Assign6 - PHY 53 Summer 2010 Assignment 6 Reading Energy 2...

Assignment 6
Reading : Energy 2
Key Concepts : Potential energy, total energy, conservation of mechanical energy,
equilibrium and turning points.
1. Some statements about the law of conservation of mechanical energy. Comment
on their validity and give your reasons.
a. The law says that the sum of the kinetic and potential energies of a system
remains constant.
b. The law says that if the net external force is zero then the sum of the
kinetic and potential energies of a system remains constant.
c. The law says that if only conservative forces do work then the sum of the
kinetic and potential energies of a system remains constant.
d. The law says that if all the external forces are conservative then the sum of
the kinetic and potential energies of a system remains constant.
2. Some short questions.
a. Prove by direct calculation that the work done by a constant force on a
particle is independent of the path followed by the particle.
b. The static friction force exerted on an object by a stationary surface does
no work on the object. Why not?
c. When you start your car moving from rest there is an obvious increase in
kinetic energy. But the external force (static friction) that causes the
acceleration does no work. Does this contradict conservation of energy?