Revision notes for AQA GCSE Physics Contact and non-contact forces. Open the guide for explanations and worked examples. Written against the AQA GCSE Physics (8463) specification, so the content matches what's examinable rather than general Physics background.
Contact and non-contact forces
What you'll learn
- What a force is, and why every force involves two objects interacting.
- The difference between contact forces and non-contact forces.
- Key GCSE examples: friction, air resistance, tension, normal contact force, gravitational, electrostatic and magnetic forces.
- How to represent forces as vectors using arrows.
Forces start with interactions
In physics, an object just means the thing you are thinking about: a book, a ball, a person, the Earth, a magnet, or even the air.
An interaction happens when two objects affect each other. Forces are not “floating around” on their own — a force is always caused by one object interacting with another object.
Definition
Force
A force is a push or pull that acts on an object due to the interaction with another object. Force is measured in newtons, symbol N, and is often given the symbol FFF.
For example, if you push a door, your hand exerts a force on the door. At the same time, the door exerts a force back on your hand. The two objects are the hand and the door.
Key Idea
Forces are object-on-object
A clear force description should say which object exerts the force, which object the force acts on, and which direction the force acts.
Example
Describing a push on a trolley
A person pushes a trolley forwards.
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Choose the object you are describing first: the trolley. The person’s hand is touching the trolley, so the trolley has a forward force from the hand.
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Reverse the two objects to find the partner force: the trolley exerts a force on the person’s hand.
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The partner force acts in the opposite direction: the trolley pushes backwards on the hand, not backwards on the trolley.
Contact forces
A force is a contact force when the two interacting objects are physically touching.
Definition
Contact force
A contact force is a force between objects that are physically touching each other.
Important examples of contact forces are:
- Friction: a force that opposes sliding, or the tendency to slide, between touching surfaces.
- Air resistance: a force that opposes motion through air. It happens because the object is colliding with air particles.
- Tension: a pulling force in a stretched string, rope, cable or chain.
- Normal contact force: the force from a surface on an object touching it. “Normal” means at right angles to the surface.
Normal contact force
If a book rests on a table, the table pushes upwards on the book. That upward push is the normal contact force from the table on the book.
The book also pushes downwards on the table. These two forces come from the contact interaction between the book and the table.
Tip
Normal means perpendicular
In “normal contact force”, normal does not mean “ordinary”. It means the force acts at right angles to the surface.
Non-contact forces
A force is a non-contact force when the two interacting objects are physically separated. They do not need to touch.
Definition
Non-contact force
A non-contact force is a force between objects that are physically separated.
Important examples of non-contact forces are:
- Gravitational force: the attractive force between objects with mass. Near Earth, the gravitational force on an object is often called its weight.
- Electrostatic force: the force between electrically charged objects. It can be attractive or repulsive.
- Magnetic force: the force involving magnets, magnetic materials, or certain electric currents. It can be attractive or repulsive.
Common Mistake
Air resistance is not non-contact
Air resistance is a contact force because the object is physically interacting with air particles, even though you cannot usually see the air.
Example
Classifying forces on a falling tennis ball
A tennis ball is falling through the air before it hits the ground.
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The Earth pulls down on the ball by gravitational force. The Earth and ball are not physically touching, so this is a non-contact force.
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The air pushes against the moving ball by air resistance. The ball is colliding with air particles, so this is a contact force.
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If the ball hits the ground, the ground pushes up on the ball. The ball and ground are touching, so this would be a normal contact force.
Force is a vector quantity
Some quantities only have a size. For example, mass has a size, such as 2 kg, but no direction.
Force is different: it has both a size and a direction.
Definition
Vector quantity
A vector quantity has both a magnitude and a direction. The magnitude of a force is its size, measured in newtons.
Forces are represented using arrows:
- The arrowhead shows the direction of the force.
- The length of the arrow shows the magnitude of the force.
- The arrow should be labelled with the type of force, or with wording such as “force from table on book”.
If one force is larger than another, its arrow should be longer, as long as the diagram is using a sensible scale.
Tip
Use the phrase from-on
A precise label often follows this pattern: force from object A on object B. For example: “gravitational force from Earth on ball”.
Interaction pairs
When two objects interact, each object experiences a force. These two forces are often called an interaction pair.
For this topic, focus on identifying:
- the two objects involved
- whether the interaction is contact or non-contact
- the direction of the force on each object
- how to draw the forces as arrows
For example, in the gravitational interaction between the Earth and a ball:
- the Earth pulls the ball downwards
- the ball pulls the Earth upwards
The forces are the same type of force, but they act on different objects.
Common Mistake
Interaction pairs do not cancel
Do not say an interaction pair “cancels out” just because the arrows are in opposite directions. The two forces act on different objects, so they are not balanced forces on one object.
Example
Representing magnetic attraction as vectors
Two magnets are separated by a small gap and attract each other.
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Identify the two interacting objects: magnet A and magnet B. They are not touching, so the force is non-contact.
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Decide the direction of each force. Because the magnets attract, the force on magnet A points towards magnet B, and the force on magnet B points towards magnet A.
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Draw one arrow on each magnet, pointing towards the other magnet. Label them “magnetic force from B on A” and “magnetic force from A on B”.
Quick comparison
Contact forces
The objects must touch.
Examples:
- friction between a shoe and the ground
- air resistance on a cyclist
- tension in a rope
- normal contact force from a table on a book
Non-contact forces
The objects are separated.
Examples:
- gravitational force from Earth on a ball
- electrostatic force between charged objects
- magnetic force between two magnets
Key Idea
The big test
To decide whether a force is contact or non-contact, ask: are the two objects causing the force physically touching?
Exam technique
In the exam
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When describing a force, write it as “force from A on B” so it is clear which object the force acts on.
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For contact versus non-contact, base your answer on whether the interacting objects are physically touching, not on whether the force is visible.
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When drawing force arrows, put the arrow on the object experiencing the force, point it in the correct direction, and make larger forces longer if sizes are being compared.
Self review
Check yourself
- Why is air resistance classed as a contact force?
- A book rests on a table. What two forces are produced by the interaction between the book and the table?
- How does a force arrow show both magnitude and direction?
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