An elastic metal rod will change its length when it
a) falls vertically under its weight
b) is pulled along its length by a force acting at one end
c) rotates about an axis at one end
d) slides on a rough surface

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only a, d are true

only b, c are true

All are true

only a, b are true

answer is B.

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Detailed Solution

When a force acting at one end pulls an elastic metal rod along its length or when it rotates about an axis at one end, the length of the rod will vary.
Elongation produced by the applied force and the centrifugal force respectively.
for a freely falling body and moving on a rough horizontal surface restoring force is not produced.

Hence the correct answer is only b, c are true.

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