Question

1. When a child descends a sliding board in a playground, does the total macroscopic mechanical energy of the system (Earth, slide, and child) maintain its value, increase, or decrease?

a. decrease
b. increase
c. maintains
d. Can be any of the above

2. Imagine that a wood block is sliding along the surface of a desk. Can the frictional forces involved in this interaction change the momentum of the two-body system thought of as the Earth (including the desk) and the block?

a. Yes, because it will provide a negative impulse to the block.
b. Yes, because it will cause a negative impulse to the Earth.
c. No, because the net force on the block will be zero.
d. No, because there is no net external force on the system.

3. A planet moves in an elliptical orbit around a star. This is a two-body system. Which of the following must stay constant during the orbit?

a. Kinetic energy of the planet
b. The angular momentum of the planet
c. The potential energy of the system
d. The force on the planet
e. Two of the above answers are correct

Answer 1

1. c because the child descend its gravitational potential energy is converted into its kinetic energy. Thus total mechanical energy is conserved.

2. d because internal forces cant change the momentum of the system as they occur in pair according to newton third law and cancel out. Only net external force changes the momentum of the system.

3. b because torque due to gravitational force is zero as it is a radial force and thus angular momentum is conserved .Also as distance of the planet from the sun star is not constant in elliptical orbit, force will not be same.