This is the full (general) form of Newton's second law
Newton's Laws & Momentum - OCR A-Level Physics
$$F = \frac{\Delta p}{\Delta t}$$
- This is the full (general) form of Newton's second lawThe resultant force on an object is equal to its rate of change of momentum. For constant mass, F = ma.
- $F = ma$ is a special case that applies only when mass is constant
- The momentum form is needed for situations where mass changes (e.g. rockets losing fuel, conveyor belts, jets of water)
- Rearranging: $F\Delta$t = $\Delta$p. This product (force times time) is called impulseThe product of force and the time for which it acts. Equal to the change in momentum.
- A larger force produces a greater rate of change of momentum
Examiner Tips and Tricks
- To show that $F = ma follows$ from F = dp/dt: start with $F = d(mv)/dt$.
- If m is constant, this becomes m(dv/dt) = ma.
- This derivation is worth 2-3 marks in longer questions.