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An object is thrown vertically upwards from the top of a cli...

An object is thrown vertically upwards from the top of a cliff with a velocity of 25ms1. Find the time, in seconds, when it is 20 metres above the cliff. [g=10ms2].

A.

0 and 1

B.

0 and 4

C.

0 and 5

D.

1 and 4

Correct answer is D

s=ut+at22

This movement is against gravity, so it is negative.

s=utgt22

s=20m,u=25ms1

20=25t10t2220=25t5t2

5t225t+20=0

5t25t20t+20=05t(t1)20(t1)=0

5t20=0 or t - 1 = 0

t=1 or 4