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Homework Help: Force and change in momentum with time

  1. Jan 10, 2006 #1
    Hey, It's been about 2 weeks since I've had a physics class and I just can't seem to put things together again.
    I have two problems that seem like they should be easy, and I'm sure they are, but am unsure on how to do them.

    A constant force of 6.00N acts on a 3.00-kg object for 10.0 s. What are the changes in the objects momentum and velocity?

    I have the equation momentum p = mv. p=(F)(delta t). Could I just do 6.00-N x 10=(3.00-kg)v and solve for v to find the velocity?

    The second problem is
    The velocity of a 625-kg car is changed from 10.0 m/s to 44.0 m/s in 68.0 s by an external constant force. What is the resulting change in momentum of the car? What is the magnitude of the force?
    I tried to do p=(625)(44-10.0) to find the momentum, but that does not seem right to me. What do i do?
  2. jcsd
  3. Jan 10, 2006 #2


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    For question one I don't think your looking for a velocity, your're looking for a change in velocity.
  4. Jan 10, 2006 #3


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    (delta p) = F (delta t) ... there has to be a change on both sides !

    or only one side, with a ratio on the other side : F = (delta p) / (delta t).

    or nowhere, as in : p = m v .

    now, in your (625)(44 - 10) , does the other side have a delta, or not?

    and whatever happened to UNITS ?!!
  5. Jan 11, 2006 #4


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    The impulse that an object experiences is equal to the change in it's momentum!
  6. Jan 15, 2006 #5
    Alright, I ended up remembering how to do these problems.
  7. Jan 15, 2006 #6
    Yeah, just remember that [tex]F=ma[/tex] only works at low velocities, but [tex]F=\frac{dp}{dt}[/tex] is true always.
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