# Are both kinetic energy and momentum conserved when 2 objects undergo a collision? [duplicate]

for example when two objects undergo a collision and no external force is acting on them. Will both kinetic energy and momentum be conserved or just momentum?

• i still dont get this.. if some KE is lost from the system of the balls as heat or sound, then that's the same as saying some of the system's $\frac{1}{2}mv^2$ is lost right?. then how can the system's $mv$ remain constant? Oct 30 '19 at 18:12