Linear momentum & its conservation
Momentum is the quantity of motion an object carries: p = mv, a vector measured in kg m/s. At AS level we state and apply the principle of conservation of momentum — in any isolated system the total momentum before an interaction equals the total momentum after — to solve one-dimensional collisions and explosions. We meet elastic collisions (kinetic energy conserved, relative speed of approach equals relative speed of separation), inelastic collisions (kinetic energy not conserved but momentum still conserved) and perfectly inelastic collisions (the objects stick together). Finally we study impulse, F Δt = Δp, read it as the area under a force–time graph, and use it to explain why airbags, seatbelts and arrestor beds save lives.
By the end you should be able to (NSSCAS Physics (AS) 1.4.3):
- Define and use linear momentum as the product of mass and velocity, p = mv, and recognise it as a vector measured in kg m/s
- State the principle of conservation of momentum: when bodies in a system interact, the total momentum remains constant provided no external force acts (an isolated system)
- Apply conservation of momentum to solve one-dimensional collisions and explosions, choosing a positive direction and handling signs correctly
- Distinguish elastic collisions (kinetic energy conserved) from inelastic collisions (kinetic energy not conserved), knowing that momentum is conserved in both
- Recognise a perfectly inelastic collision as one in which the objects stick together, losing the most kinetic energy
- Recall that for a perfectly elastic collision the relative speed of approach equals the relative speed of separation
- Explain that momentum is always conserved in interactions between bodies while some change in kinetic energy may take place
- Define impulse as F Δt, relate it to the change in momentum (F Δt = Δp), and find impulse as the area under a force–time graph
- Apply the concept of impulse to safety considerations such as airbags, seatbelts and arrestor beds
Miss Elizabeth and Mike talk through the whole topic — with the figure and working drawn live.