Contact and Non-Contact Forces

This lesson covers the two main categories of forces — contact forces and non-contact forces — as required by the AQA GCSE Physics specification (4.5.2). A force is a push, a pull, or a twist that can change the shape, speed, or direction of an object. Forces are measured in newtons (N) and are vector quantities, meaning they have both magnitude and direction.

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What Is a Force?

A force is an interaction between two objects that can cause an object to:

• Change speed — speed up or slow down.
• Change direction — turn or curve.
• Change shape — stretch, compress, bend, or deform.

Forces are measured using a newton meter (also called a spring balance). The SI unit of force is the newton (N).

Key facts:

• A force is a vector quantity — it has both magnitude and direction.
• Forces always occur in pairs — when object A exerts a force on object B, object B exerts an equal and opposite force on object A (Newton's third law).
• A force arrow shows the direction and relative size of a force.

Exam Tip: The AQA specification requires you to describe forces as interactions between pairs of objects. For example, "the Earth pulls the apple downwards (gravity)" and "the apple pulls the Earth upwards." Always identify BOTH objects involved in the interaction.

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Contact Forces

A contact force is a force that acts between two objects that are physically touching. The two objects must be in direct contact for the force to act.

Contact Force	Description	Example
Friction	Opposes the motion of two surfaces sliding past each other	Brakes on a bicycle
Air resistance (drag)	A type of friction that acts on objects moving through air	A parachute slowing a skydiver
Normal contact force	The support force from a surface acting perpendicular to the surface	A book resting on a table
Tension	The pulling force transmitted through a string, rope, cable, or wire	A rope in a tug-of-war
Applied force	A push or pull exerted by a person or object	Pushing a shopping trolley
Upthrust	The upward force exerted by a fluid on a submerged or floating object	A boat floating on water

Friction

Friction is a contact force that opposes motion between two surfaces. Friction acts in the opposite direction to the direction of motion.

Key points about friction:

• Friction always opposes the direction of movement or attempted movement.
• Friction can be useful — e.g. grip between tyres and road, brakes.
• Friction can be unwanted — e.g. friction in engine parts causes wear and wastes energy as heat.
• Friction can be reduced by lubrication, streamlining, or using smoother surfaces.

Air Resistance (Drag)

Air resistance is a type of friction that acts on objects moving through air. It depends on:

• The speed of the object — faster objects experience more air resistance.
• The surface area of the object — larger surface areas experience more air resistance.
• The shape of the object — streamlined shapes reduce air resistance.

Exam Tip: When describing forces on a falling object, always mention that air resistance increases as the object speeds up. At terminal velocity, air resistance equals weight, so the resultant force is zero and the object moves at constant speed.

Normal Contact Force

The normal contact force (also called the reaction force or normal force) is the force exerted by a surface on an object resting on it. It acts perpendicular (at right angles) to the surface.

For a book sitting on a table:

• The weight of the book acts downwards.
• The normal contact force from the table acts upwards.
• These two forces are balanced, so the book remains stationary.

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Non-Contact Forces

A non-contact force is a force that acts between two objects that are not physically touching. The objects can be separated by a gap (which may be empty space or filled with a medium).

Non-Contact Force	Description	Example
Gravitational force (weight)	The attraction between any two masses	The Earth pulling an apple downwards
Electrostatic force	The attraction or repulsion between charged objects	A charged balloon sticking to a wall
Magnetic force	The attraction or repulsion between magnets or magnetic materials	A magnet attracting an iron nail

Gravitational Force (Weight)

Weight is the force of gravity acting on an object. It is calculated using:

W = m x g

Where:

• W = weight (N)
• m = mass (kg)
• g = gravitational field strength (N/kg)

On Earth, g = 9.8 N/kg (often rounded to 10 N/kg in calculations).

Mass (kg)	Weight on Earth (N)	Weight on Moon (N)
1	9.8	1.6
5	49	8.0
10	98	16.0
70	686	112.0

Exam Tip: The gravitational field strength on Earth is 9.8 N/kg but on the Moon it is only about 1.6 N/kg. Mass stays the same wherever you go, but weight changes because it depends on the gravitational field strength. This is a very common exam distinction.

Electrostatic Force

Electrostatic forces act between objects that carry an electric charge.

• Like charges repel (positive repels positive; negative repels negative).
• Opposite charges attract (positive attracts negative).
• Electrostatic forces can act through a vacuum.

Magnetic Force

Magnetic forces act between magnets or between a magnet and a magnetic material (iron, steel, cobalt, nickel).

• Like poles repel (north repels north; south repels south).
• Unlike poles attract (north attracts south).
• Magnetic forces can act through a vacuum and through many materials.

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Force Diagrams

Force diagrams are used to show all the forces acting on an object. There are two main types:

Free Body Diagrams

A free body diagram shows a single object with all the forces acting on it drawn as labelled arrows. The object is usually drawn as a simple shape (dot or box).

graph TD
    W["Weight (W)"] --> O["Object"]
    O --> N["Normal contact force (N)"]
    F["Friction (F)"] --> O
    O --> A["Applied force (A)"]

    style O fill:#2c3e50,color:#fff
    style W fill:#e74c3c,color:#fff
    style N fill:#27ae60,color:#fff
    style F fill:#e67e22,color:#fff
    style A fill:#2980b9,color:#fff