Forces
Newton's laws, momentum, stopping distances, and pressure.
In this topic
Forces and Their Effects
A force is a push or pull acting on an object. Forces are measured in newtons (N) and are vectors (they have magnitude and direction).
Contact forces: friction, air resistance, tension, normal contact force Non-contact forces: gravity, electrostatic, magnetic
Weight = mass × gravitational field strength (W = mg) On Earth, g ≈ 10 N/kg
Resultant force: the single force that has the same effect as all forces acting together. If forces are balanced (resultant = 0), the object stays still or moves at constant velocity.
Key Points
- Balanced forces = no acceleration (Newton's 1st Law)
- Unbalanced forces = acceleration (Newton's 2nd Law: F = ma)
- Every action has an equal and opposite reaction (Newton's 3rd Law)
- Free body diagrams show all forces acting on an object
Example Questions
2A car of mass 1500 kg accelerates at 2 m/s². Calculate the resultant force.
F = ma = 1500 × 2 = 3000 N
[2 marks]
3Explain why a skydiver reaches terminal velocity.
Initially, gravity > air resistance, so the skydiver accelerates downwards. As speed increases, air resistance increases. Eventually air resistance equals weight — the forces are balanced, resultant force is zero, and the skydiver falls at a constant speed (terminal velocity).
[3 marks]
Momentum
Momentum is a property of moving objects.
Momentum = mass × velocity (p = mv) Unit: kg m/s
Conservation of momentum: In a closed system, the total momentum before = total momentum after an event (collision or explosion).
Impulse = change in momentum = force × time (FΔt = mΔv)
This is why: cars have crumple zones (increase collision time → reduce force), airbags, seatbelts.
Key Points
- Momentum is a vector — direction matters
- In collisions, momentum is always conserved
- Longer collision time = smaller force (F = Δp/Δt)
- Safety features increase collision time to reduce injury force
Example Questions
3A 70 kg person is in a car travelling at 15 m/s. The car crashes and stops in 0.5 s. Calculate the average force on the person.
Change in momentum = 70 × 15 = 1050 kg m/s. Force = 1050 / 0.5 = 2100 N
[3 marks]