Question

A ball is rolled off a horizontal roof at 6m/s . after leaving the roof how long will the ball take to reach a speed of 10m/s

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Answer 1

Net speed must be 10. We know $v_{horizontal}=6$  then $v_{vertical}$ must be:

$\sqrt{v_h^2+v_v^2}=\sqrt{10}\\ 6^2+v_v^2=100\\v_v^2=64\\v_v=8m/s$

Time taken to reach vertical speed to 8m/s will be:

$v=u+at\\v=gt\\8=10t\\t=0.8s$

Hope it was useful :) Please report if wrong.