The 20 kg block resting on a surface with coefficient of friction as = 0.3. A force F = (10 + s) N, where s is in meters, acts on the block in the direction shown. The spring is originally unstretched and the block is at rest. Determine the power developed by the force at the instant s= 0.5 m.

1) 5.32 W.
2) 12.34 W.
3) 4.43 W.
4) 2.65 W.

When s = 0.5 m,
F = 20 + 0.5 = 20.5 N;
P = F.v2 = (20.5cos20o)(0.23) = 4.43 J.