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M1 level help

  1. Oct 27, 2007 #1
    [SOLVED] M1 level help

    Hi.

    Could someone please help me with the following M1 level question? I would really appreciate any help as I am really stuck at the moment.

    A car starts from rest at a point S on a straight racetrack. The car moves with constant acceleration for 20 s, reaching a speed of 25 m/s. The car then travels at a constant speed of 25 m/a for 120 s. Finally it moves with constant deceleration, coming to rest at a point F.

    (a) Sketch a speed-time graph to illustrate the motion of the car.


    I did this part.

    The distance between S and F is 4 km.

    (b) Calculate the total time the car takes to travel from S to F.


    I wrote down:

    4000 = (0.5 x 20 x 25) + 0.5(120 + (F - 20))x25 and this leads to 50F = 2500.

    However, I am not sure if this is correct, or what to do next.

    The correct answer is 200 seconds.

    Thank you.

    Cathy
     
  2. jcsd
  3. Oct 27, 2007 #2

    learningphysics

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    When I solve your equation: 4000 = (0.5 x 20 x 25) + 0.5(120 + (F - 20))x25

    I get F = 200.
     
  4. Oct 27, 2007 #3
    Oh yeah - I get it now!

    Thanks for your help!

    Could someone please help me with the next part of the question too? I am completely stuck over what to do.

    A motorcycle starts at S, 10 s after the car has left S. The motorcycle moves with constant acceleration from rest and passes the car at a point P which is 1.5 km from S. When the motorcycel passes the car, the motorcycle is still accelerating and the car is moving at a constant speed. Calculate

    (c) the time the motorcycle takes to travel fro S to P,

    (d) the speed of the motorcycle at P.


    Thank you.

    Cathy
     
  5. Oct 27, 2007 #4

    learningphysics

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    Find the time at which the car reaches 1500m. call this tc. This is the same time that the motorcycle reaches 1500m. The time the motorcycle takes to go from S to P is just this tc - 10...

    Then you can solve for the speed of the motorcycle, using the area under the v-t graph for the motorcycle... or just using your kinematics formulas... either way, you'll end up doing.

    d = [(v1+v2)/2] *t

    1500 = [(0 + v2)/2] * (tc-10)

    d = (0 + v2)/2 * (
     
  6. Oct 28, 2007 #5
    Thanks so much for your help!

    Cathy
     
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