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Posted by on Tuesday, December 20, 2011 at 5:50pm.

A stone is thrown vertically upwards from a platform 60meters above the ground. The initial velocity of the stone is 20m/s.The height of the stone above the ground at t seconds is given by: H(t)=20t-5t^2+60m.How long does it take for the stone to hit the ground? At what velocity does the stone hit the ground?

  • Math - , Wednesday, December 21, 2011 at 12:02am

    what does it mean when it hits the ground? height H(t) = 0. so, since

    H(t) = -5t^2 + 20t + 60
    = -5(t^2 - 4 - 12)
    = -5(t-6)(t+2)

    when do you think it hits the ground?

    Then, since

    V(t) = 20 - 10t

    you can figure the velocity

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