A small block of mass 20 kg rests on a bigger block of mass 30 kg, which lies on a smooth horizontal plane. Initially the whole system is at rest. The coefficient of friction between the blocks is 0.5. A horizontal force F = 50 N is applied on the lower block. Find the work done (in J) by frictional force on upper block in t = 2sec.
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(40)
Sol. Assume 20 kg and 30 kg block to move together
a = 
= 1 m/s 2
frictional force on 20 kg block is
f = 20 × 1 = 20 N
The maximum value of frictional force is
f max = 
× 200 = 100 N
Hence no slipping is occurring.
The value of frictional force is f = 20 N.
Distance travelled in t = 2 seconds.
S = 
× 1 × 4 = 2m.
Work done by frictional force on upper block is
W fri = 20 × 2 = 40 J
Work done by frictional force on lower block is = – 20 × 2 = – 40 J
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