Block A weighs 1.20 N and block Bweighs 3.60 N. The coefficient of kinetic frictionbetween all surfaces is 0.300. Find the magnitudeof the horizontal force F necessary to drag blockB to the left at constant speed a) if A rests on Band moves with it and b) if A is held at rest.

Explanation:

a )  In this case A rests on B and both rest on horizontal surface

Both moves together .

Total reaction due to weight of both of them

R = 1.2 + 3.6 = 4.8 N

friction force by horizontal surface = μ R , μ is coefficient of friction .

= .3 x 4.8 = 1.44 N

Force equal to frictional force will be required to put both of them in uniform motion .

b ) If A is held stationary , friction force will arise at both , the upper and lower surface of B .

At upper surface friction force =  μ x weight of A

= .3 x 1.2 = .36 N

At lower surface friction force = μ x weight of A +B

= .3 x 4.8

= 1.44

Total frictional force on B

= 1.8 N  N .

So 1.8 N force will be required to put B in uniform keeping A stationary.