Q.

A block P of mass m is placed on a horizontal frictionless plane. A second block of same mass m is placed on it and is connected to a spring of spring constant k, the two blocks are pulled by distance A. Block Q oscillates without slipping. What is the maximum value of frictional force between the two blocks          [2004]

1 kA2  
2 kA  
3 μsmg  
4 zero  

Ans.

(1)

Here acceleration is same for both P and Q as block Q oscillates but does not slip. The (P-Q) system oscillates with angular frequency ω. The spring is stretched by A.

Angular frequency of the system,

ω=km+m=k2m

Maximum acceleration of the system in SHM

amax=Aω2=A(k2m)2=kA2m

This acceleration to the lower block is provided by friction

 Maximum force of friction

fmax=mamax=m(kA2m)=kA2