a boy stands on the edge of a cliff of height 60m.he throwws

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a boy stands on the edge of a cliff of height 60m.he throwws a stone upward so that its distance ,h
a boy stands on the edge of a cliff of height 60m.he throwws a stone upward so that its distance ,h m,above the cliff top is given by h=20t-5t^
(b)calculate the time which elapses before the stone hits the beach
(c)calculate the speed with which the stone hits the beach

（1）20t-5t^2=-60,t=6 seconds
(2)based on the equation of mechanic energy,
the velocity v0=20m/s,so
m*g*h=0.5*m*(vt^2-v0^2)
that is
10*60=0.5*(vt^2-20^2)
vt=40meters per second
再问： .he throwws a stone upward so that its distance 是什么意思
再答：个人理解为，整句的意思是：他向上抛石头，这样石头高于峭壁的距离h由h=20t-5t^2来表示。h是distance的同位语。
再问：那为什么-60=20t-5t^2
再答：设从抛出到落地的时间为t，因为h是以向上为正，因此海滩的位置是-60米。

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