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Q.

A ball is dropped from a building of height 45 m. Simultaneously another ball is thrown up with a speed 40 m/s. The relative speed of the balls varies with time as

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a

t2

b

t0

c

t1

d

1/t

answer is A.

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Detailed Solution

According to the problem, for the ball dropped from the building, u1​=0 and u2​=40m/s

Velocity of the ball after time t,

v1​ = u1 ​− gt

v1 ​= −gt

And for another ball which is thrown upward,

u2 ​= 40m/s

Velocity of the ball after time t,

v2​ = u2​ − gt = (40 − gt)

Therefore,

Relative velocity of one ball with respect to another ball is,

v12​ = v1 ​− v2​ = −gt − [−40 − gt]

v12 ​= v1 ​− v2​ = −gt + 40 + gt = 40m/s

Therefore, relative speed is indepent of time.

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