Quick questions on Momentum: conservation, impulse and collisions - Edexcel A-Level Physics

Newton's second law in its fundamental form is $F = \frac{\Delta p}{\Delta t}$, the rate of change of momentum. For constant mass this reduces to $F = ma$, but the momentum form is the more general statement and is needed for variable-mass problems such as rockets.

Conservation of momentum applies equally to collisions (objects coming together) and explosions (a body flying apart). In an explosion the total momentum is zero before and after, so the fragments carry equal and opposite momenta. Always set a positive direction first and treat velocities with consistent signs.

Impulse explains crash safety: extending the contact time $\Delta t$ for a given change in momentum reduces the average force. Crumple zones, airbags, and bending the knees on landing all lengthen the impact time and so cut the peak force.

Define linear momentum and give its unit. [1 mark]

A $2.0$ kg trolley at $3.0$ m per second collides and sticks to a stationary $1.0$ kg trolley. Find their common speed. [2 marks]

Explain how an airbag reduces the force on a passenger in a crash. [2 marks]