Physics collision question.

Question

Two objects move towards each other, then collides and separates. If no external force acts on

this system, which of the following statement is true?

a. Px, Py, and total momentum are conserved but kinetic energy is not conserved

b. Px, Py, total momentum and kinetic energy are conserved

c. Kinetic energy is conserved but Px, Py and total momentum is not conserved

d. Px, Py, total momentum and kinetic energy are not conserved

A short explanation would be helpful too!

Answer 1

Momentum is ALWAYS conserved when no external forces act on the system. Also, momentum in a x-direction (P_x) is always conserved, as is P_y.

Total energy is ALWAYS conserved when no external forces act on the system however, energy can be transformed from one type to another meaning that the kinetic energy is not always conserved. If the collision is ELASTIC, then kinetic energy is conserved. We are not told that the collision is elastic so we cannot assume that kinetic energy is conserved.