Asseration : A plank A is placed on a rough surface over which a block B is placed. In the shown situation, elastic cord is unstretched. Now a gradually increasing force F is appled slowly on A until the relative motion between the block and plank starts.
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At this moment cord is making an angle` theta` with the vertical. Work done by force F is equal to energy lost against fricton `f_(2)`, plus potential energy stored in the cord.
Reason : work done by static friction `f_(1)` on the system as a whole is zero.

Options

A. If both Assertion and Reason are true and the Reason is correct explanation of the Assertion.
B. If both Assertion and Reason are true and the Reason is not the correct explanation of the Assertion.
C. If Assertion is true, but the Reason is false
D. If Assertion is false but the Reason is true.

Answer

A

Explanation

`(W_(f1))_(on A) = - f_(1) s`
`(W_(f1))_(on B) = - f_(1) s`
`:.` Net work done by `f_(1) =0`
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