 0.15 seconds a constant force acts on the particle, with Fx= −8 N and Fy= Fz = 0 N. What is the magnitude of the momentum of the particle (in kg*m/s) at the end of this 0.15 second interval?

    2. Relevant equations
    pf = pi + Fnet*deltat

    3. The attempt at a solution

    I am at a complete lose as to what to do. I think I have to add the velocity and the Fx together. After that, I have no idea.
  2. jcsd
  3. Sep 8, 2013 #2


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    Hi Alucard2487! Welcome to PF! :smile:
    i] you can only add things of the same srot, eg two velocities or two momentums or two forces …

    you can't add a velocity to a force :wink:

    ii] force is rate of change of momentum, so how much and in which direction does the given force change the momentum? and what was the original momentum?

    show us what you get :smile:
  4. Sep 8, 2013 #3


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    Your relevant equation is indeed relevant.

    you are after pf [pf]

    You can calculate pi [pi]
    You were given Fnet [Fnet]
    You were given deltat [Δt]

    Go ahead and calculate.
  5. Sep 8, 2013 #4
    I was also wondering if there is a good youtube channel that I could watch. My professor doesn't have a good grasp on the English language, so it's hard to understand her.
  6. Sep 8, 2013 #5
    I figured it out. What I did was this: .5kg(14)+(-8)(.15). I ended up with 5.8kg*m/s which was the correct answer.
  7. Sep 9, 2013 #6


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    probably better to find a good online book :wink:
  8. Sep 9, 2013 #7
    Any recommendations? We use "Matter and Interactions" in class and I find the book to be a bit dull.
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