Question

Which of these is always conserved in a collision in a closed system?

A) momentum and energy
B) momentum and mechanical energy
C) energy only
D) momentum and kinetic energy

Answer 1

B) momentum and mechanical energy

Step-by-step explanation:

In classical physics, a closed system is a system that does not exchange any matter with the surrounding, and on which there are no external forces acting on it.

Due to this definition, we have the following consequences:

- In a collision in a closed system, momentum is always conserved. This is because there is exchange of matter with the surroundings (so, no mass can be lost), and also because there are no external forces acting on the objects inside the system. In fact, the variation of the total momentum of the system is related to the net force acting on the system:

`F=(\Delta p)/(\Delta t)`

but since the net force F is zero, then the variation of momentum,
`(\Delta p)/(\Delta t)`, is also zero, therefore the momentum is constant.

- Also, mechanical energy is conserved. Mechanical energy is the sum of potential energy and kinetic energy of the system: again, since there are no external forces acting on the system, energy cannot be dissipated, so the total energy of the system cannot change.

Answer 2

Answer;

A) momentum and energy

Explanation;

• There are two types of collisions that relate to momentum: elastic and inelastic.
• In a closed system, which means that there are no external forces acting on the objects that collide, both types of collisions follow the Law of Conservation of Momentum, which states that the total amount of momentum before a collision is equal to the total amount of momentum after a collision.
• In an elastic collision, not only is momentum is conserved, but also kinetic energy, while in an inelastic collision, momentum is conserved, but the total kinetic energy of the system is not conserved.