Scalar and vector quantities; contact and non-contact forces

SPEC 4.5.1.1 / 4.5.1.2 • FORCES — SCALARS, VECTORS & FORCE TYPES

Scalars, Vectors & Force Types

The vocabulary of Module 5. Get these definitions exact and you've banked easy marks across every Paper 2 force question.

Welcome to Module 5 — Forces

Forces is the biggest module in the entire spec — 13 lessons, the most-examined calculations on Paper 2 (W = mg, F = ma, work done, momentum), and three required practicals' worth of method-writing material.

13

Lessons

3

Required Practicals

P2

Examined On

Why this lesson first

Before any calculation, every force question expects you to classify quantities correctly. Scalar or vector? Contact or non-contact? Get the vocabulary wrong here and you'll lose marks for the rest of the module. This is foundational — 8 minutes that pay off across 12 future lessons.

Scalar Quantities

A scalar has size only — no direction. If you can answer the question "how much?" with just a number and a unit, it's a scalar.

Scalar Quantity

A physical quantity that has magnitude (size) only.

The 5 scalars to memorise

MassDistanceSpeedTimeEnergy

Example: a mass of 5 kg is fully described by "5 kg". You don't need to say "5 kg north" — mass has no direction.

Vector Quantities

A vector has size AND direction. If a quantity changes meaning when you reverse the direction, it's a vector.

Vector Quantity

A physical quantity that has magnitude AND an associated direction.

The 5 vectors to memorise

ForceDisplacementVelocityAccelerationMomentum

Example: a force of 5 N to the right is different from a force of 5 N to the left — same size, opposite effect. Direction matters.

The Two Pairs Examiners Test

Two scalar/vector pairs come up again and again on Paper 2. Mixing them up loses marks every series.

ScalarVectorWhat the vector adds

Distance Total path length travelledDisplacement Straight-line distance from start to finish, with directionDirection. Walk 3 m east then 3 m west → distance 6 m, displacement 0 m.

Speed Rate of change of distanceVelocity Rate of change of displacement, with directionDirection. A car going round a roundabout at constant speed has changing velocity (direction changes).

Exam Tip

If a question asks for velocity, you must include a direction (e.g. "5 m/s east" or "5 m/s downward"). Just writing "5 m/s" is the answer for speed only.

The Examined Trap — Distance vs Displacement

This is where the most marks get dropped on the scalar/vector topic. A real exam pattern: students measure the curved route on a scale diagram and call it displacement. That's distance — and it loses the mark.

Mark-scheme phrasing

Credited answer for "what's the difference?": "distance is a scalar and displacement is a vector" OR "distance has magnitude only; displacement has magnitude and direction". Either earns the mark — but you must include the word "direction" for displacement.

For "Determine displacement" questions

Real Paper 2 questions ask you to "Determine the magnitude of the displacement" using a scale diagram. Method: ignore the path; measure the straight-line distance from start to finish with a ruler, then apply the scale. If asked for direction too (e.g. "displacement = ___ m, direction = ___"), give a compass bearing or describe with respect to a reference line.

Vectors as Arrows

Vectors are drawn as arrows. Two things must be clear:

Length represents the magnitude (use a scale, e.g. 1 cm = 1 N)

Arrowhead direction represents the direction of the quantity

Tip-to-tail addition

To add two vectors, draw them tip-to-tail. The resultant is a single arrow from the start of the first to the tip of the last. You'll use this method in lesson 5.03 (resultant forces).

Contact Forces

A force is a push or pull on an object due to interaction with another object. Every force is either contact or non-contact. Force is a vector — measured in newtons (N), and always has a direction.

Contact Force

A force where the two objects are physically touching.

The 4 spec-named contact forces

Friction

Air resistance

Tension

Normal contact force

Test: are the two objects physically touching at the point where the force acts? Then it's a contact force.

Mark scheme also accepts

For "give an example of a contact force", mark schemes also credit: upthrust, lift, thrust, water resistance, and "normal reaction force" (same thing as normal contact force). Use a spec-named one first, but these are safe back-ups.

Non-Contact Forces

Non-contact forces act through a field, with no physical contact required. The Sun pulls Earth across empty space — that's gravity acting through a gravitational field.

Non-Contact Force

A force where the two objects are physically separated and the force acts through a field.

The 3 non-contact forces to name

Gravitational

Electrostatic

Magnetic

Test: Force acts at a distance with nothing in between? It's non-contact.

Exam Tip

If a question asks you to state two non-contact forces, pick from this exact list of three. Do not write "wind" (that's air resistance acting as a contact force) or "upthrust" (a contact force exerted by a fluid).

Newton's 3rd Law — Force Pairs

Every single force has a partner. When object A exerts a force on object B, object B exerts an equal and opposite force on A. The two forces act on different bodies.

Why they don't cancel

The two forces in a Newton's 3rd law pair act on different objects (one on the rocket, one on the gas). Forces only cancel when they act on the same object. That's the misconception examiners hammer.

The PEE Framework

For every describe / explain / compare question in Physics, structure your answer with PEE. This is the framework you'll see in every worked example across the course.

PPointState the physics fact or claim precisely.

EEvidenceCite the specific equation, principle, or named example.

EExplainGive the mechanism — use "because" to link cause and effect.

For calculations

For numerical questions you'll switch to ESUA (Equation → Substitute → Unit → Answer). Many extended-response questions blend both: calculate first, then explain in PEE.

Examiner Alerts

Alert 1 — Gravity is NOT a contact force

The most-flagged misconception on this entire topic. When asked to "name a contact force", students reliably write gravity. Wrong — gravity acts across empty space, so it's a non-contact force. Equally common: students just repeat a force already mentioned in the question (e.g. naming "friction" when the stem already gave friction). Pick a different spec-named contact force: friction, air resistance, tension, or normal contact force.

Alert 2 — "Equal so they cancel"

For Newton's 3rd law force pairs, students write "the forces are equal so they cancel". Wrong. The two forces act on different bodies — they cannot cancel each other out. Forces only "cancel" when both forces act on the same object (that's a resultant of zero — lesson 5.03). The mark scheme demands you name both bodies in the force pair: e.g. "the rocket pushes the gas down; the gas pushes the rocket up".

Practice Question

3 marks

Note: weight equations (W = mg) are covered in detail in lesson 5.02 — this question is the cleanest scalar/vector test in the spec.

P

Point. Mass is a scalar quantity, measured in kilograms (kg); weight is a vector quantity, measured in newtons (N).

E

Evidence. Mass is the amount of matter in an object — it has size but no direction. Weight is the force on an object due to gravity — it acts downward toward the centre of the planet.

E

Explain. Mass is constant wherever you take the object, because the amount of matter doesn't change. Weight depends on gravitational field strength, because weight = mass × g — so the same object weighs less on the Moon (g = 1.6 N/kg) than on Earth (g = 9.8 N/kg), even though its mass is unchanged.

Weak vs Strong Answers

The same exam question, two student responses. Spot the mark-scheme language.

Q: A rocket launches by burning fuel. Explain how Newton's 3rd law applies. (3 marks)

✗ Weak

"The rocket pushes itself up. The force of the fuel makes it move."

Doesn't name a force pair. Treats "force of fuel" as if energy is a force. Misses the 3rd law structure entirely.

✓ Strong

"The rocket pushes the burning gas downwards; the gas pushes the rocket upwards. The two forces are equal in size and opposite in direction, and act on different bodies."

Both bodies named. Forces equal and opposite stated. "Different bodies" makes clear they don't cancel.

The mark-scheme template

For any Newton's 3rd law answer: "A pushes/pulls B [direction]; B pushes/pulls A [opposite direction]. Equal in size, opposite in direction, on different bodies." Use this exact structure every time.

Checkpoint — You Should Now Be Able To...

✓ Classify quantities

Decide whether any physical quantity is a scalar or a vector. Name the 5 scalars (mass, distance, speed, time, energy) and the 5 vectors (force, displacement, velocity, acceleration, momentum).

✓ Draw vectors as arrows

Represent a vector with an arrow whose length shows magnitude (to scale) and whose arrowhead shows direction. Know that vectors are added tip-to-tail.

✓ Classify forces

Distinguish contact (friction, air resistance, tension, normal contact) from non-contact (gravitational, electrostatic, magnetic). Know that force is a vector, in newtons.

✓ Apply Newton's 3rd law

Explain a force pair by naming both bodies and the directions, and state that the forces act on different bodies so don't cancel.

Summary

Scalars vs Vectors

Scalar (magnitude only): mass · distance · speed · time · energy

Vector (magnitude + direction): force · displacement · velocity · acceleration · momentum

Pairs to know: distance ↔ displacement · speed ↔ velocity

Forces — Contact vs Non-Contact

Contact (touching): friction · air resistance · tension · normal contact (reaction)

Non-contact (through a field): gravitational · electrostatic · magnetic

Force is a vector , measured in newtons (N)

Newton's 3rd Law — Force Pairs

"When A pushes/pulls B, B pushes/pulls A with an equal and opposite force"

The two forces act on different bodies — so they don't cancel

Always name both bodies in your answer

Exam Tip

The single biggest pattern across mark schemes for this topic: specifying direction. Vectors need a direction. Newton's 3rd law needs two directions. "5 m/s" is speed; "5 m/s east" is velocity. Make direction explicit every time.