9709 · 4.3
Momentum — FAQ
Frequently asked questions for 9709 Momentum. Direct answers first, then deeper explanation — then practise with marking.
Is momentum always conserved?
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).
Can kinetic energy increase in a collision?
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'.
What's the difference between the units kg m s⁻¹ and N s?
They are equivalent. From Newton's Second Law, , 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.