Teaching Physics with the Physics Suite

Edward F. Redish

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Molly on the Skateboard

Rebecca has put her puppy, Molly, on a skateboard, and has attached a rope to the skateboard in order to give Molly a ride. At time t = 0, Rebecca starts pulling on the rope. She is pulling upward at an angle of 37o. Once she is up to speed (at time t1), she runs along at a constant rate until a time t2. A little after that, her mother yells at her and she stops.

(a) While Rebecca is pulling, draw free-body diagrams for Molly and the skateboard. Label the forces so as to identify the kind of force (N, T, f, W) and the actors (the one feeling the force and the one causing it). You can ignore the sliding friction between the skateboard wheels and the floor.

(b) Sketch appropriate qualitative graphs representing Molly’s position, velocity, acceleration, and the friction force Molly is experiencing.

(c) Molly weighs 20 lbs, the skateboard weighs 3 lbs, and Rebecca gets the skateboard up to a speed of 2 m/s after a time of 3 s. Do you have enough information to figure out the force that Rebecca exerts between 0 and t1? Between t1 and t2? If you don't explain what additional information you need. If you do, calculate both forces.

Page last modified October 8, 2009: D38