Retrieval Practice
Newton's Laws & Momentum — AQA A-Level Physics
Q1. State Newton's first law of motion.
An object remains at rest or moves with constant velocity unless acted on by a resultant force.
Q2. State Newton's second law in two forms.
- F = ma (force equals mass times acceleration).
- F = delta-p / delta-t (force equals rate of change of momentum).
Q3. State Newton's third law of motion.
If object A exerts a force on object B, then object B exerts an equal and opposite force on object A.
Q4. What four conditions must a Newton's third law pair satisfy?
Same type of force, same magnitude, opposite direction, acting on different objects.
Q5. Define momentum and state its unit.
- Momentum = mass times velocity.
- Unit: kg m s^-1.
Q6. State the principle of conservation of momentum.
Total momentum before a collision equals total momentum after, provided no external resultant force acts.
Q7. What is impulse and how is it calculated?
- Impulse = force times time = change in momentum.
- Impulse = F delta-t = delta-p = mv - mu.
Q8. How do you find impulse from a force-time graph?
Calculate the area under the force-time curve.
Q9. What distinguishes an elastic collision from an inelastic collision?
- In an elastic collision, kinetic energy is conserved.
- In an inelastic collision, kinetic energy is not conserved (some is converted to heat, sound, etc.).
- Momentum is conserved in both.
Q10. How do crumple zones reduce injury in a car crash?
- They increase the time over which the car's momentum changes to zero.
- From F = delta-p / delta-t, longer time means smaller force on the passengers.
Q11. Why are weight and normal reaction on the same object NOT a Newton's third law pair?
- They are different types of force (gravitational vs contact) and both act on the same object.
- Third-law pairs must be the same type and act on different objects.
Q12. A ball rebounds off a wall. Why is the impulse greater than if it just stopped?
The ball reverses direction, so the change in velocity (and momentum) is larger: delta-v = v - (-u) = v + u rather than just v.