Forces and Motion

This guide covers the balanced/unbalanced forces, friction and circular motion parts of subtopic 1.5 Forces, from Topic 1, Motion, forces and energy, for Cambridge O Level Physics 5054, 2026–2028 series. The elastic deformation, moments and centre of gravity parts of 1.5 are covered separately in Elastic Deformation, Moments and Centre of Gravity.

Where this fits in 5054

Forces are what connect the description of motion in Kinematics and Motion Graphs to why objects speed up, slow down or change direction. Newton’s laws here are used throughout the rest of Topic 1 — Momentum is built directly from F = ma, and the energy transfers in Energy, Work and Power are caused by forces doing work.

Syllabus coverage

CAMBRIDGE O LEVEL PHYSICS 5054

• Identify and use different types of force, including weight (gravitational force), friction, drag, air resistance, tension (elastic force), electrostatic force, magnetic force, thrust (driving force) and contact force (1.5)
• Identify forces acting on an object and draw free-body diagram(s) representing the forces (1.5)
• State Newton’s first law: an object either remains at rest or continues to move in a straight line at constant speed unless acted on by a resultant force (1.5)
• State that a force may change the velocity of an object by changing its direction of motion or its speed (1.5)
• Determine the resultant of two or more forces acting along the same straight line (1.5)
• Recall and use resultant force = mass × acceleration, F = ma (1.5)
• State Newton’s third law: when object A exerts a force on object B, then object B exerts an equal and opposite force on object A (1.5)
• Know that Newton’s third law describes pairs of forces of the same type acting on different objects (1.5)
• Describe friction as a force that may impede motion and produce heating (1.5)
• Understand the motion of objects acted on by a constant weight or driving force, with and without drag (1.5)
• Explain how an object reaches terminal velocity (1.5)
• Define thinking distance, braking distance and stopping distance of a moving vehicle (1.5)
• Explain the factors affecting thinking and braking distance, including speed, tiredness, alcohol, drugs, load, tyre surface and road conditions (1.5)
• Describe, qualitatively, motion in a circular path due to a force perpendicular to the motion (1.5)

5054 is not tiered — every candidate covers all of the above. The quantitative circular motion equation F = mv²/r is explicitly not required; only the qualitative relationships below are examined.

Types of force and free-body diagrams

A force is a push or pull that can change an object’s motion. The syllabus names nine types you should recognise: weight (gravitational force), friction, drag, air resistance, tension (elastic force), electrostatic force, magnetic force, thrust (driving force), and contact force (the general push/pull between touching surfaces).

A free-body diagram shows only the object of interest, with an arrow for every force acting on it, each arrow’s length roughly representing the force’s size and its direction showing the force’s direction. Drawing one correctly, before doing any calculation, is what makes it possible to identify the resultant force.

Newton’s first and third laws

Newton’s first law: an object either remains at rest, or continues to move in a straight line at constant speed, unless acted on by a resultant force. In other words, no resultant force means no change in motion — constant velocity (which includes zero velocity) is the “natural” state, not something requiring a continuous push to sustain.

A force can change an object’s velocity in either of two ways: by changing its speed, or by changing its direction — an object moving in a circle at constant speed is still accelerating, because its direction (and therefore its velocity) is constantly changing.

Worked example. A 4 N force and a 7 N force act on an object in the same direction along a straight line. A 3 N force acts on it in the opposite direction. Find the resultant force.

resultant = (4 + 7) − 3 = 8 N, in the direction of the 4 N and 7 N forces

Newton’s third law: when object A exerts a force on object B, object B exerts an equal and opposite force on object A. The two forces in a Newton’s-third-law pair are always the same type of force, and always act on two different objects — never on the same object. A book resting on a table exerts a (gravitational) weight force on the Earth and experiences a weight force from the Earth in return, while separately the table pushes up on the book with a contact force and the book pushes down on the table with an equal contact force — two different pairs, not one.

F = ma

Newton’s second law connects resultant force, mass and acceleration:

resultant force = mass × acceleration        F = ma

Worked example. A resultant force of 15 N acts on a 3 kg object. Find its acceleration.

a = F / m = 15 / 3 = 5 m/s²

Friction, drag and terminal velocity

Friction is a force that may impede motion and produce heating — it opposes relative motion between two surfaces in contact. Drag and air resistance are the fluid equivalent, opposing motion through a liquid or gas.

When an object moves under a constant driving force with drag acting against it, the drag force increases as speed increases. Eventually drag grows large enough to exactly balance the driving force — at that point the resultant force is zero, so (by Newton’s first law) the object stops accelerating and continues at a constant terminal velocity.

Stopping distance for a vehicle has two parts:

• Thinking distance — the distance travelled during the driver’s reaction time, before the brakes are applied.
• Braking distance — the distance travelled once the brakes are applied, until the vehicle stops.
• Stopping distance = thinking distance + braking distance.

Both are affected by real factors examiners expect you to name: speed (both distances increase — thinking distance because more distance is covered in the same reaction time, braking distance because kinetic energy increases with the square of speed), tiredness, alcohol and drugs (increase thinking distance, by slowing reaction time), and load, tyre condition and road conditions (increase braking distance, by reducing the grip available to decelerate the vehicle).

Circular motion (qualitative)

An object moving in a circular path has a resultant force acting perpendicular to its motion, constantly changing its direction without (necessarily) changing its speed. The syllabus asks for three qualitative relationships, with mass, force, speed and radius otherwise held constant:

• Speed increases if force increases (mass and radius constant).
• Radius decreases if force increases (mass and speed constant) — a tighter circle needs a larger centre-seeking force for the same speed.
• An increased mass requires an increased force to keep speed and radius constant.

The equation F = mv²/r is not required at this level — only these qualitative trends.

Common mistakes

• Treating “no resultant force” as “no forces at all.” An object can have several forces acting on it and still not accelerate, provided they sum to zero — Newton’s first law is about the resultant, not the absence of individual forces.
• Pairing a Newton’s-third-law force with the wrong force on the same object. A book’s weight and the table’s contact force on the book are not a Newton’s-third-law pair (different types of force, same object) — the true pairs each involve the same type of force on two different objects.
• Assuming terminal velocity means zero force. It means zero resultant force — the driving force and drag are both still acting, just equal and opposite.
• Confusing thinking distance and braking distance factors. Reaction-time factors (tiredness, alcohol, drugs) affect thinking distance; grip-related factors (tyres, road, load) affect braking distance; speed affects both.
• Trying to use F = mv²/r. It’s explicitly excluded from this syllabus — answer circular motion questions with the qualitative relationships only.

Quick revision checklist

• The nine named force types, and how to draw a free-body diagram
• Newton’s first law, and the two ways a force can change velocity (direction or speed)
• Newton’s third law: same force type, different objects, always a pair
• F = ma, and using it to find resultant force, mass or acceleration
• Terminal velocity: why drag increasing with speed leads to a constant final speed
• Thinking distance vs braking distance, and which factors affect each
• The three qualitative circular-motion relationships (F = mv²/r not required)

Related resources

• Kinematics and Motion Graphs — the motion these forces act on
• Elastic Deformation, Moments and Centre of Gravity — the rest of subtopic 1.5
• Momentum — F = Δp/Δt, an alternative form of Newton’s second law
• Cambridge O Level Physics subject hub

Written against Cambridge O Level Physics 5054, 2026–2028 series. Always check the current syllabus for your examination year.