Forces | OCR GCSE Physics A, Higher tier (J249)

Forces

374 questions
22 subtopics
Paper 3
Paper 3

Forces is examined in Physics A Paper 3.

It covers measuring distance and time, and calculating speed, converting units, and rates, vectors and scalars in motion, distance-time and velocity-time graphs, average speed, and the equations of uniform motion, how objects interact, and forces as vectors, newton's first law, resolving forces, and free body diagrams, resultant forces, and balanced forces, newton's second law, and inertia, momentum, and force as a change in momentum, work done, and stored energy, power, and Newton's third law, circular motion and changing velocity, stretching, bending and compressing; elastic and plastic, force and extension, linear and non-linear, the spring constant, and work done in stretching, gravitational fields, weight and free fall, calculations with weight and gravitational field strength, turning forces and the moment of a force, levers and gears and pressure in fluids, and the force on a surface.

Sample questions from Forces

Answer each one closed book first, then open the answer.

Measuring distance and time, and calculating speed

A cyclist travels 450 m in 30 s. What is her average speed?

Show the answer
450 ÷ 30 = 15 m/s.
Vectors and scalars in motion

What must be stated for a velocity to be fully described?

Show the answer
Both its size, such as 15 m/s, and its direction, such as south.
How objects interact, and forces as vectors

When two objects interact, how many forces are produced?

Show the answer
Two, one acting on each of the two objects.
Resolving forces, and free body diagrams

Why does the acceleration of a falling skydiver get smaller as she speeds up?

Show the answer
Air resistance grows as her speed grows, so the downward resultant force becomes smaller.
Momentum, and force as a change in momentum

A 2 kg trolley moving at 3 m/s collides with a stationary 4 kg trolley and they move off joined together. What is their velocity afterwards?

Show the answer
The total momentum of 6 kg·m/s is now carried by 6 kg, so they move at 1 m/s.
Power, and Newton's third law

Why do the two forces of an interaction pair never cancel each other out?

Show the answer
They act on different objects, so they can never be combined into a resultant on one object.
Force and extension, linear and non-linear

What does it mean if force and extension have a linear relationship?

Show the answer
Equal increases in force always produce equal increases in extension, so the graph of force against extension is a straight line through the origin.
Gravitational fields, weight and free fall

What is meant by free fall?

Show the answer
Falling under the pull of gravity alone, with no other force such as air resistance acting on the object.

The 22 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Measuring distance and time, and calculating speed	Instruments for measuring length and time, why light gates beat a hand-held stopwatch, how reaction time is reduced, and finding speed from distance, time and the gradient of a distance-time graph.	28
Converting units, and rates	The standard units of distance and time, converting kilometres per hour to metres per second, minutes to seconds and centimetres to metres, and why units must match in an equation.	21
Vectors and scalars in motion	The difference between a scalar and a vector, why distance is a scalar and displacement a vector, how velocity differs from speed, and journeys where the two disagree.	10
Distance-time and velocity-time graphs	What horizontal, sloping and curved lines mean on a distance-time graph, what the gradient and area of a velocity-time graph give, and calculations of acceleration and distance from them.	16
Average speed, and the equations of uniform motion	How average speed is found for a changing journey, distance, speed and time calculations, acceleration from a change in velocity, and the equation linking final velocity, starting velocity, acceleration and distance.	23
How objects interact, and forces as vectors	The four ways objects interact and which act at a distance, normal contact force and friction, the equal and opposite pair of forces produced by every interaction, free body force diagrams, and finding the resultant of forces acting along one line.	21
Newton's first law	What happens to an object with no resultant force on it, what a resultant force can change, and why spacecraft, books, cyclists and sliding loads behave as they do.	10
Resolving forces, and free body diagrams	Resolving a force into two components at right angles, scale drawings and finding the size and direction of a resultant, forces in equilibrium, terminal velocity for a skydiver before and after the parachute opens, and why a car has a top speed that streamlining raises.	16
Resultant forces, and balanced forces	Finding the resultant of forces along one line, worked examples for rockets, trolleys, swimmers and lifts, what balanced forces mean, and why a zero resultant does not always mean rest.	16
Newton's second law, and inertia	The relationship between resultant force, mass and acceleration, what one newton means, calculations of force, mass and acceleration, and how inertial mass explains why loaded objects are harder to move.	16
Momentum, and force as a change in momentum	How momentum is calculated and its unit, momentum as a vector, conservation of momentum in collisions and when skaters push apart, the force needed to change velocity in a given time, and why crumple zones and padded mats lower the force by lengthening the stopping time.	16
Work done, and stored energy	How work done is calculated and its unit, why holding a bag still does no work, where energy goes when work is done against friction, and the energy in a moving object's kinetic store.	16
Power, and Newton's third law	The definition and unit of power with calculations for motors, lamps and stairs, the law describing interaction pairs, and why the two forces of a pair never cancel each other.	16
Circular motion and changing velocity	Why an object moving in a circle at steady speed has a changing velocity and is accelerating, the direction of the resultant force, and what supplies it for cars, strings and satellites.	10
Stretching, bending and compressing; elastic and plastic	The three ways forces change shape and why at least two forces are needed, the forces that stretch a spring, bend a diving board and compress a pillar, elastic and plastic deformation and how to tell them apart, and why an overstretched spring stays longer.	16
Force and extension, linear and non-linear	Extension and how to calculate it, direct proportion up to the limit of proportionality, the elastic limit, linear and non-linear force-extension graphs, and the spring-stretching investigation with its variables, precautions, results table, hypothesis, length against force graph and conclusion.	29
The spring constant, and work done in stretching	What the spring constant measures and its unit, force = spring constant × extension with calculations and stiffness comparisons, work done in stretching as stored energy, work done = ½ × spring constant × extension², the area under a force-extension graph, and the effect of doubling the extension.	17
Gravitational fields, weight and free fall	Gravitational fields around every mass and why they only attract, field strength on the Earth and the Moon, weight as a force measured in newtons with a newtonmeter, how mass differs from weight, the centre of mass, and free fall at about 10 m/s² whatever the mass.	21
Calculations with weight and gravitational field strength	The relationship between weight, mass and gravitational field strength, its rearrangements, worked examples on the Earth and a moon, and how weight and mass behave when the field strength changes.	10
Turning forces and the moment of a force	What a pivot is and when a force makes an object turn, the direction of rotation, the moment of a force and its unit, calculations of moment, force and perpendicular distance, and the principle of moments applied to balanced see-saws.	17
Levers and gears	What a lever and a force multiplier are, how a lever, a long spanner and a crowbar multiply force, where the pivot, load and effort sit in a wheelbarrow, and what gears do.	11
Pressure in fluids, and the force on a surface	Why an inflated balloon holds a rounded shape, why wide snowshoes stop a walker sinking into deep snow and how a hydraulic system multiplies a force.	18

Forces is 374 of the 1,584 questions in the guide.Get the guide, £7

How the guide is worked

Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.

Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.

Read the full method, the return schedule and the research behind it.