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Finding net force using velocity graph?

  1. Jan 26, 2013 #1
    Finding net force using velocity graph?????

    The figure(Figure 1) shows the velocity graph of a 2.5object as it moves along the x-axis.


    06.EX08.jpg

    What is the net force acting on this object at t=7 ?



    The attempt at a solution

    Since F=ma, I tried first finding a by dividing 6 m by 7 s. Then I got 0.857 m/s2 x 2.5 kg = 2.1429 ≈ 2.1 ....but it's wrong :(

    Please help.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jan 26, 2013 #2

    rude man

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    Re: Finding net force using velocity graph?????

    What you need to do is plot acceleration on the same graph. Acceleration = ax*dvx/dt. So at first the particle accelerates positively a certain amount (how much?), then stops accelerating, then accelerates negatively another certain amount (again, how much?).

    Now, keeping in mind Fx = max, read off ax on your newly created chart and use this formula.

    What comment might you have regarding the ax graph for x < 6 s?
     
  4. Jan 26, 2013 #3
    Re: Finding net force using velocity graph?????

    I got a= -2 at t = 7.
    I did F = 2.5 x 2 = 5.0 ...It's still wrong
     
  5. Jan 26, 2013 #4

    rude man

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    Re: Finding net force using velocity graph?????

    How did you get a = -2 at t = 7? That's really off.
     
  6. Jan 26, 2013 #5
    Re: Finding net force using velocity graph?????

    Nevermind. I think it's a = 6? I took the slope = 12 / 2 = 6
     
  7. Jan 26, 2013 #6
    Re: Finding net force using velocity graph?????

    correction: -6 ?
     
  8. Jan 26, 2013 #7
    Re: Finding net force using velocity graph?????

    I used a = -6, and i got the right answer! Thanks for you help
     
  9. Jan 26, 2013 #8
    Re: Finding net force using velocity graph?????

    Force is a change in momentum, the momentum changes for the first 3s and the last 2s - The force is constant since the decrease in velocity is linear. Find the change in velocity over the period of time when you know the momentum is changing, then you get the force. You know this is exact since the change in velocity throughout each time period is constant (not changing) - i.e. it is not an approximation.
     
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