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    A tennis ball is projected vertically upwards with a speed of 6.3m/s from a platform that is 28m above the floor. At what time does it strike the floor?
    I said:
    v=0
    u=6.3
    a=-9.8
    t=?
    And I used v=u+at and got 0.642 seconds but I think that's wrong.
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    (Original post by Ahrimylove)
    A tennis ball is projected vertically upwards with a speed of 6.3m/s from a platform that is 28m above the floor. At what time does it strike the floor?
    I said:
    v=0
    u=6.3
    a=-9.8
    t=?
    And I used v=u+at and got 0.642 seconds but I think that's wrong.
    You don't know that the final velocity is 0. Try drawing a diagram, a ball is thrown from a platform 28m above the floor, and you are working out the time for when it hits the floor, does this tell you anything about the displacement?
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    (Original post by Ahrimylove)
    A tennis ball is projected vertically upwards with a speed of 6.3m/s from a platform that is 28m above the floor. At what time does it strike the floor?
    I said:
    v=0
    u=6.3
    a=-9.8
    t=?
    And I used v=u+at and got 0.642 seconds but I think that's wrong.
    Your right that your wrong. What you calculate is fir the upward motion only
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    If you set (taking upwards as positive)
    S=-28m since the vertical displacement when the ball hits the ground will be -28m because we are taking upwards as positive
    u=6.3
    v=V
    a=-9.8
    t=T
    Now just set up an equation with time as one of the variables and solve for T
    This will lead to solving a quadratic equation. You will get 2 answers and you have to reject one and it should be obvious which one to reject.
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    (Original post by B_9710)
    If you set (taking upwards as positive)
    S=-28m since the vertical displacement when the ball hits the ground will be -28m because we are taking upwards as positive
    u=6.3
    v=V
    a=-9.8
    t=T
    Now just set up an equation with time as one of the variables and solve for T
    This will lead to solving a quadratic equation. You will get 2 answers and you have to reject one and it should be obvious which one to reject.
    Okay so I formed a quadratic equation using s=ut+1/2at^2 and my equation was 4.9t^2-6.3t-28=0. I solved this and got 2.95 as my positive answer.
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    (Original post by Ahrimylove)
    Okay so I formed a quadratic equation using s=ut+1/2at^2 and my equation was 4.9t^2-6.3t-28=0. I solved this and got 2.95 as my positive answer.
    I haven't done it but I will do it when I get back (I'm out) and will tell you if you're right.
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    (Original post by Ahrimylove)
    Okay so I formed a quadratic equation using s=ut+1/2at^2 and my equation was 4.9t^2-6.3t-28=0. I solved this and got 2.95 as my positive answer.
    I got 3.12s as the answer but I formed the same quadratic as you so you mus that've made a mistake in solving the quadratic that you given.
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    Got same as B_9710
 
 
 
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