In which one of the following situations is zero net work done? a) a ball rolls down an inclined plane. b) a physics student stretches a spring. c) a projectile falls toward the surface of earth. d) a box is pulled across a rough floor at constant velocity. e) a child pulls a toy across a rough surface causing it to accelerate.

Option D

Explanation:

The work done can be given by the mechanical energy used to do work, i.e., Kinetic energy and potential energy provided to do the work.

In all the cases, except option D, the energy provided to do the useful work is not zero and hence work done is not zero.  

In option D, the box is being pulled with constant velocity, making the acceleration zero and thus Kinetic energy of the system is zero. Hence work done in this case is zero.

muscle endurance

explanation:

i can do 50 in a row ( but i get so tired after that)

need to see circuit to get time constant