When is kinetic energy conserved?
A) in elastic collisions
B) in inelastic collisions
C) in any collision in which the objects do not stick together
D) in all collisions
The correct answer and explanation is:
The correct answer is A) in elastic collisions.
Kinetic energy is conserved in elastic collisions. An elastic collision is one where both momentum and kinetic energy are conserved. This occurs when there is no loss of kinetic energy due to factors such as heat or deformation. The objects involved in elastic collisions rebound off each other without any permanent changes to their shape or internal energy. A classic example of an elastic collision is the collision of two billiard balls, where they bounce off each other, and no kinetic energy is lost to heat or sound.
In contrast, inelastic collisions (answer B) involve the conversion of some kinetic energy into other forms of energy, such as heat, sound, or potential energy due to deformation. Although momentum is still conserved in inelastic collisions, kinetic energy is not. In some cases, when the objects stick together, the collision is termed perfectly inelastic, and all the kinetic energy is converted into other forms, with no kinetic energy remaining in the system.
Answer C) in any collision in which the objects do not stick together is incorrect because, even if the objects do not stick together, kinetic energy may still be lost in an inelastic collision. The key factor is the conservation of momentum and whether energy is lost to other forms during the interaction.
Finally, answer D) in all collisions is also incorrect because, as mentioned, only elastic collisions conserve kinetic energy. In inelastic collisions, some of the kinetic energy is transformed into other types of energy, so kinetic energy is not conserved.
Thus, kinetic energy is only conserved in elastic collisions, where both momentum and kinetic energy remain constant throughout the process.