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Homework Help: Projectiles - Are my answers correct?

  1. Sep 14, 2012 #1
    Just two questions, if anyone has the time to check over my answers (no mark scheme is available).

    Q1. A particle P moves on the x-axis. The acceleration of P at time t seconds, t ≥ 0 is (3t+5) ms^-2 in the positive x-direction. When t = 0, the velocity of P is 2 ms^-1 in the positive x-direction. When t = T, the velocity of P is 6ms^-1 in the positive x direction. Find the value of T.


    integrating the acceleration to find the velocity, I get ((3t^2)/2) + 5t + C, finding the constant by using v = 2 when t = 0, the constant is 0

    so v = 6, when t = T
    6 = (3T^2)/2 + 5T + 2

    solving this to get T = 3/2 or T = -4
    as t ≥ 0, T = 3/2


    A particle P moves along the x-axis. At time t seconds the velocity of P is v ms^-1 in the positive x-direction, where v = 3t^2 - 4t + 3. When t = 0, P is at the origin 0. Find the distance of P from O when P is moving with minimum velocity.

    Taking the derivative of v, we get 6t - 4, letting that = 0 we get t = 2/3.

    then we integrate the velocity to get an equation of the displacement, and P is at O when t = 0 the constant is 0, so the displacement: x = t^3 - 2t^2 + 3t

    substuting t = (2/3) we get the distance to be (38/27)

    If anyone could actually check over these (I need correct answers!) it'd be really good help!

    Thank yoU!
  2. jcsd
  3. Sep 14, 2012 #2


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    Gold Member

    For Q1, I get T= 2/3 and -4

    You should be able to put your answers back into the equation for a sanity check.
  4. Sep 14, 2012 #3
    Sorry! That was a typo, is everything else ok.
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