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What is Fnet?

  1. Jan 17, 2008 #1
    [SOLVED] What is Fnet?

    I am given a problem with an initial velocity then a final velocity, the weight and how long it took to change that velocity. They want us to get the change in momentum, but I don't know how to compute the Fnet to use in the equation. Any suggestions on the steps I should take?
  2. jcsd
  3. Jan 17, 2008 #2

    Doc Al

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    Staff: Mentor

    To find the change in momentum, just calculate the final momentum and the initial momentum and find the difference. (Remember that momentum is a vector, so direction counts.)

    What's the definition of momentum?
  4. Jan 17, 2008 #3
    Thank you Doc Al for replying.

    Momentum is p = (gamma)(mass)(velocity)

    Having said that I still don't understand how to compute Fnet given two vectors, a mass, and a time.

    I would just like to know how to compute the net force, Fnet, given the circumstances.
    Last edited: Jan 17, 2008
  5. Jan 17, 2008 #4

    Doc Al

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    Is this a question about relativistic dynamics? Or ordinary non-relativistic physics?

    Are you familiar with the impulse-momentum theorem? http://hyperphysics.phy-astr.gsu.edu/Hbase/impulse.html" [Broken]
    Last edited by a moderator: May 3, 2017
  6. Jan 17, 2008 #5
    Actually I was not.

    That link for impulse might help me out.

    The force average is the mass(change in velocity)/(change in time).

    The question is as follows:(took out number, substitiued x so you can't solve it for me so i can try to figure it out)
    You were driving a car with velocity <x, x, x> m/s. You quickly turned and braked, and your velocity became <x, 0, x> m/s. The mass of the car was x kg. The maneuver took x seconds. What was the average net (vector) force acting on the car?

    So to get that do i just use the force average formula?
  7. Jan 17, 2008 #6

    Doc Al

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    Yes, that will work. Take care to find the change in velocity correctly (subtract them as vectors).
  8. Jan 17, 2008 #7
    Thank you very much Doc.

    I solved the equation thanks to your help and now know what Fnet is. You are awesome.
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