position and movement

a girl walks along a straight road from a point A to a point B with an average speed 1m/s. She then returns from B to A along the same road with an average speed 2m/s. Find the average speed of the girl for the whole journey.可吾可以寫埋步驟+解釋?

仲有我想問有無方法轉換a-t graph, v-t graph, s-t graph同埋他們的關係

另外,我想問下點樣可以一睇題目,就知道用邊條equations of uniformly accelerated motion(e.g. s=1/2(u+v)t, v=u+at, etc.)

thank you!!!!!!!!

1 個解答

評分
  • 天同
    Lv 7
    9 年前
    最愛解答

    Let d be the distance between A and B

    hence time t1 to travel from A to B = distance/speed = d/1 = d

    time t2 to travel from B to A = d/2

    Total distance travelled in the whole journey = 2d

    total time taken = t1 + t2 = d + d/2 = 3d/2

    Therefore, average speed = 2d/(3d/2) = 4/3 m/s = 1.33 m/s

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    In s-t graph, the slope gives the velocity at any instant of time. Just plot the values of the slopes at some critical time points, you could obtain the corresponding v-t graph.

    SImilarly, the slope of a v-t graph gives the acceleration at that instant of time. Just plot the values of the slopes of the v-t graph at some critical time points, you could obtain the corresponding a-t graph.

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    There are altogether 5 quantities: u, v, a, s, and t

    and each equation of motion relates any 4 of these quantities.

    See what quantities, including the one that needs to be found, you have been given, and select the equation that contains all these quantities.

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