Teaching Physics with the Physics Suite Edward F. Redish

Where's the force?

 A 50 gram mass is hanging from a spring whose unstretched length is 10 cm and whose spring constant is 2.5 N/m. In the list below are described five situations. In some of the situations, the mass is at rest and remains at rest. In other situations, at the instant described, the mass is in the middle of an oscillation initiated by a person pulling the mass downward 5 cm from its equilibrium position and releasing it. Ignore both air resistance and internal damping in the spring. At the time the situation occurs, indicate whether the force vector requested points up (U), down (D), or has magnitude zero (0). The force on the mass exerted by the spring when the mass is at its equilibrium position and is at rest. The force on the mass exerted by the spring when the mass is at its equilibrium position and is moving downward. The net force on the mass when the mass is at its equilibrium position and is moving upward. The force on the mass exerted by the spring when it is at the top of its oscillation. The net force on the mass when it is at the top of its oscillation.

Note to the instructor: This is a surprisingly challenging problem, especially the last part. Some students seem to have difficulty paying attention to the flow of time and the associated forces.

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