Question

a ball is thrown vertically with a velocity u from top of tower . if it strike the ground with velocity 3u the time taken by the ball to reach the ground is

Answer 1

The formula to find time when a body is thrown up is 2u/g∴∴∴
2u/g=2u/10       ( take g as 10 m/s² for simplicity)
to find time taken to reach ground from top of tower to ground,
v=u+at
3u=u+gt
3u-u=gt
2u=gt
2u/g=t
t=2u/10
∴total time=2u/10+2u/10=4u/10=0.4u seconds


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Answer 2

The formula is   v = u + gt

u=u     and     v=3u as the final velocity is acting downwards it is negative

so v=  -3u

we modify the formula as -v  =  u  -   gt

sub the values     -3u  =  u  -   gt

-3u -u  =  -gt

-4u  =-gt

t = 4u/g is the correct answer