Edexcel IAL Physics revision · AS — Mechanics
Dynamics
Newton's laws stated precisely, then applied to connected bodies and collisions. AS raises the bar in one way above all: the second law is now about momentum, not just F = ma.
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What the syllabus demands
- —State and apply Newton's three laws of motion
- —Use F = ma for single bodies and connected systems
- —Define linear momentum; state force as rate of change of momentum
- —Apply conservation of momentum in one and two dimensions
- —Distinguish elastic collisions (KE conserved) from inelastic (KE not conserved)
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Newton's first law
- An object remains at rest or continues at constant velocity unless acted on by a resultant force.
- Newton's second law
- The resultant force on an object equals the rate of change of its momentum: F = Δp ÷ Δt (which reduces to F = ma for constant mass).
- Newton's third law
- When body A exerts a force on body B, body B exerts a force on body A that is equal in magnitude, opposite in direction, and of the same type.
- Elastic collision
- A collision in which both momentum and total kinetic energy are conserved. In a perfectly elastic collision the relative speed of approach equals the relative speed of separation.
The equations
More equations to practise: the Edexcel IAL formula sheet.
Where the marks die
Common mistakes to check
- 01
Third-law 'pairs' that act on the same object. Weight and the normal contact force on a book are NOT a third-law pair — they act on the same body and are different types. The pair of the book's weight is the book pulling the Earth up.
- 02
Testing elasticity with momentum. Momentum is conserved in every collision — the test for elastic is whether kinetic energy is conserved. Compute KE before and after.
- 03
Connected bodies treated with the wrong mass: for the whole system use the total mass; for one body, include the tension. Mixing the two gives self-consistent nonsense.
- 04
Quoting F = ma as Newton's second law at A-Level. The full statement is rate of change of momentum — required whenever mass changes or the examiner asks for the law itself.
One worked example, done properly
Question
A 0.40 kg ball moving at 5.0 m/s strikes a wall and rebounds at 3.0 m/s. The contact lasts 0.20 s. Calculate the average force on the ball.
Method
- 1.Take the initial direction as positive: u = +5.0, v = −3.0 m/s.
- 2.Δp = m(v − u) = 0.40 × (−3.0 − 5.0) = −3.2 kg m/s.
- 3.F = Δp ÷ t = −3.2 ÷ 0.20.
F = −16 N (16 N acting away from the wall)