Star 發問於 科學及數學其他 - 科學 · 1 十年前

al physics

A ball is projected up a smooth inclined plane which makes an angle of 60 degrees with the horizontal. It takes 4 seconds for the ball to return to the starting point. What is the length of the inclined plane? Take g=10meter per second square.

Ans: 17.3m

Please teach me with explanation...Thank you very much for your help !!!

更新:

would you mind to explain why time for upward motion = time for downward motion?

Thank you!!!

1 個解答

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  • cipker
    Lv 5
    1 十年前
    最愛解答

    Time for upward motion = time for downward motion = 2s

    Upward motion:

    accleration = gsin 60

    s = 1/2at^2 = 0.5 g sin 60 2^2 = 17.3m

    2010-06-19 18:10:54 補充:

    Sorry it should be

    downward motion:

    acceleration = gsin 60

    s = 1/2at^2 = 0.5 g sin 60 2^2 = 17.3m

    2010-06-19 20:51:13 補充:

    Consider a ball being thrown vertically upwards, I guess you would agree that the time for upward journey is the same for downward journey.

    2010-06-19 20:51:18 補充:

    If we turn to the situation on inclined plane, the acceleration along the plane is always g sin60. In this case, the situation is exactly the same as a ball being thrown vertically upwards EXCEPT is acceleration is smaller than g.

    2010-06-19 20:59:43 補充:

    Hence, the time for upward motion is the same as the downward motion.

    You can prove it mathematically , or more conveniently consider the velocity time graph of the ball (take upward as positive)

    http://www.flickr.com/photos/27778998@N04/47143256...

    2010-06-19 20:59:51 補充:

    Sometimes, it is VERY VERY useful to draw a graph and use graphical method to solve the problem, especially mc questions.

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