Contact and non-contact forces — AQA GCSE Physics
Test yourself on Contact and non-contact forces with AQA GCSE practice questions.
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Contact and non-contact forces explained
A force is a push or a pull on an object, and it always arises from an interaction with another object.
Read the full explanation
Nothing has a force by itself; there is always a second object doing the pushing or pulling. Forces split into two families. Contact forces act only while the two objects are physically touching, so friction, air resistance, tension, upthrust and the normal contact force all belong here. Non-contact forces act through a field while the objects are physically separated, which covers gravitational, electrostatic and magnetic forces. A brick resting on the bottom of a pool shows the idea: its weight acts downwards with no contact, while upthrust from the water and the normal contact force from the floor both act upwards through contact.
Examples of contact forces include friction, air resistance, tension and normal contact force.
Contact forces need the two objects to be touching. Friction acts along the surfaces where two objects slide or try to slide past each other, always opposing the relative motion. Air resistance, and water resistance, are the friction of a fluid on an object moving through it, and grow larger as the object speeds up. Tension is the pull along a stretched rope, cable or spring, pulling inwards on whatever is attached at each end. The normal contact force is the push of a surface on an object resting against it, acting at right angles to that surface. Upthrust, the upward push of a fluid on an immersed object, is also a contact force and is accepted in answers.
Examples of non-contact forces are gravitational force, electrostatic force and magnetic force.
Non-contact forces act between objects that are physically separated, through a field that surrounds each object. Gravitational force acts between any two masses, is always attractive, and points towards the centre of the other mass; it is what holds a satellite in orbit and gives every object its weight. Electrostatic force acts between charged objects, attracting unlike charges and repelling like charges, which is why two negatively charged rods push each other apart. Magnetic force acts between magnetic poles and between a magnet and a magnetic material, with unlike poles attracting and like poles repelling. In each case the force gets weaker as the separation increases.
Force is a vector quantity.
Force is a vector, so it has a magnitude in newtons and a direction, and both are needed for a complete answer. On a diagram it is shown by an arrow whose length represents the size and whose head shows the direction. The vector nature controls how forces combine: along a straight line, forces pointing the same way add and forces pointing opposite ways subtract. A 400 N driving force with 250 N of drag gives a resultant of 150 N forwards, not 650 N. Equal and opposite forces give a resultant of zero. Choose a positive direction first, state it, and treat forces the other way as negative; then the sign of the answer tells you the direction of the resultant.
Students should be able to describe the interaction between pairs of objects which produce a force on each object. The forces to be represented as vectors.
Forces always come in interaction pairs. When two objects interact, each exerts a force on the other: the forces are equal in magnitude, opposite in direction, of the same type, and crucially they act on different objects. The Earth pulls a satellite towards its centre and the satellite pulls the Earth with an equal force the other way. A charged rod held near an earthed conductor attracts it while being attracted itself. A hammer pushing a nail is pushed back by the nail. Drawn as vectors, the pair is two arrows of equal length pointing in opposite directions, each drawn on the object it acts on, which is why they never cancel each other out on any single object.
Your focus
- Name the second object responsible for a given force, writing it in the form 'the force of A on B'.
- Sort a list of named forces into contact and non-contact, giving the test used.
- Identify all the forces acting on an object in a described situation and classify each one.
Show all 15 objectives
- Name three contact forces other than friction and air resistance.
- Identify all the contact forces acting on an object resting on the bottom of a pool.
- Describe the direction in which the normal contact force and friction each act on a sliding object.
- Name the three non-contact forces and give a situation in which each acts.
- Predict whether two charged objects or two magnetic poles will attract or repel, and justify it.
- State how the size of a non-contact force changes as the separation of the objects increases.
- Justify the classification of force as a vector rather than a scalar.
- Combine two forces acting along the same line, stating the direction of the result.
- Draw two forces to scale on a diagram so that their relative sizes are clear.
- Decide whether two named forces form an interaction pair, applying the tests that they act on different objects and are of the same type.
- Draw the two forces of an interaction pair as labelled vectors on the correct objects.
- Explain why the two forces of an interaction pair do not cancel each other out.
Contact and non-contact forces exam tips
Marking Points
- one mark for a force being a push or a pull on an object due to its interaction with another object
- one mark for contact forces acting only while the objects are physically touching
- one mark for non-contact forces acting while the objects are physically separated
- one mark for correctly classifying a named force, such as normal contact force as a contact force and weight as non-contact
- one mark for a named contact force other than those already given in the question, such as tension, normal contact force or upthrust
- one mark for the two objects being physically in contact while the force acts
- one mark for using the full name, normal contact force or normal reaction force, rather than 'reaction' alone
- one mark for identifying every contact force acting in a described situation, such as upthrust and normal contact force on a brick lying on the bottom of a pool
- one mark for naming gravitational, electrostatic or magnetic force
- one mark for the objects not being in contact, the force acting through a field
- one mark for a correct direction, such as the gravitational force on a satellite acting towards the centre of the Earth
- one mark for like charges or like poles repelling while unlike ones attract
- one mark for a force having both a magnitude in newtons and a direction
- one mark for representing a force by an arrow whose length shows the size
- one mark for adding forces acting in the same direction and subtracting those acting in opposite directions
- one mark for giving the direction of the resultant as well as its magnitude
- one mark for naming both interacting objects, for example the Earth and the satellite
- one mark for each object exerting a force on the other, equal in magnitude and opposite in direction
- one mark for the two forces acting on different objects, so they never cancel on one object
- one mark for two arrows of equal length drawn in opposite directions, each on the object it acts on
Examiner Tips
- 💡Name both objects when describing a force, for example 'the pull of the Earth on the brick'.
- 💡Ask whether the two objects touch; that single test decides the category every time.
- 💡In a diagram question, list every force acting before you start drawing, so none is missed.
- 💡Check which forces the question has already named, because repeating one of them scores nothing.
- 💡Lift, thrust and water resistance are all accepted, so you have more than three safe answers available.
- 💡Say 'normal contact force' in full; it takes two extra words and secures the mark.
- 💡If a question about charged rods asks for a reason, say they carry the same charge; the mark for the reason usually depends on the first answer being right.
- 💡Name the specific non-contact force, not just 'a field force'.
- 💡Give the direction towards the centre of the Earth for gravitational forces on diagrams of orbits.
- 💡Write down which direction you are treating as positive before you start adding.
- 💡Give the direction in words as well as by sign, so there is no doubt what a negative answer means.
- 💡Match arrow lengths to force sizes on any diagram you draw, even a rough one.
- 💡Write each force as 'the force of A on B', which makes it obvious the pair acts on different objects.
- 💡Check that both forces in your pair are the same type, both gravitational or both contact.
- 💡Draw the two arrows separately, one on each object, rather than head to head in the gap between them.
Common Mistakes
- saying that a moving object 'has force' or 'carries force'
- classifying friction or air resistance as non-contact
- calling upthrust a non-contact force because the water is not solid
- treating gravity as needing contact with the ground
- naming a force without naming the second object involved in the interaction
- answering 'drag', which examiners ignore because it repeats air resistance already given
- offering gravity or weight, which are non-contact forces
- writing 'reaction' on its own instead of normal contact force
- describing friction as a force that acts in the direction of motion
- leaving upthrust out when listing the forces on a submerged object
- naming friction, air resistance or upthrust as non-contact forces
- claiming a magnet must touch an iron object before it exerts a force
- writing that gravity pushes objects down rather than pulling them towards the centre of the Earth
- assuming electrostatic forces are only attractive
- saying the force disappears entirely at a distance rather than becoming weaker
- adding a 5 N force to the left and a 3 N force to the right to get 8 N
- giving the size of a resultant with no direction
- arguing that force is a scalar because newtons are just numbers
- changing the positive direction part-way through a calculation
- drawing all force arrows the same length regardless of size
- drawing both arrows of the pair on the same object
- saying the two forces cancel out so nothing ever moves
- naming the weight of a resting book and the normal contact force on it as an interaction pair, when both act on the book
- drawing the two arrows with different lengths
- describing only the force on one object and never naming the partner force