Overview
What you’ll learn
- Apply Newton’s laws to describe the effect of balanced and unbalanced forces, the ways a force changes motion, and action–reaction pairs (stating the laws is not required).
- Identify the forces acting on a body and draw free-body diagrams.
- Apply resultant force = mass × acceleration to solve problems.
- Explain the effects of friction on the motion of a body.
Key points
- 1Balanced forces (resultant zero): the body keeps steady velocity — at rest it stays at rest, moving it keeps moving straight at constant speed.
- 2Unbalanced (resultant) force changes motion: speeds up, slows down, or changes direction.
- 3F = ma: resultant force (N) = mass (kg) × acceleration (m/s²); double the force → double the acceleration.
- 4Free-body diagram: the one object, arrows for every force on it, each labelled, drawn from the body.
- 5Action–reaction pairs: equal size, opposite direction, on different bodies (tyre pushes road back; road pushes tyre forward).
- 6Friction opposes relative motion — it slows things but also enables walking, braking and gripping.
Notes · 1 of 5
Balanced and unbalanced forces
No resultant → no change in velocity. Ever.
When the forces on a body balance, the resultant force is zero and the velocity does not change: a book on a table stays put; a parachute falling at steady speed keeps that speed. This is the counter-intuitive half of Newton)s thinking — steady motion needs no resultant force. When the forces do not balance, the resultant force changes the velocity: speeding up, slowing down, or bending the direction.
| Situation | Resultant force | Motion |
|---|---|---|
| book on a table | zero (weight balanced by normal force) | at rest — stays at rest |
| parachute at steady speed | zero (weight balanced by drag) | falls straight at constant speed |
| rocket just after launch | upwards (thrust > weight) | accelerates upwards |
| braking car | backwards (brakes > drive) | decelerates |
An action–reaction pair is two forces that are equal in size and opposite in direction, acting on different bodies. A tyre pushes the road backwards; the road pushes the tyre forwards — that forward push is what drives the car. The pair never cancel, because they act on different objects; only forces on the same body can balance each other.
Notes · 2 of 5
Action–reaction pairs
Equal, opposite — and on different bodies.
Every force comes with a partner: if body A pushes body B, then body B pushes body A equally and oppositely. Walking: your foot pushes the ground backwards, the ground pushes you forwards. A gun firing: the bullet is pushed one way, the gun recoils the other. Swimming: hands push water back, water pushes swimmer on. Three tests identify a true pair: equal size, opposite direction, different bodies — and all three must hold.
| Action (on B, by A) | Reaction (on A, by B) |
|---|---|
| foot pushes ground backwards | ground pushes foot forwards |
| rocket pushes gas downwards | gas pushes rocket upwards |
| swimmer pushes water backwards | water pushes swimmer forwards |
| book pushes table downwards | table pushes book upwards |
Notes · 3 of 5
Free-body diagrams
One body, every force on it, drawn from the body.
A free-body diagram isolates one object and draws every force acting on it as a labelled arrow starting from the body: weight downwards, normal force upwards, friction and drive along the motion. The test of a good diagram: for every arrow you draw, you can name what object exerts that force on your body. If you cannot, it is not a force on this body — and forces the body exels on others (the reaction half of a pair) must not appear.
Notes · 4 of 5
Resultant force and F = ma
The leftover force sets the acceleration.
Add the forces along the direction of travel (with signs) to find the resultant force, then apply F = ma: resultant force (N) = mass (kg) × acceleration (m/s²). The acceleration points the same way as the resultant. The equation scales honestly: double the resultant force on the same mass gives double the acceleration; the same force on double the mass gives half the acceleration.
Notes · 5 of 5
Friction: nuisance and necessity
It opposes motion — and makes motion useful.
Friction is the contact force that opposes relative motion between surfaces. As a nuisance it slides against you: it wears out machine parts, wastes energy as heat, and resists moving furniture. As a necessity it makes things possible: without friction you could not walk (your foot would slip backwards), brake (brake pads grip by friction), or hold a pencil. Reducing friction uses lubricants, ball bearings or smoother surfaces; increasing it uses grippier materials and treaded patterns.
The two jobs meet in every moving object. A cyclist pedalling at steady speed has drive balancing friction and air resistance — balanced, so no acceleration. Stop pedalling and the same friction and drag become unbalanced, and the cyclist slows. Same forces, opposite consequences, depending on whether they are balanced.
Revise
Key terms
The words examiners expect you to use precisely.
- resultant force
- The single force that has the same effect as all the forces on a body combined.Zero resultant = no change in velocity.
- balanced forces
- Forces that add to zero resultant.A body with balanced forces keeps its steady velocity.
- free-body diagram
- A sketch of one body with labelled arrows for every force acting on it.Forces the body exerts on others are left out.
- action–reaction pair
- Two equal, opposite forces acting on different bodies.Tyre on road: the forward push on the tyre is paired with a backward push on the road.
- friction
- The contact force opposing relative motion between two surfaces.Slows sliding bodies — and enables walking, brakes and grips.
- air resistance (drag)
- The friction-like force from air pushing against a moving object.Grows with speed; balances weight for a steady parachute descent.
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- lubricant
- A substance (oil, grease) that reduces friction between surfaces.One of the standard friction-reduction methods, with bearings and smoother surfaces.
- newton (N)
- The unit of force: 1 N gives a 1 kg mass an acceleration of 1 m/s².F = ma defines it: N = kg × m/s².
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying a moving object needs a forward force to keep moving.
Steady velocity needs zero resultant force.
Instead: No resultant = no change in velocity; forward forces only exist to overcome friction and drag.
Mistake: Using F = ma with any single force instead of the resultant.
Only the leftover force accelerates.
Instead: Subtract opposing forces first, then F = ma with the resultant.
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Mistake: Saying zero resultant force means the object is at rest.
It means zero acceleration.
Instead: Balanced forces allow any constant velocity — including 30 m/s.
Mistake: Putting action–reaction pairs on the same free-body diagram and cancelling them.
The pair acts on different bodies.
Instead: One body per diagram: the forward push on the tyre belongs there; the backward push on the road does not.
Mistake: Drawing weight acting from above the object.
Arrows start from the body and point away in the force's direction.
Instead: Weight arrow starts on the body, points down; normal force points up from the surface.
Mistake: Adding friction to the driving direction.
Friction opposes relative motion.
Instead: Friction (and drag) point against the movement for a sliding object.
Mistake: Believing friction is always bad.
Walking, brakes and grips all need it.
Instead: Friction opposes relative motion — sometimes a nuisance, sometimes essential.
Mistake: Doubling mass and expecting doubled acceleration for the same force.
Mass is the divisor in a = F ÷ m.
Instead: Same force on double the mass gives half the acceleration.
Mistake: Quoting acceleration without a direction.
Acceleration is a vector — it has a direction (the resultant's).
Instead: State forward, backward, downward with the value.
Mistake: Mixing up mass and weight inside F = ma.
F = ma needs mass in kg, not weight in N.
Instead: Use the mass; find it from weight by m = W ÷ g if needed.
Mistake: Thinking balanced forces mean all forces are equal in size.
Balance is about the sum in each direction.
Instead: Forces balance when they cancel as vectors — 30 N left and 10 N + 20 N right also balance.
Mistake: Saying friction acts backwards on a car)s driven wheels.
Friction on the driven wheel from the road points forward — it is the drive.
Instead: The tyre pushes the road backwards; friction from the road on the tyre acts forwards.
Check yourself
Quick quiz
20 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Apply Newton’s laws to describe the effect of balanced and unbalanced forces, the ways a force changes motion, and action–reaction pairs (stating the laws is not required).
The forces on a trolley are balanced. The trolley
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5 multiple-choice questions and structured questions with answer lines. Chapter 4 (balanced and unbalanced forces, free-body diagrams, resultant force = ma, friction).
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Questions
Frequently asked questions
Does steady motion need a forward force?
No. Zero resultant force means zero change in velocity — the object keeps whatever steady velocity it has. Forward forces exist only to cancel friction and air resistance.
Which force goes into F = ma?
The resultant — add up all forces along the direction of travel with signs first. Using a single force while ignoring friction is the classic lost mark.
Why don’t action and reaction cancel each other?
They act on different bodies. The road pushes the tyre forward; the tyre pushes the road backward. Only forces on the same body can balance.
Is friction always the enemy?
No — it opposes relative motion, which is bad for bearings but essential for walking, brakes and grips. Engineers manage friction, not remove it.