Syllabus coverage
- Explain the concepts of force, mass, weight, inertia and acceleration and how they create and affect linear motion
- Identify and explain the forces acting during physical activity, including gravitational force or weight, air resistance, friction, reaction, action or muscular force, and balanced and unbalanced forces
- Describe and apply Newton's first law (law of inertia)
- Describe and apply Newton's second law (law of acceleration)
- Describe and apply Newton's third law (law of reaction)
Part of the AS Level PE 8386 syllabus 8386.
The five concepts you have to explain
Force is a push or a pull that changes the motion, shape or direction of a body. It is measured in newtons (N). In sport the force that starts a movement is usually muscular force; the forces that resist it are weight, air resistance and friction.
Mass is the amount of matter in a body, measured in kilograms (kg). Mass does not change with location.
Weight is the force of gravity acting on that mass. weight = mass × gravitational field strength, and it is measured in newtons. Gravity is taken as 9.81 m/s², or sometimes 10 for a rough calculation.
Inertia is the tendency of a body to resist a change in its state of motion. A body at rest stays at rest and a moving body keeps moving in the same direction unless an external force acts on it. Inertia depends on mass: the greater the mass, the greater the inertia and the harder it is to change the body's motion.
Acceleration is the rate of change of velocity, measured in m/s². A performer accelerates when the forces acting on them are unbalanced in the direction of the movement.
| Concept | Definition | Unit | What it explains in sport |
|---|---|---|---|
| Force | A push or a pull that changes a body's motion, shape or direction | Newtons (N) | The muscular force a sprinter drives into the blocks to start moving |
| Mass | The amount of matter in a body | Kilograms (kg) | Why a heavier player is harder to push into a faster acceleration |
| Weight | The force of gravity acting on a mass (mass × 9.81) | Newtons (N) | The downward force that a high jumper has to overcome to leave the ground |
| Inertia | The resistance of a body to a change in its state of motion; it depends on mass | No separate unit | Why a large rugby forward takes longer to get moving and longer to stop |
| Acceleration | The rate of change of velocity | m/s² | How quickly a performer reaches top speed after pushing off |
The forces acting during physical activity
Cambridge lists six things to look for when you identify the forces acting on a performer. Learn them as a checklist and run down it whenever a question says 'state the forces acting'.
Forces are usually shown as arrows on a diagram: the arrow shows the direction of the force and the length shows its size.
- Balanced forces = no acceleration. The performer stays still or keeps moving at constant velocity.
- Unbalanced forces = acceleration or deceleration in the direction of the larger force.
- Friction is not always a negative force. Without friction between the foot and the ground, a performer could not push off at all.
| Force | What it is | Sporting example |
|---|---|---|
| Gravitational force / weight | The downward pull of gravity on the performer's mass | A basketball player returns to the ground after a jump; a shot putter's throw is pulled down into its flight path |
| Air resistance | The force of the air pushing against a body moving through it; it acts in the opposite direction to the motion | A track and field athlete feels air resistance pushing back against them at top speed; a competitive swimmer feels the same force through the water |
| Friction | The force between two surfaces in contact that opposes movement | The friction between running spikes and the track allows a sprinter to push backwards and move forwards; it also slows a bowler's run-up if studs are wrong |
| Reaction | The equal and opposite force a surface or object pushes back with | The ground pushes back against a sprinter's foot with a force equal to the force the sprinter applies |
| Action (muscular force) | The force produced by a muscle contracting, applied to a bone | The gastrocnemius plantar flexing the ankle to drive a player off the ground |
| Balanced and unbalanced forces | Balanced forces are equal and opposite, so no change in motion; unbalanced forces produce a change in speed or direction | A netball player standing still has balanced forces; the same player sprinting forwards has unbalanced forces because the muscular force exceeds air resistance and friction |
Newton's first law: the law of inertia
Newton's first law states that a body will remain at rest, or continue moving at a constant velocity in a straight line, unless acted upon by an external force.
In sport this explains two things: why a stationary performer will not move until an external force is applied, and why a moving performer carries on until something slows them down. The resistance to that change is inertia, and inertia is greater when mass is greater.
A large rugby union forward has more inertia than a small scrum half. That is why the forward takes longer to reach top speed out of a standing start, and why it takes a bigger external force to stop them once they are moving.
| Situation | What the first law says is happening | Sporting example |
|---|---|---|
| Starting from rest | The performer stays still until an external muscular force acts | A swimmer is motionless on the blocks until the leg muscles push against the block |
| Constant velocity | When the driving force balances the resisting forces, velocity stays constant | A cross-country runner at steady pace in a straight line, with air resistance and friction balanced by muscular force |
| Changing motion | An external force is needed to change speed or direction | A hockey player uses a stick and muscular force to change direction because the body would otherwise continue in a straight line |
| Greater mass, greater inertia | A larger mass resists a change of motion more strongly | A heavyweight Olympic weightlifter accelerates more slowly than a lightweight lifter applying a similar force |
Newton's second law: the law of acceleration
Newton's second law states that the rate of change of momentum of a body is proportional to the force applied, and the change takes place in the direction of that force. In its simple form: force = mass × acceleration.
force = mass × acceleration
The law gives three relationships you must be able to use and explain: a bigger force produces a bigger acceleration, a bigger mass produces a smaller acceleration for the same force, and an acceleration is always in the direction of the unbalanced force.
Worked example 1. A rugby union player of mass 80 kg accelerates at 3.5 m/s² as they drive out of a ruck. Force = 80 × 3.5 = 280 N.
Worked example 2. A netball player of mass 65 kg accelerates at 4 m/s² during a drive to the circle edge. Force = 65 × 4 = 260 N.
Worked example 3. The same 260 N muscular force applied to a heavier 90 kg player gives acceleration = 260 ÷ 90 = 2.9 m/s². Same force, bigger mass, less acceleration — this is the comparison Cambridge looks for.
Worked example 4. A shot putter's weight: mass 110 kg × 9.81 = 1079.1 N downward, which is the force they must overcome with their legs before the throw.
Newton's third law: the law of reaction
Newton's third law states that for every action there is an equal and opposite reaction.
In sport the reaction force is what allows a performer to move. The performer applies a muscular force to the ground or to an object, and the ground or object pushes back with an equal force in the opposite direction.
A sprinter drives their foot backwards into the block. The block pushes forwards with an equal force, and that reaction force is what accelerates the sprinter out of the blocks. A competitive swimmer pushes water backwards with their hands and feet, and the water pushes them forwards, which is why the stroke has to be long and strong.
- Action and reaction forces are equal in size, opposite in direction and act on different bodies.
- Newton's third law is why a performer can only move forward by pushing something backwards.
- The reaction force is often called the ground reaction force in biomechanics.
| Action force | Reaction force | Sporting example |
|---|---|---|
| Sprinter pushes backwards and down into the blocks | Blocks push forwards and up against the feet | The sprinter is driven out of the blocks |
| Competitive swimmer pushes water backwards with the hands | Water pushes the swimmer forwards | Forward propulsion through the water |
| Basketball player drives down into the floor to jump | Floor pushes up against the feet | The player leaves the ground |
| Volleyball player strikes the ball downwards | The ball pushes back against the hand | The hand is forced slightly upwards and backwards on contact |
Putting the three laws together in an analysis
Long-answer questions reward you for moving between the laws rather than describing each one separately. A sprint start is the cleanest example.
The sprinter is at rest in the set position (first law: at rest until an external force acts). The gun goes and the leg muscles apply a large muscular force against the blocks. The blocks produce an equal and opposite reaction force (third law), which is larger than the resisting forces of weight, friction and air resistance, so the forces are unbalanced and the sprinter accelerates (second law, force = mass × acceleration). As speed rises, air resistance rises until the driving force and the resistances balance, and the sprinter reaches a constant maximum velocity (back to the first law).
- First law explains the start and the plateau at constant velocity.
- Second law explains the acceleration phase and every calculation using force = mass × acceleration.
- Third law explains where the accelerating force comes from in the first place.
- Balanced forces give constant velocity; unbalanced forces give acceleration.
Key terms you must be able to define
- Force
- A push or a pull that changes a body's motion, shape or direction; measured in newtons (N).
- Mass
- The amount of matter in a body, measured in kilograms; it does not change with location.
- Weight
- The force of gravity acting on a body's mass, calculated as mass × gravitational field strength (9.81 m/s²) and measured in newtons.
- Inertia
- The tendency of a body to resist a change in its state of motion; it increases as mass increases.
- Acceleration
- The rate of change of velocity, measured in m/s²; it occurs when the forces acting on a body are unbalanced.
- Air resistance
- The frictional force of air against a moving body, acting in the opposite direction to the motion.
- Friction
- The force between two surfaces in contact that opposes movement; between the foot and the ground it also provides the grip needed to push off.
- Reaction force
- The equal and opposite force a surface or object pushes back with when another force is applied to it.
- Action force (muscular force)
- The force produced by a muscle contracting and pulling on a bone; in sport it is the force that moves the body.
- Balanced forces
- Forces that are equal in size and opposite in direction, so no change in motion occurs.
- Unbalanced forces
- Forces that are not equal, producing acceleration or deceleration in the direction of the larger force.
- Newton's first law (inertia)
- A body remains at rest, or continues moving at constant velocity in a straight line, unless acted upon by an external force.
- Newton's second law (acceleration)
- The rate of change of momentum is proportional to the force applied, in the direction of that force; expressed as force = mass × acceleration.
- Newton's third law (reaction)
- For every action there is an equal and opposite reaction.
Applied to your sport
Cambridge requires you to apply this knowledge to physical activities. Use these as models, then write your own for the sport you play.
A sprinter pushes backwards into the blocks; the blocks push forwards with an equal force (third law). That unbalanced force accelerates the sprinter (second law) until air resistance builds up and balances the driving force, at which point velocity becomes constant (first law).
A swimmer at rest on the blocks stays at rest until the leg muscles push against the block (first law). In the water, the hands push water backwards and the water pushes the swimmer forwards with an equal force (third law), which is why an effective catch and pull produces forward propulsion.
A 90 kg forward has greater inertia than a 70 kg back, so for the same muscular force the forward accelerates more slowly and is also harder to stop once moving, which is why a heavier player is used to carry the ball into contact.
A lifter of mass 110 kg must apply an upward muscular force greater than their weight of about 1079 N to move the bar off the floor; once that force exceeds the weight, the forces are unbalanced and the bar accelerates upwards.
A player standing still on court has balanced forces, so they stay still. When they drive towards the circle edge with a force greater than friction and air resistance, the forces become unbalanced and they accelerate.
Flashcards
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Exam-style questions
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Mark scheme — tick each point you actually made
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Exam technique for this topic
- Name the law before you apply it: 'Newton's first law, the law of inertia, states that...'. A stated law plus an application is worth more than a description of the movement alone.
- In force = mass × acceleration questions, write the formula, substitute, then give the unit. Newtons (N) can be earned separately from the numerical answer.
- When a question asks for 'the forces acting', run the checklist: gravitational force or weight, air resistance, friction, reaction, action or muscular force, and balanced or unbalanced forces.
- Weight is not mass. If you are given a mass in kilograms and asked about weight, multiply by 9.81 first.
- Check the direction of your forces. Cambridge expects the reaction force to be drawn or described as opposite to the action force.