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M1 acceleration help

  1. Sep 23, 2007 #1

    Could someone please help me to do part (b) of the following question? I would really appreciate any help as I am really stuck.

    A boy is walking at a constant speed of 1.8 m/s along a straight road. He passes a telephone booth where his sister is making a telephone call. His sister takes 30 s to complete the call and then sets off in pursuit of the boy. She accelerates uniformly from rest at 3 m/s^2 until she is running at a speed of 9 m/s. She maintains this constant speed until she reaches her brother.

    (a) On the same diagram sketch the speed-time graphs for the boy and his sister.

    I did this part.

    (b) Calculate the time taken by the girl to reach her brother.

    Thank you.

  2. jcsd
  3. Sep 23, 2007 #2


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    What have you tried so far?

    I'll give you a hint: what does the area of the speed-time graph represent?
  4. Sep 23, 2007 #3
    Thanks for replying.

    It represents the distance travelled.

  5. Sep 23, 2007 #4


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    So what is meant by "girl to reach her brother" ?
    Well it means that they have travelled same distance, i.e find the value of time t, when the area of girl = area of boy. Do you know how to go from lines on a graph to functions and do integration?

    I have no time to draw pictures and so on, I am quite sure that you know how this works.

    If not, just use the fact that the boy will have area: 1.8*t and the find the time for the girl to reach 9 m/s with the acceleration given and then just continue.
  6. Sep 23, 2007 #5
    Thanks so much for your help!

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