A level Physics · Edexcel 9PH0 · Topic 6A
Impulse and conservation of momentum
Key points
- Impulse = force × time = change of momentum: (unit N s, same as kg m s).
- The area under a force–time graph gives the impulse. Increasing the impact time (crumple zones, airbags, bending knees) reduces the average force for the same momentum change.
- Conservation of momentum applies in two dimensions: momentum is conserved separately in each perpendicular direction. Resolve, conserve, then recombine with Pythagoras.
- Kinetic energy and momentum are linked by .
- Elastic collisions conserve kinetic energy; inelastic collisions do not (energy goes to heat, sound and deformation). Total energy is always conserved.
- To decide if a collision is elastic, calculate total kinetic energy before and after and compare.
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Impulse connects a force acting for a time to the change of momentum it produces, which explains how safety features work. This sub-topic also applies conservation of momentum to collisions at angles and classifies collisions using kinetic…
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