Question

Name and explain three ways in which momentum and kinetic energy are not conserved when they should be.

Answer 1

The momentum and kinetic energy will not be conserved in following three cases

1) When there is a friction involved

2) when there is gravity acting on objects

3) when net external force is not zero.

Here are the examples

Suppose two pendulums, one held at 90 degree on the left and other held 90 degree on the right, when we let them go, they collide together at the bottom. Here momentum is not conserved because there is gravity acting on pendulums

Actually, it depends on what we consider as a system. Only then we can tell if momentum is conserved or not.

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Kinetic energy is not conserved in inelastic collisions. Whenever, two objects stick together after collision, some energy is lost to heat or any other form. So, kinetic energy will be lost

other case can be when collision is not perfectly elastic, then also energy is lost. The energy lost will be less as compared to case of inelastic but energy will be lost, nevertheless.