OCR GCSE Physics A: Interacting forces, vectors and free body diagrams (J249) – Revision Notes

These notes condense statements P2.2a to P2.2h and P2.2p of the OCR GCSE (9-1) Gateway Science Physics A specification (J249, Version 5.0, July 2026, for first assessment in 2018). None of these statements is Higher Tier only. For full explanations read the study guide, then try the practice questions. Related: OCR GCSE Physics: Forces revision notes, the OCR GCSE Physics hub and the checklist. Check your level with the free 10-minute diagnostics.

Key definitions

Term	Meaning
Force	A push or pull on an object, measured in newtons (N)
Vector	A quantity with size and direction
Contact force	Needs touching: normal contact force, friction, drag, tension
Non-contact force	Acts across a gap: gravity, electrostatic, magnetic
Resultant force	One force that could replace all the forces on the object with the same overall effect
Equilibrium	Resultant force is zero
Free body diagram	One object, with arrows for the forces acting on it only
System	A chosen object or group treated as one
Terminal velocity	Constant velocity reached when drag equals the driving force or weight
Interaction pair	Two forces from one interaction: equal, opposite, same type, on different objects

Interactions (P2.2a, P2.2b)

• Contact: normal contact force (surface pushes at right angles to itself) and friction (opposes sliding).
• Non-contact: gravity, electrostatics, magnetism.
• Every interaction involves two objects and gives a force on each.
• Wording to use: “the force of A on B” and “the force of B on A”.

Free body diagrams (P2.2c, P2.2f, P2.2g, P2.2h)

Method in steps:

1. Choose the object. Draw it as a box or dot.
2. Add an arrow for each force acting on it, from the object, labelled.
3. Scale the lengths to the sizes.
4. Leave out forces the object exerts on others, and leave out other objects.
5. Compare the arrows in opposite directions to find the resultant.

Typical forces to include:

Situation	Forces on the object
Book on a table	Weight (down), normal contact force (up)
Skydiver falling	Weight (down), drag (up)
Car at steady speed	Thrust/driving force (forward), drag and friction (backward), weight (down), normal contact force (up)
Hanging mass on a string	Weight (down), tension (up)
Block sliding on a table	Weight, normal contact force, friction (against motion), push or pull

Newton’s first law (P2.2d)

• Resultant zero: stays at rest, or keeps the same speed and direction.
• Resultant not zero: velocity changes (speed, direction, or both).
• Constant velocity needs no resultant force.
• A stationary object has forces on it, and they balance.

Resultants and vector diagrams (P2.2e, P2.2g)

Case	Method
Same line, same direction	Add
Same line, opposite directions	Subtract: bigger minus smaller, direction of the bigger
At right angles	Head-to-tail scale drawing, or Pythagoras: R = √(a² + b²)
Direction of the resultant	tan θ = opposite / adjacent

Resolution is the reverse: draw the force as the diagonal of a rectangle and read the horizontal and vertical sides to scale. Scale drawings are limited to parallel and perpendicular vectors.

Equilibrium test: vectors placed head to tail form a closed shape.

Terminal velocity (P2.2f)

1. Start: weight greater than drag, so a resultant downward and speed rises.
2. Speed rises, so drag rises, so the resultant falls.
3. Drag equals weight: resultant zero, constant velocity.
4. Parachute opens: drag greater than weight, upward resultant, slows.
5. Drag equals weight again: smaller terminal velocity.

Newton’s third law (P2.2p)

• Pair forces: same size, opposite direction, same type, on different objects.
• Pair forces never cancel, since each acts on a different object.
• A book’s weight and the table’s push on it are not a pair.
• Pair of the Earth’s pull on a book: the book’s pull on the Earth.

Worked reminders

Same line. A barge: tow rope force 540 N forward, water resistance 410 N backward. Resultant = 540 − 410 = 130 N forward.

Perpendicular. 21 N west and 20 N south. R = √(21² + 20²) = 29 N. Angle south of west = tan⁻¹(20/21) = 44°.

Equilibrium. 9 N right, 4 N left and 5 N left on a block. Net = 9 − 4 − 5 = 0 N.

Naming interaction pairs

Interaction	Force 1	Force 2 (its pair)
Foot on the ground	Ground pushes foot forward (friction)	Foot pushes ground backward (friction)
Magnet and nail	Magnet pulls nail	Nail pulls magnet
Charged rod and paper	Rod attracts paper (electrostatic)	Paper attracts rod (electrostatic)
Hand pushing a wall	Hand pushes wall	Wall pushes hand (normal contact)
Moon and Earth	Earth pulls Moon (gravity)	Moon pulls Earth (gravity)
Swimmer in water	Swimmer pushes water backward	Water pushes swimmer forward

Method: swap the two objects in the sentence, keep the type of force, reverse the direction.

Model answers for the first law

• “A hockey puck slides at constant velocity on ice.” Resultant force is zero, so no force is needed to keep it moving. Any friction is very small.
• “A car slows down.” There is a resultant force backward, from braking and drag, so its velocity changes.
• “A satellite changes direction.” A resultant force acts on it, because a change of direction is a change of velocity.
• “A parcel on a stationary shelf.” Weight and normal contact force balance, so the resultant is zero and it stays at rest.

Always link the motion to the resultant force. Do not describe it with “the forces cancel” unless you say which forces and on which object.

Free body diagram checklist

Before you leave a diagram, check that you have: only one object; every arrow starting at the object; a label on every arrow; matching lengths for balanced forces; no force that the object exerts on something else; and no extra arrow for the resultant.

Resolution reminder

A 130 N force is drawn at an angle whose triangle has sides in the ratio 5 : 12 : 13. Scale 1 cm to 10 N: the arrow is 13.0 cm. Its sides measure 12.0 cm and 5.0 cm. The components are 120 N and 50 N. Check: 120² + 50² = 16 900 = 130².

Command words in this topic

Command word	What the answer needs
State	One fact, no reasoning
Draw	Arrows from the object, labelled, lengths to scale
Describe	What happens, in order, using the forces
Explain	A reason using a law: first or third
Calculate	Working, answer and unit
Determine	Use a scale drawing or a calculation to get a value

Must-know distinctions

• Weight and normal contact force (same object, balanced) versus a third-law pair (different objects).
• Equilibrium (no resultant force) versus stationary (zero velocity). Equilibrium can mean constant velocity.
• Drag (against motion through a fluid) versus friction between solid surfaces.
• Free body diagram (forces on one object) versus an interaction diagram (both objects).

Quick self-test

1. Name two non-contact forces.
2. Which contact force acts at right angles to a surface?
3. State Newton’s first law.
4. A cyclist moves at a steady 6 m/s on level ground. What is the resultant force?
5. Find the resultant of 7.5 N and 10 N acting at right angles.
6. A box has 90 N pushing it right and 55 N of friction acting left. State the resultant.
7. Why do a book’s weight and the table’s normal contact force not form a third-law pair? Give two reasons.
8. A skydiver weighs 700 N and falls at terminal velocity. State the drag.
9. A magnet pulls a nail with 0.4 N. What force does the nail exert on the magnet?
10. What shape does a vector diagram of three forces in equilibrium make?
11. A lift cable pulls up with 4500 N and the lift’s weight is 4200 N. State the resultant force and its direction.
12. A sledge is in equilibrium on level snow. A rope pulls it forward with 64 N. Give the friction force.

Answers

1. Gravity, magnetism or electrostatics (any two).
2. Normal contact force.
3. An object stays at rest or keeps moving at constant velocity unless a resultant force acts on it.
4. Zero.
5. √(7.5² + 10²) = √156.25 = 12.5 N.
6. 90 − 55 = 35 N to the right.
7. Both forces act on the book, and one is gravitational while the other is a contact force.
8. 700 N upward.
9. 0.4 N, towards the nail.
10. A closed triangle.
11. 4500 − 4200 = 300 N upward.
12. 64 N, backward (opposing the pull, with all other forces balanced).

Where marks are usually lost

• Drawing arrows that start on a surface, or forces that belong to another object.
• Unequal arrow lengths for forces that balance.
• Describing a pair of forces without naming both objects.
• Giving weight and the normal contact force as a third-law pair.
• Saying “there is no force” instead of “the forces are balanced” or “the resultant is zero”.
• Adding perpendicular forces arithmetically.
• Missing the direction of a resultant, or missing the unit.
• Writing “the car keeps moving because the force continues” rather than using the first law.
• Skipping the stage in terminal velocity where drag rises as speed rises.

Official syllabus

OCR GCSE (9-1) Gateway Science Physics A (J249), Version 5.0, July 2026, for first assessment in 2018, published by OCR: statements P2.2a to P2.2h and P2.2p.