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Expression for the force on a particle

  1. Mar 4, 2008 #1
    1. The problem statement, all variables and given/known data
    A particle of mass m is at rest at t=0. Its momentum for t>0 is given by Px=6t^2 kg m/s, where t is in s. Find an expression for Fx(t), the force exerted on the particle as a function of time.

    2. Relevant equations


    3. The attempt at a solution

    The question seems really simple but momentum confuses me. I know that momentum is related to the area under the Fx(t)curve between Ti and Tf by Pfx-Pix or the change in momentum, but I don't know where to go from there. Something to get me headed in the right direction would be greatly appreciated!
  2. jcsd
  3. Mar 4, 2008 #2


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    Hint: Newton's second law states that:

    [tex] \vec{F} = \frac{d \vec{p}}{dt} [/tex]
  4. Mar 4, 2008 #3
    so then it would be F=12t. That seems a little too easy. When they say Px would I also have to find Py in order to find the mag of P?
  5. Mar 4, 2008 #4


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    Maybe it is. What level of physics are you taking? Is it calculus-based? It's clear that you already know how to differentiate. If your prof expected you to be familar with calculus and to be aware that F = dp/dt was the true (most general) form of Newton's law, then yes, he has assigned a trivial problem. However, if you prof did not expect you to be familiar with calculus or F = dp/dt, then maybe he thought he had given you a stumper.

    By the way, WERE you aware that F = dp/dt before I told you? If not, can you see that F = ma follows from this relation provided the mass of the particle is constant?

    No. Why would you? They don't ask you for P or Py. They only ask for Px.
  6. Mar 4, 2008 #5
    I am in calculus physics and I talked to my teacher and he didn't realize that he assigned such an easy problem :rofl:

    I was aware that F = dp/dt before you told me but I didn't use it because I thought I was missing something because it seemed too simple of a problem considering it was among the more difficult problems at the end of the chapter.
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