AQA GCSE Combined Science (8464), Higher tier · Physics › Forces › Forces and their interactions
Practise Contact and non-contact forces. 11 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
A force is a push or a pull that acts on an object because of its interaction with another object. You need to sort forces into contact and non-contact forces, name the forces acting in a situation, and describe how interacting objects exert forces on each other.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Define a force as a push or pullA force acts on an object because of its interaction with another object, and is measured in newtons (N).
3
Name examples of contact forcesFriction, air resistance, tension and normal contact force act when objects are touching.
4
Name examples of non-contact forcesGravitational, electrostatic and magnetic forces act even when the objects are physically separated.
5
Identify the forces acting in a situationFor example, a book on a table has its weight acting down and a normal contact force acting up.
5
Show forces as vector arrowsDraw each arrow from the object, in the direction of the force, with its length showing the size.
6
Describe the forces between interacting objectsEach object exerts a force on the other; the two forces are equal in size and opposite in direction.
Notes
What a force is
A force is a push or a pull that acts on an object due to its interaction with another object.
Force is measured in newtons (N). It is a vector quantity: it has size and direction.
Forces can change an object's speed, its direction of motion or its shape.
Contact forces: the objects are touching
Friction: opposes motion when two surfaces slide, or try to slide, over each other.
Air resistance (drag): opposes the motion of an object moving through air. Water also exerts drag.
Tension: the pulling force in a stretched rope, string, cable or spring.
Normal contact force: the push of a surface on an object resting on or against it, acting at right angles (normal) to the surface.
Upthrust, the upward push of a liquid or gas on an object in it, is also a contact force.
Non-contact forces: the objects are physically separated
Gravitational force: the attraction between masses, e.g. the Earth pulling you down (your weight).
Electrostatic force: the force between electric charges. Like charges repel; unlike charges attract.
Magnetic force: the force between two magnets, or between a magnet and a magnetic material such as iron.
Forces come in pairs
When two objects interact, each object exerts a force on the other.
The two forces are equal in size and opposite in direction, and each acts on a different object.
Example: the Earth pulls you down with a force equal to your weight, and you pull the Earth up with a force of the same size.
Cheatsheet
Force = push or pull due to an interaction between objects; unit newton (N)
Force is a vector
Contact forces: friction, air resistance (drag), tension, normal contact force, upthrust
Non-contact forces: gravitational, electrostatic, magnetic
Normal contact force acts at 90° to the surface
Friction and air resistance oppose motion
Interacting objects: equal and opposite forces, acting on different objects
How to answer each type of question
Classify forces as contact or non-contact
1 to 2 marks3
Ask whether the objects must touch for the force to act.
Touching: contact force. Not touching: non-contact force (gravitational, electrostatic or magnetic).
Example. (a) Which of these is a non-contact force? air resistance / friction / magnetic force / tension (b) Give one other example of a non-contact force.
Show the model answer
(a) magnetic force (1) (b) gravitational force (weight) or electrostatic force (1)
Name the forces acting on an object
2 to 4 marks5
Start with weight (down), then look for anything touching the object.
A surface gives a normal contact force; a rope gives tension; sliding gives friction; moving through air gives air resistance.
Give the direction of each force if asked.
Example. A crate is pulled at a steady speed across a rough, level floor by a horizontal rope. Name the four forces acting on the crate and give the direction of each.
Show the model answer
weight, downwards (1) normal contact force from the floor, upwards (1) tension in the rope, forwards (1) friction, backwards (opposite to the motion) (1)
Describe the interaction between two objects
2 to 3 marks6
Say that each object exerts a force on the other.
Say that the forces are equal in size.
Say that they act in opposite directions, and name the type of force.
Example. Two balloons are given the same type of charge. They are hung from threads close together and move apart. Describe the forces between the balloons.
Show the model answer
Each balloon exerts an electrostatic (non-contact) force on the other balloon (1). The two forces are equal in size (1). They act in opposite directions, pushing the balloons apart (repulsion) (1).
Shortcuts and memory tricks
Non-contact forces are GEM: Gravitational, Electrostatic, Magnetic.
Contact test: would the force stop if you moved the objects slightly apart? If yes, it is a contact force.
'Normal' means 'at right angles': the normal contact force is always perpendicular to the surface.
Where marks are lost
Calling weight a contact force. Weight is caused by gravitational attraction, a non-contact force.
Labelling a force arrow 'gravity'. Write 'weight' or 'gravitational force'.
Saying the two forces in an interacting pair act on the same object. They act on different objects.
Drawing friction in the direction of motion. Friction acts opposite to the motion (or attempted motion).
Exam technique
Use the exact names: friction, air resistance, tension, normal contact force, weight, electrostatic force, magnetic force.
On force diagrams, draw each arrow starting on the object, pointing the way the force acts, with lengths that show the relative sizes.
In 'describe the interaction' questions, say that each object exerts a force on the other and that the forces are equal and opposite.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Name two other non-contact forces.
Gravitational force (weight) and electrostatic force.
Name one contact force and one non-contact force that act on the kite-surfer.
Contact: tension in the lines. Non-contact: weight (gravitational force).
State whether friction is a contact force or a non-contact force.
Contact.
A maglev (magnetic levitation) train uses magnets to float just above its track, without touching it. Magnets in the track also drive the train forwards, again without any contact between the train and the track. Name the force that holds the train up.
Magnetic force.
A student rubs a plastic comb on their hair. The student then holds the comb close to, but not touching, small pieces of paper on a table. The pieces of paper jump upwards and stick to the comb. Name the non-contact force responsible for this effect.
Electrostatic force.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy6 marks
(a) Which of these forces is a non-contact force? Tick (✓) one box.[1]
Air resistance
Friction
Magnetic force
Tension
(b) Name two other non-contact forces.[2]
(c) Describe the difference between a contact force and a non-contact force.[2]
(d) Force is a vector quantity. What does this mean?[1]
Show the answer and mark scheme
(a)Answer: Magnetic force
(b)Answer: Gravitational force (weight) and electrostatic force.
gravitational force / weight / gravity
electrostatic force
(c)Answer: Contact forces act between objects that touch; non-contact forces act between objects that are separated.
contact force: the objects are (physically) touching
non-contact force: the objects are (physically) separated / not touching
(d)Answer: It has magnitude and direction.
force has magnitude (size) and direction
Question 2Medium6 marks
A kite-surfer stands on a board and is pulled across the sea by a large kite. The kite is attached to the kite-surfer by lines.
(a) Name one contact force and one non-contact force that act on the kite-surfer.[2]
(b) The weight of the kite-surfer is caused by an interaction between the kite-surfer and the Earth. Describe the force that the kite-surfer exerts on the Earth as a result of this interaction.[2]
(c) The kite lines exert a force of 450 N on the kite-surfer, pulling towards the kite. Give the size and direction of the force that the kite-surfer exerts on the kite lines.[2]
Show the answer and mark scheme
(a)Answer: Contact: tension in the lines. Non-contact: weight (gravitational force).
contact: tension (in the lines) / air resistance / drag / normal contact force (from the board) / friction (from the board)
non-contact: weight / gravitational force
(b)Answer: A gravitational force on the Earth, equal in size to the kite-surfer’s weight and directed upwards towards the kite-surfer.
a (non-contact) gravitational force on the Earth
equal in size to the kite-surfer’s weight and in the opposite direction / upwards (towards the kite-surfer)
(c)Answer: 450 N, directed away from the kite.
450 (N)
away from the kite / in the opposite direction (to the force on the kite-surfer)
Question 3Hard6 marks
A student rubs a plastic comb on their hair. The student then holds the comb close to, but not touching, small pieces of paper on a table. The pieces of paper jump upwards and stick to the comb.
(a) Name the non-contact force responsible for this effect.[1]
(b) Explain, in terms of forces, why the pieces of paper jump upwards towards the comb.[3]
(c) A student says: ‘The paper must be attracted because the comb actually touches it through the air.’ Explain why this statement is incorrect.[2]
Show the answer and mark scheme
(a)Answer: Electrostatic force.
electrostatic force
(b)Answer: The charged comb attracts the paper electrostatically; once this force exceeds the paper's weight, the resultant force is upwards and the paper accelerates up.
rubbing the comb on hair gives the comb an electric charge
the charged comb exerts an attractive electrostatic force on the paper
once this upward force becomes bigger than the weight of the paper, there is a resultant force upwards, so the paper accelerates upwards off the table
(c)Answer: The comb and paper are separated by air; a non-contact force acts across that gap without the objects touching.
the comb and the paper are not touching — they are separated by a gap of air
a non-contact force, by definition, can act across a gap without the objects touching, which is exactly what is happening here