Posted by **Lusi.** 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 -
**Steve**, 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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