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Question
A ball is dropped on the floor from a height of 10 m. It rebounds to a height of 2.5 m. If the ball is in contact with the floor for 0.01 sec, the average acceleration during contact is
Options:
A .  2100m/sec2 downwards
B .  2100m/sec2 upwards
C .  1400m/sec2 
D .  700m/sec2
Answer: Option B
:
B
Velocity at the time of striking the floor,
u=2gh=2×9.8×10=14m/s (-ve since downwards)
Velocity with which it rebounds.
v=2gh2=2×9.8×2.5=7m/s (+ve since upwards)
Change in velocity v=7(14)=21m/s
Acceleration = vt=210.01=2100m/s2(upwards since positive)

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