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Homework Help: 1 string, 2 balls, motion problem

  1. Oct 13, 2012 #1
    1. The problem statement, all variables and given/known data
    http://www.xtremepapers.com/papers/...S Level/Mathematics (9709)/9709_w09_qp_41.pdf

    number 6III
    http://www.xtremepapers.com/papers/...S Level/Mathematics (9709)/9709_w09_ms_41.pdf

    2. Relevant equations

    3. The attempt at a solution
    Okay, the distance of P from ground is 3m and the distance of Q from ground is 7m, already calculated. However, im confused when the marking scheme says that the initial velocity for Q is -2m/s? why is that?

    3 = 2tP + 5tP^2 initially i wrote this
    7=2t+5t^2 for Q, the problem is why is the intial velocity is -2?
  2. jcsd
  3. Oct 13, 2012 #2
    P & Q are moving in different directions? If one has a "positive" velocity, what will the other have?
  4. Oct 13, 2012 #3
    But since the string breaks, the balls should all be moving downwards, right?
  5. Oct 13, 2012 #4


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    Science Advisor

    It doesn't. I will admit that the answer as shown is ambiguous. The problem asked for two speeds and only the speed of P is given. The speed of P (not "initial" but at the time the string broke) is -2 m/s (downward) so the speed of Q is 2 m/s (upward).
  6. Oct 13, 2012 #5
    Ok, but why is speed of Q taken as -2, though? A bit confused. Can elaborate more?
  7. Oct 14, 2012 #6
    Yes, but you are asked to find the speed of the particles when the string brakes.

    Before string brakes, the particles are moving. P goes down (it's heavier) and Q goes up. When the string breaks, then they both go down.

    I've attached a photo. Position 1 is at rest, position 2 before string breaking and position 3 after breaking.

    Attached Files:

  8. Oct 14, 2012 #7
    If you mean why did they put -2 at (iii), it's because the initial velocity has different direction than the path s. The direction of the path goes from up to down. For particle P it's +2 because the initial velocity (2 m/s) has the same direction as s.
  9. Oct 14, 2012 #8
    Ah that explains it!. thanks all much appreciated
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