Physicshard · Past Paper
A stone is dropped from a rising balloon moving up at 10 m/s. If the balloon is at 40m height, the time taken by stone to hit ground is (g=10):
A2 s
B4 s
C6 s
D8 s
✓ Correct Answer: B — 4 s
Using s = ut + 0.5at^2, -40 = 10t - 5t^2 => 5t^2 - 10t - 40 = 0 => t^2 - 2t - 8 = 0 => (t-4)(t+2)=0. So t=4s.
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