OxfordAQA International GCSE Physics 3.1 Forces and their Effects Exam Questions

About OxfordAQA International GCSE Physics 3.1 Forces and their Effects

Forces and their Effects is the first and widest section of OxfordAQA International GCSE Physics. It covers types of force, scalars and vectors, weight, stretching springs, distance-time and velocity-time graphs, resultant forces and Newton's three laws. The section then applies these ideas to transport safety: stopping distances and braking. The separate Physics course adds momentum, terminal velocity, centre of mass and moments and levers. Many equations from the equation sheet are used here, and the ideas link to work and energy in 3.2. Examiners often ask for multi-step F = m x a and momentum calculations, graph interpretation and explanations of safety features.

What you need to know

• Forces Contact forces include friction, air resistance, tension and normal contact force; non-contact forces include gravitational, electrostatic and magnetic forces. Scalars have magnitude only (speed, distance); vectors have magnitude and direction (velocity, displacement, force, acceleration, momentum).
• Weight and springs W = m x g, with g given in the exam. Within the limit of proportionality F = k x e, where e is extension. Required practical: investigate the relationship between force and extension for a spring.
• Motion graphs The gradient of a distance-time graph is speed. On a velocity-time graph the gradient is acceleration and the area under the line is the distance travelled. v = s / t and a = delta v / t.
• Newton's laws First law: zero resultant force means constant velocity or staying at rest. Second law: F = m x a. Third law: when two objects interact they exert equal and opposite forces on each other. Find resultant forces in a line or with scale drawings.
• Momentum (separate Physics) p = m x v. Momentum is conserved in collisions and explosions in a closed system. F = delta p / t, so air bags, seat belts, crumple zones, crash mats and cycle helmets increase the time of impact and reduce the force.
• Stopping distances Stopping distance = thinking distance + braking distance. Reaction time is increased by tiredness, distractions, drugs and alcohol. Braking distance increases with speed, wet or icy roads and worn brakes or tyres; the brakes do work against friction and heat up.
• Terminal velocity (separate Physics) A falling object accelerates until drag equals weight, so the resultant force is zero and it falls at terminal velocity. Parachutes increase drag and lower terminal velocity; streamlining reduces drag.
• Centre of mass and moments (separate Physics) A suspended object hangs with its centre of mass below the pivot; use a plumb line to find the centre of mass of an irregular lamina. M = F x d with d the perpendicular distance. Balanced: total clockwise moment = total anticlockwise moment. An object topples if its weight acts outside its base.

Common mistakes in 3.1 Forces and their Effects

• Using distance along a slope instead of perpendicular distance for moments. d must be the perpendicular distance from the pivot to the line of action of the force.
• Saying Newton's third law pairs cancel out. They act on different objects, so they cannot cancel.
• Saying an object at terminal velocity has no forces on it. The forces are balanced, so the resultant force is zero.
• Saying air bags reduce the change in momentum. The change in momentum is the same; the longer time reduces the rate of change of momentum and so the force.
• Forgetting that a negative acceleration means slowing down. Deceleration is negative acceleration.

Try three 3.1 Forces and their Effects exam-style questions