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Homework Help: Projectiles Exam Question

  1. Apr 22, 2007 #1
    1. The problem statement, all variables and given/known data

    I was practicing exams for PHY 1 but I don't understand how to do this question, even with the mark scheme.

    A cricketer bowls a ball from a height of 2.3m. The ball leaves the hand horizontally with a velocity u. After bouncing once, it passes just over the stumps at the top of its bounce. The stumps are 0.71m high and are situated 20m from where the bowler releases the ball.

    (Diagram shown)

    a) Show that from the momnt it is released, the ball takes about 0.7 s to fall 2.3 m.

    This is pretty easy, resolving vertically,
    u = 0 m/s
    a= -9.81 m/s/s
    s= 2.3 m

    Using v^2= u^2 + 2as
    u^2 cancels out.
    2 x -9.81 x 2.3 = 45.126
    Square root ans = v = -6.72 m/s
    Using t = v-u/a
    -6.72-0/-9.81 = 0.685s

    Then it says, b) How long does it take the ball to rise 0.71 m after bouncing? (3 mark question)

    Then c) Use your answers to parts (a) and (b) to calculate the initial horizontal velocity u of the ball. You may assume that the horizonatl velocity has remained constant (2 marks)

    d) In reality the horixontal velocity would not be constant. State one reason why.

    The answer to this is simple too: There will be significant air resistance at the bounce.

    Please could someone help with parts b and c.

    Thanks, Nasser
  2. jcsd
  3. Apr 22, 2007 #2

    um, what grade is this?:eek:
  4. Apr 22, 2007 #3
    What a canny problem. i believe this assumes knowledge of coefficient of restitution which can be computed from the ratios of Yf/Yi, specifically the sqrt of same. All velocities are subject, including horizontal, to the same reduction, in other words Vxf=0.55Vxi.
    Last edited: Apr 22, 2007
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