What you'll learn
This topic forms the foundation of forces in AQA GCSE Physics. You'll learn to distinguish between forces that require physical touch and those that act over a distance, identify examples of each type, and understand how they affect objects in everyday situations. This knowledge is essential for tackling questions on force diagrams, resultant forces, and motion.
Key terms and definitions
Contact force — a force that acts only when objects are physically touching each other
Non-contact force — a force that acts at a distance without physical contact between objects
Friction — a contact force that opposes motion between two surfaces sliding or trying to slide past each other
Normal contact force — a support force acting perpendicular to a surface when an object rests on it
Gravitational force — a non-contact force of attraction between all masses, most noticeably between objects and the Earth
Electrostatic force — a non-contact force between electrically charged objects, which can be attractive or repulsive
Magnetic force — a non-contact force between magnetic poles or between a magnet and a magnetic material
Tension — a contact force transmitted through a string, rope, cable or wire when pulled tight
Core concepts
Understanding contact forces
Contact forces only exist when two objects are touching. The force is transmitted through the physical contact between the surfaces or materials.
Examples of contact forces:
- Friction — occurs when surfaces move or try to move past each other. It always acts in the opposite direction to motion or attempted motion. Air resistance and drag are types of friction
- Normal contact force — the support force from a surface. When you sit on a chair, it pushes up on you with a normal contact force equal to your weight
- Tension — exists in stretched strings, ropes or cables. When you pull on a rope, tension acts along its length
- Applied force — a push or pull directly applied by one object on another, such as pushing a shopping trolley
- Compression — a squeezing force when objects are pushed together
Friction deserves special attention as it appears frequently in GCSE questions. The magnitude of friction depends on:
- The nature of the surfaces in contact (rough surfaces produce more friction)
- How hard the surfaces are pressed together (the normal contact force)
Friction can be useful (car brakes, walking without slipping) or problematic (energy waste in machinery, increased fuel consumption).
Understanding non-contact forces
Non-contact forces act between separated objects without any physical touching. They are sometimes called "field forces" because they act through force fields.
Gravitational force:
All masses attract each other through gravity. On Earth, we experience gravity as our weight — the force pulling us toward the planet's centre.
Key points about gravitational force:
- Acts between all masses, but only noticeable when at least one mass is very large (like a planet)
- Always attractive, never repulsive
- Weight = mass × gravitational field strength (W = mg)
- On Earth, g = 9.8 N/kg (or approximately 10 N/kg for calculations)
- Weight varies with location (less on the Moon, more on Jupiter) but mass stays constant
Electrostatic force:
This force acts between electrically charged objects. Like charges (both positive or both negative) repel each other, while opposite charges attract.
Examples include:
- A charged balloon sticking to a wall
- Lightning formation in thunderclouds
- Static shocks when touching metal after walking on carpet
The electrostatic force becomes stronger when:
- The charges are larger
- The objects are closer together
Magnetic force:
This acts between magnets or between magnets and magnetic materials (iron, steel, cobalt, nickel).
Key characteristics:
- Like poles repel (north-north or south-south)
- Unlike poles attract (north-south)
- The force gets weaker with increasing distance
- Acts through non-magnetic materials (magnetism works through paper, plastic, etc.)
Force fields and interactions
All non-contact forces act through force fields — regions of space where an object experiences a force.
The three types of fields you need to know:
- Gravitational fields surround all masses
- Electric fields surround all charged objects
- Magnetic fields surround all magnets and current-carrying wires
Field strength decreases with distance from the source. This explains why:
- You weigh slightly less at the top of a mountain (further from Earth's centre)
- Magnets pick up paper clips when close but not from across the room
- Charged objects only attract nearby pieces of paper
Interaction pairs
According to Newton's third law, forces always come in pairs. When object A exerts a force on object B, object B exerts an equal and opposite force on object A.
This applies to both contact and non-contact forces:
Contact force example:
- When you push a wall (you exert a force on the wall), the wall pushes back on you with an equal force
- These are two separate forces acting on two different objects
Non-contact force example:
- The Earth pulls you down with gravitational force (your weight)
- You pull the Earth up with an equal gravitational force
- The Earth doesn't accelerate upward noticeably because its mass is enormous
Common misconception: interaction pairs do NOT cancel out because they act on different objects. Forces only cancel when they act on the same object.
Identifying forces in real situations
For any GCSE question, you should be able to identify all forces acting on an object and classify them as contact or non-contact.
Example: A book resting on a table
Forces acting on the book:
- Weight (gravitational force) — non-contact, acts downward
- Normal contact force from table — contact, acts upward
These forces are balanced (equal in size, opposite in direction), so the book remains stationary.
Example: A car accelerating along a road
Forces acting on the car:
- Weight (gravitational) — non-contact, acts downward
- Normal contact force from road — contact, acts upward
- Driving force from engine — contact (through tyres), acts forward
- Friction and air resistance — contact, act backward
The driving force exceeds the resistive forces, creating a resultant force forward and causing acceleration.
Representing forces
Forces should be represented as arrows in diagrams:
- Arrow direction shows force direction
- Arrow length represents force magnitude (longer = larger force)
- Label each arrow clearly
For free body diagrams (showing only the forces on one object):
- Draw the object as a simple shape (box or dot)
- Draw force arrows starting from the object
- Do not include forces the object exerts on other things
Worked examples
Example 1: Classifying forces
Question: A skydiver falls through the air before opening their parachute. Identify all the forces acting on the skydiver and state whether each is a contact or non-contact force. [3 marks]
Answer:
- Weight / gravitational force — non-contact force [1 mark]
- Air resistance / drag / friction — contact force [1 mark]
(Award 1 mark for correctly identifying both forces as contact/non-contact)
Examiner tip: Always give the proper physics name for forces. "Gravity pulling down" is less precise than "weight" or "gravitational force."
Example 2: Calculating weight
Question: An astronaut has a mass of 80 kg.
(a) Calculate the astronaut's weight on Earth where g = 9.8 N/kg. [2 marks]
(b) The astronaut travels to Mars where g = 3.7 N/kg. Calculate their weight on Mars. [2 marks]
(c) Explain why the astronaut's weight changes but their mass does not. [2 marks]
Answer:
(a) W = mg [1 mark] W = 80 × 9.8 = 784 N [1 mark]
(b) W = mg [1 mark] W = 80 × 3.7 = 296 N [1 mark]
(c) Mass is the amount of matter in the astronaut, which doesn't change with location [1 mark]. Weight is the gravitational force acting on the mass, which depends on the gravitational field strength, which is different on Mars and Earth [1 mark].
Examiner tip: Always show the formula first, then substitute values, then calculate. This ensures method marks even if your arithmetic is wrong.
Example 3: Friction
Question: A student drags a wooden block across a bench at constant speed. The student pulls with a force of 4.5 N.
(a) State the size of the friction force. [1 mark]
(b) Explain your answer. [2 marks]
(c) The student repeats the experiment with the same block on a rougher surface. They must pull with a force of 7.2 N to maintain constant speed. Explain why a larger force is needed. [2 marks]
Answer:
(a) 4.5 N [1 mark]
(b) At constant speed, the block is not accelerating, so the resultant force is zero [1 mark]. Therefore friction must equal the pulling force [1 mark].
(c) The rougher surface produces more friction / greater friction force [1 mark]. A larger pulling force is needed to balance this increased friction and maintain constant speed / zero resultant force [1 mark].
Common mistakes and how to avoid them
Confusing mass and weight — Mass is measured in kg and never changes. Weight is measured in N and depends on gravitational field strength. Don't say "my weight is 60 kg."
Forgetting that air resistance is a contact force — Students often classify air resistance as non-contact because you can't "see" the contact. Air molecules do touch the object, so it's a contact force.
Thinking interaction pairs cancel out — Newton's third law pairs act on different objects, so they cannot cancel. Only forces on the same object can produce a zero resultant force.
Mislabeling forces — Avoid vague terms like "movement force" or "gravity." Use precise terminology: applied force, driving force, weight, gravitational force.
Forgetting units — Weight must be in newtons (N), mass in kilograms (kg), and gravitational field strength in N/kg. Always include units in calculations.
Drawing force arrows incorrectly — Force arrows must start from the object experiencing the force, not from the object causing it. The arrow for weight starts at the object and points toward Earth's centre.
Exam technique for "Contact and non-contact forces"
Command words matter: "State" needs a simple answer (1 mark). "Explain" requires because/therefore reasoning linking cause and effect (typically 2+ marks). "Describe" needs more detail about what happens.
Force diagram questions: Use a ruler for straight arrows. Make arrow lengths proportional to force sizes when asked to draw to scale. Always label every arrow.
Calculation questions: Write the formula (W = mg), substitute numbers with units, then calculate. This structured approach secures method marks even with arithmetic errors.
Extended response questions: When explaining forces in situations, identify all forces first, classify them as balanced/unbalanced, then link to the motion (acceleration, constant velocity, or stationary).
Quick revision summary
Forces are pushes or pulls measured in newtons. Contact forces (friction, tension, normal contact force, applied force) require objects to touch. Non-contact forces (gravitational, electrostatic, magnetic) act at a distance through fields. Weight equals mass times gravitational field strength (W = mg). All non-contact forces get weaker with increasing distance. Forces always come in interaction pairs acting on different objects. Balanced forces produce no acceleration; unbalanced forces cause acceleration in the direction of the resultant force.