Block A in the figure weighs 1.90N and block B weighs 3.90N . The coefficient of

Question

Block A in the figure weighs 1.90N and block B weighs 3.90N . The coefficient of kinetic friction between all surfaces is 0.500.
Figure 1: http://i.imgur.com/GWjiZTO.jpg

Part A: Find the magnitude of the horizontal force F? a necessary to drag block B to the left at a constant speed of 9.50cm/s if A  rests on B  and moves with it (figure a).

Part B: Find the magnitude of the horizontal force F? b necessary to drag block B to the left at a constant speed of 9.50cm/s if A  is held at rest by a string (figure b).

Part C: In part (A), what is the friction force on block A?

Explanation / Answer

A)

Here , For moving the blocks with constant speed ,

F = friction

F = 0.5*(1.90+3.90)

F = 2.9 N

magnitude of the horizontal force F is 2.9 N

B)

Here , If the Block A is held at rest ,

then force needed to move must balance friction due to A also

F = 0.5*(1.9+3.9)+0.5*1.9

F = 3.85 N

magnitude of the horizontal force F is 3.85 N

c)

Here , as the relative motion is ZERO and their is no acceleration

Fricition on block A is ZERO

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