A bullet moving with a speed of 100 ms-1 can just penetrate two planks of equal thickness. Then, the number of such planks penetrated by the same bullet when the speed is doubled will be

1) 6

2) 10

3) 4

4) 8

 Answer: 4) 8

Solution:

Equation on motion v2 = u2+ 2as, here acceleration is negative

Therefore, v2 = u2 – 2as

v(final velocity) = 0

Therefore, u2 = 2as

i.e. s is proportional to u2 = s1/s2 = u12/u22

Therefore, s2 = (u22/u12) x s1 = [(200)2/(100)2] x 2 = 8 planks

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