2021 A/L Physics Past Paper · Question 04 (English Medium)

Answer: (5) total linear momentum

What the question asks

Find the quantity that is conserved in any type of collision.

The only condition is that no external force acts on the system.

Relevant theory

For a system of interacting objects, the momentum remains constant, provided no external force acts on the system.

According to Newton's second law resultant force is proportional to the rate of change of momentum.

Reading the options

Test each option against the law.

Kinetic energy and mechanical energy are lost as heat and sound when objects stick together.

Potential energy and angular velocity have no such law here.

Only total linear momentum stays constant.

A quick check with two trolleys that stick together.

Before the collision:

$$p = M(2v) + M(v) = 3Mv$$

After the collision they move together at $\tfrac{3}{2}v$:

$$p = 2M \times \tfrac{3}{2}v = 3Mv$$

The momentum is the same. The kinetic energy is not.

Why the other options fail

Kinetic energy is lost as heat and sound in an inelastic collision.

The law keeps momentum, and says nothing about potential energy.

Mechanical energy includes kinetic energy, which is lost in an inelastic collision.

The law keeps a momentum, not an angular velocity.

Conclusion

When no external force acts, the total linear momentum is the same before and after any collision.

The answer is (5).

Momentum is kept in every collision when no outside force acts.

Kinetic energy is kept only when the collision is elastic.

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