Conceptual Physics Questions?

Question

1.) The feet of a standing person of mass m exert a force equal to mg on the floor; and the floor exerts an equal and opposite force upwards on the feet, which we call the normal force. During the extension phase of a vertical jump, the feet exert a force on the floor that is greater than mg, so the normal force is greater than mg. We can use this result and Newton's second law to calculate the acceleration of the jumper: a=Fnet/m=(n-mg)/m. Using energy ideas, we know that work is performed on the jumper to give him or her kinetic energy. But the normal force can't perform any work here, because the feet don't undergo any displacement. How is energy transferred to the jumper?

2.) In most circumstances, the normal force acting on an object and the force of static friction do no work on the object. However, the reason that the work is zero is different for the two cases. In each case, explain why the work done by the force is zero.

Can I please get detailed responses for these? Thanks!

Answer 1

There is energy from the leg muscles that do work and travel during the extension of the jump. To compute the work using force, there would have to be a FBD of parts of the leg.

2. The normal force is perpendicular to the displacement, where static friction is parallel to the direction of displacement. Once the mass is o longer static, the friction becomes kinetic and work is done by that force converting the energy to heat.

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