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Question

A car starts from rest and accelerates at 5 m/s2. At t=4 s, a ball is dropped out of a window by a person sitting in the car. What is the velocity and acceleration of the ball at t=6 s?

(g=10 m/s2)

A
202 m/s,10 m/s2
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B
20 m/s,5 m/s2
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C
20 m/s,0 m/s2
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D
202 m/s,0 m/s2
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Solution

The correct option is A 202 m/s,10 m/s2
Velocity of the car at t=4 s,

v=u+at=0+5×4=20 m/s

The ball is dropped at this instant, this means the horizontal velocity of the ball will be vh=20 m/s.

This velocity will remain constant as there is no horizontal acceleration.

Now, vertical velocity at t=6 s i.e. Δt=2 s,

vv=uv+avΔt=0+10×2=20 m/s.

Further, the net velocity is,

vnet=v2h+v2v=202+202=202 m/s.

The motion of the ball is under gravity, so,

anet=10 m/s2

Hence, option (A) is the correct answer.

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