PhysicsLAB AP Free Response Question
2007 Form B - B1

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A child pulls a 15 kg sled containing a 5.0 kg dog along a straight path on a horizontal surface. He exerts a force of 55 N on the sled at an angle of 20º above the horizontal, as shown in the figure above. The coefficient of friction between the sled and the surface is 0.22.
(a) On the dot below that represents the sled-dog system, draw and label a free-body diagram for the system as it is pulled along the surface.

 (b) Calculate the normal force of the surface on the system.

 (c) Calculate the acceleration of the system.

 (d) Calculate the work done by the child’s pulling force as the system moves a distance of 7.0 m.

(e) At some later time, the dog rolls off the side of the sled. The child continues to pull with the same force. On the axes below, sketch a graph of speed v versus time t for the sled. Include both the sled’s travel with and without the dog on the sled. Clearly indicate with the symbol tr the time at which the dog rolls off.

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