OxfordAQA IGCSE Physics: Forces and Their Effects — Revision Notes

Condensed for the final weeks. For the full explanation, use the Forces and Their Effects study guide.

Motion

speed = distance / time        acceleration = (v - u) / t

(The equation sheet for this specification has no symbol for initial velocity and no uniform-acceleration (“SUVAT”) equation – work acceleration out from the gradient of a velocity-time graph instead.)

Distance–time graph: gradient = speed. Velocity–time graph: gradient = acceleration, area = distance.

Reading area as gradient is the most common error on this topic.

Scalars (distance, speed, mass, energy) have magnitude only; vectors (displacement, velocity, force, momentum) have direction too.

Forces

Newton’s laws: constant velocity without a resultant force; F = ma; equal and opposite forces on different bodies.

Resultant force determines the motion. Balanced forces mean constant velocity — which includes being at rest, but also includes moving steadily. Forces along the same line combine algebraically: 8 N right and 3 N left give a resultant of 5 N right.

Third-law pairs are equal, opposite, the same type of force, and act on two different bodies — never on the same object. A book resting on a table has its weight balanced by the table’s normal contact force, but those two forces act on the same object (the book), so they are a first-law balance, not a third-law pair.

Worked example. A 1200 kg car accelerates from rest to 24 m/s in 8.0 s. Find the driving force if drag is 400 N.

a = (v - u) / t = (24 - 0) / 8.0 = 3.0 m/s^2
F(resultant) = m a = 1200 x 3.0 = 3600 N
Driving force = resultant + drag = 3600 + 400 = 4000 N

The driving force is not the resultant — the resultant is what remains once drag has been subtracted. If the car instead moved at a constant 24 m/s, the resultant would be zero and driving force would exactly equal drag, 400 N.

Terminal velocity, in order:

1. Weight acts downwards, so the object accelerates.
2. Air resistance increases with speed.
3. When air resistance equals weight, resultant force is zero.
4. Acceleration is zero, so velocity is constant — the object still moves.

Momentum and safety

p = mv        F = (mv - mu) / t

Momentum is conserved in a closed system and is a vector, so opposite directions must be assigned opposite signs.

Crumple zones, airbags and seatbelts all work identically: they increase the time over which momentum changes, so the force is reduced. The momentum change itself is fixed by the collision — only the time can be altered.

Stopping distance

stopping = thinking + braking

Thinking distance rises with speed, tiredness, alcohol and distraction. Braking distance rises with speed, worn tyres and wet or icy roads.

Braking distance increases with the square of speed, because kinetic energy is proportional to v². Doubling speed quadruples braking distance.

Moments and stability

moment = force x perpendicular distance from pivot

Principle of moments: in equilibrium, clockwise moments equal anticlockwise moments.

An object topples when the line of action of its weight falls outside its base. Stability improves with a lower centre of mass and a wider base.

Hooke’s law

F = k x

Valid up to the limit of proportionality. Beyond the elastic limit, deformation is permanent.

Elastic potential energy stored = area under the force–extension graph = ½Fx for a linear region.

Exam traps

• Confusing gradient and area on a velocity–time graph.
• Saying an object stops at terminal velocity.
• Treating braking distance as proportional to speed.
• Omitting “perpendicular” from the moment definition.
• Saying an airbag absorbs the force.
• Saying balanced forces mean the object is stationary.
• Treating the driving force as if it were the resultant force.
• Naming weight and normal contact force on a resting object as a third-law pair — they act on the same object, so they cannot be.

Self-test

1. What do gradient and area give on a velocity–time graph?
2. Explain terminal velocity in four steps.
3. Why does braking distance rise so steeply with speed?
4. How does a seatbelt reduce injury?
5. When does an object topple?
6. A 1200 kg car accelerates from rest to 24 m/s in 8.0 s against 400 N of drag. Find the driving force.
7. Why are weight and normal contact force on a book resting on a table not a third-law pair?

Answers: 1. Gradient is acceleration; area is distance travelled. 2. Weight causes acceleration; air resistance increases with speed; when air resistance equals weight the resultant force is zero; acceleration becomes zero so velocity is constant. 3. Braking distance depends on kinetic energy, which is proportional to v², so doubling speed quadruples it. 4. It increases the time over which the passenger’s momentum changes, reducing the force since force is rate of change of momentum. 5. When the line of action of its weight falls outside the base of support. 6. a = (24 − 0) ÷ 8.0 = 3.0 m/s²; resultant F = ma = 1200 × 3.0 = 3600 N; driving force = 3600 + 400 = 4000 N. 7. They act on the same object (the book) — a third-law pair must act on two different bodies, so this is a first-law balance instead.