PhysicsLAB AP Free Response Question
2008 Form B - B2

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A 4700-kg truck carrying a 900-kg crate is traveling at 25 m/sec to the right along a straight, level highway, as shown above. The truck driver then applies the brakes, and as it slows down, the truck travels 55 meters in the next 3.0 sec. The crate does not slide on the back of the truck.
 (a) Calculate the magnitude of the acceleration of the truck, assuming it is constant.

(b) On the diagram below, draw and label all the forces acting on the crate during braking.

 (c) i. Calculate the minimum coefficient of friction between the crate and truck that prevents the crate from sliding.

 (c) ii. Indicate whether this friction is static or kinetic.

Now assume the bed of the truck is frictionless, but there is a spring of spring constant 9200 N/m attaching the crate to the truck, as shown below.
The truck is initially at rest.
 (d) If the truck and crate have the same acceleration, calculate the extension of the spring as the truck accelerates from rest to 25 m/s in 10 sec.

 (e) At some later time, the truck is moving at a constant speed of 25 m/sec and the crate is in equilibrium. Indicate whether the extension of the spring is greater than, less than, or the same as in part (d) when the truck was accelerating. Explain your reasoning.

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