Contact and Non-Contact Forces

This lesson covers the two main categories of forces — contact forces and non-contact forces — as required by AQA GCSE Combined Science Trilogy (8464), section 6.5.1. You will learn how to classify forces, understand weight and its calculation, and interpret free body diagrams. Forces are central to nearly every physics question, so a solid grasp of this material is essential.

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What Is a Force?

A force is an interaction between two objects that can:

• Change the speed of an object (speed it up or slow it down).
• Change the direction of an object.
• Change the shape of an object (stretch, squash, bend or twist it).

Forces are measured in newtons (N) using a newton meter (spring balance). Force is a vector quantity — it has both magnitude and direction.

Key Fact: Forces always exist in pairs. When object A exerts a force on object B, object B always exerts a force back on object A. This is Newton's third law, covered in Lesson 8.

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Contact Forces

A contact force acts between two objects that are physically touching. The objects must be in direct contact.

Contact Force	Description	Example
Friction	Opposes motion between two surfaces sliding past each other	Brake pads gripping a wheel
Air resistance (drag)	Friction acting on an object moving through air	A parachute slowing a skydiver
Normal contact force	Perpendicular support force from a surface	A table supporting a book
Tension	Pulling force in a string, rope, cable or wire	A cable holding a lift
Upthrust	Upward force from a fluid on a submerged or floating object	A boat floating on water

Friction in Detail

Friction is a contact force that opposes relative motion between two surfaces. It acts in the opposite direction to the direction of movement (or attempted movement).

graph LR
    subgraph "Box sliding right on a surface"
        F1["Applied force -->"]
        F2["<-- Friction"]
    end

    style F1 fill:#2980b9,color:#fff
    style F2 fill:#e74c3c,color:#fff

Friction can be:

• Useful: Stopping a car (brakes), walking (grip between shoe and ground).
• Unwanted: Wasting energy as heat in engines, causing wear on moving parts.

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Non-Contact Forces

A non-contact force acts between two objects that are not physically touching. The objects can be separated by empty space or by another material.

Non-Contact Force	Description	Example
Gravitational force (gravity)	Attractive force between any two masses	Earth pulling an apple downwards
Magnetic force	Force between magnets or between a magnet and a magnetic material	Two bar magnets repelling
Electrostatic force	Force between electrically charged objects	A charged balloon sticking to a wall

Exam Tip (AQA 8464): A common exam question asks you to classify forces as contact or non-contact. Remember: friction, air resistance, normal contact force, tension and upthrust are all contact forces. Gravity, magnetism and electrostatic force are non-contact.

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Weight, Mass and Gravitational Field Strength

Weight is the force acting on an object due to gravity. It is a non-contact force.

The equation for weight is:

$W = m \times g$W=m×g

Where:

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

On Earth, $g \approx 9.8$g≈9.8 N/kg (often rounded to 10 N/kg in calculations).

Quantity	Type	Unit	Measured with
Mass	Scalar	kilograms (kg)	Balance
Weight	Vector (always acts downwards)	newtons (N)	Newton meter / spring balance

Worked Example

A bag has a mass of 8 kg. Calculate its weight on Earth (g = 9.8 N/kg).

Solution:

$W = m \times g = 8 \times 9.8 = 78.4$W=m×g=8×9.8=78.4 N

The weight of the bag is 78.4 N downwards.

Exam Tip: Always state that weight acts downwards (towards the centre of the Earth). Many students forget to include the direction and lose a mark.

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Free Body Diagrams

A free body diagram shows all the forces acting on a single object, each represented by a labelled arrow. The arrow length is proportional to the magnitude of the force, and the arrow direction shows the direction of the force.

graph TD
    W["Weight (W)"] --> O["Object"]
    O --> N["Normal contact force (N)"]
    style O fill:#2c3e50,color:#fff
    style W fill:#e74c3c,color:#fff
    style N fill:#27ae60,color:#fff

Rules for drawing free body diagrams:

1. Draw the object as a simple shape (box or dot).
2. Draw one arrow for each force acting on the object.
3. Arrows must start from the object and point in the direction the force acts.
4. Label each arrow with the name and magnitude of the force.
5. Make arrow lengths proportional to the force magnitudes.

Common Forces on a Free Body Diagram

Situation	Forces to include
Object resting on a table	Weight (down), normal contact force (up)
Object falling through air	Weight (down), air resistance (up)
Car driving on a road	Driving force (forward), friction/drag (backward), weight (down), normal force (up)

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Gravitational Field Strength on Other Bodies

The value of $g$g depends on the mass of the planet or moon:

Location	g (N/kg)
Earth	9.8
Moon	1.6
Mars	3.7
Jupiter	24.8

An object's mass stays the same everywhere, but its weight changes depending on the gravitational field strength.

Worked Example

An astronaut has a mass of 75 kg. Calculate their weight (a) on Earth and (b) on the Moon.

Solution:

(a) On Earth: $W = 75 \times 9.8 = 735$W=75×9.8=735 N

(b) On the Moon: $W = 75 \times 1.6 = 120$W=75×1.6=120 N

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Common Mistakes

• Confusing mass and weight — mass is in kg (scalar), weight is in N (vector).
• Forgetting that weight acts downwards.
• Failing to identify all forces on a free body diagram — always check for weight, normal contact force, friction and air resistance.
• Classifying gravity as a contact force — it is a non-contact force.
• Using g = 10 N/kg when the question states g = 9.8 N/kg (read the question carefully).

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Summary