Momentum — common mistakes

In questions involving a force that varies with time, the impulse is the area under the force-time graph. Be careful with signs when calculating the change in momentum, especially if an object rebounds. If a ball hits a wall at velocity $u$ and rebounds with velocity $v$, the change in momentum is $m(-v) - mu = -m(v+u)$ (assuming initial direction is positive).

Always draw a 'before' and 'after' diagram. Clearly label masses, velocities, and your chosen positive direction. This simple step prevents the most common errors related to signs.

No. Momentum is only conserved in a closed system, which means there are no external resultant forces acting on the objects involved. For example, if friction is present and not negligible, it is an external force, and the total momentum of the colliding objects will not be conserved (it will decrease).

In a standard collision between passive objects, kinetic energy can only be conserved (elastic) or decrease (inelastic). However, if the collision triggers an internal release of energy (like a small explosion on impact), then the total kinetic energy of the system could increase. This is rare in A-Level problems unless it's explicitly described as an 'explosion'.

They are equivalent. From Newton's Second Law, $F=ma$, so a Newton (N) is a kg m s⁻². Therefore, a Newton second (N s) is (kg m s⁻²) × s = kg m s⁻¹. Both are accepted units for momentum and impulse.