PhysicsLAB AP Free Response Question
2012 B1

Block A of mass 2.0 kg is pulled along a horizontal table by a force of 15 N, which is applied by a light string that passes over a light frictionless pulley, as shown below. The coefficient of kinetic friction between the block and the surface is 0.25.
(a) On the dot below, which represents the block, draw and label the forces (not components) that act on the block as it is pulled across the table.

 (b) Calculate the magnitude of the acceleration of the block.

The applied force is removed. Block B of mass 1.5 kg is now attached to the string, as shown above. The system  is released from rest so that the 1.5 kg box descends and the 2.0 kg block is again pulled across the table.
 (c) Calculate the acceleration of the 1.5 kg block as it descends.

 (d) Calculate the tension in the string connecting the two blocks.

 (e) Calculate the distance that the 1.5 kg block descends in 0.40

 (f) If this system is set up in a laboratory and the acceleration of the 1.5 kg block is experimentally determined, the experimental value is found to be smaller than the value calculated above. If the given value for the coefficient of friction is correct and air resistance is negligible, explain briefly, but specifically, why the experimental value of the acceleration is smaller.

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