Friction

Blocks shown in figure moves with constant velocity $$10 \ m/s$$ towards right. All surfaces is constant are rough. The friction force applied by B on A is:
A $$40$$ kg box rests on the horizontal floor of a truck which is initially at rest on a horizontal road. The coefficient of kinetic friction between the floor and the box is $$0.2$$. The truck at time $$t=0$$ accelerates constantly at $$5$$ $$ms^{-2}$$ and the box is $$6$$ m away from the rear opening of the truck.
A car is moving on a horizontal circular road $$0.1 km$$ with constant speed. If coefficient of friction between tyres of car and road is $$0.4$$, then speed of car may be $$(g = 10 m/s^2)$$
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A $$25$$ kg body is resting on a horizontal plane having coefficient of friction $$0.2$$. A force of $$200$$N is required to pull the body horizontally. The acceleration developed by the body is: