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Q.

A stone is allowed to fall from the top of a tower 100 m high and at the same time another stone is projected vertically upwards from the ground with a velocity of 25 m/s. calculate where and when the two stones will meet.


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a

100 m, 10 s

b

80 m, 5 s

c

80 m, 4 s

d

100 m, 5 s 

answer is C.

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Detailed Solution

A stone is allowed to fall from the top of a tower 100 m high and at the same time another stone is projected vertically upwards from the ground with a velocity of 25 m/s. The two stones will meet after 4 s at a height of 80 m from the ground.
Given data in the problem,
Height of tower, = 100 m
For first stone          Initial velocity, = 0    By using the second equation,
= ut +12at2 =>a=g Where,
= distance = initial velocity = uniform acceleration = time By putting all the values in the above equation,
=>= 0 + 12gt2 , where x is the height where two stones meet.
=>= 0 + 12 × 10 × t2 =>= 5 × t2  … (1)
Now, for second stone      Initial velocity, = 25 m/s
By using the second equation,
= ut +12at2 Where acceleration a=g(acceleration due to gravity)  Where,
= distance = initial velocity = uniform acceleration = time By putting all the values in the above equation,
100 – x = ut + 12gt2 100 – x = (25) × t + 12 × 10 × t2 100 – x = 25 × t - 5 × t2 … (2)
Now adding equation 1 and 2      25= 100              = 4s     putting value of t in equation 1, we get          = 5 × 42                 = 80 m  The stones will meet up at a height of 80 m and time is 4 s.
 
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