Laws of MotionHard

Question

Two block of mass m1 and m2  are connected with a mass less spring and placed over a plank moving with an acceleration ′a′ as show in figure. The coefficient of friction between the blocks and platform is μ.

Options

A.spring will be stretched if a > μg
B.spring will be compressed if a ≤ μg
C.spring will neither be compressed nor be stretched for a ≤ μg
D.spring will be in its  natural lenght under all conditions

Solution

Let a ≤ μg (i.e. friction is static)
Then both the blocks are rest w.r.t plank
Therefore spring will be in  its natural length.
Now let a > μg (i.e. friction is kinetic)
Then both the blocks are moving with same acceleration w.r.t plank.
In this case spring force is equal to zero

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