Forces and motion · GCSE Physics
Contact and non-contact forces
Teacher-written GCSE Physics revision on contact and non-contact forces: friction, tension, normal contact, gravity, electrostatic and magnetic forces, free-body diagrams and resultant force.
Contact forces need touching. Non-contact forces act at a distance. A resultant force changes speed or direction; zero resultant means stationary or steady velocity.
The important bits
What you need to know
- 1
A force is a vector: it has magnitude and direction. The unit is the newton (N). Mass, energy, time and speed are scalars.
- 2
Contact forces need the objects to touch: friction, air resistance (drag), tension, normal contact (reaction), upthrust and compression.
- 3
Non-contact forces act at a distance: gravitational, electrostatic and magnetic. Gravity acts on masses; electrostatic on charges; magnetic on magnetic materials or currents.
- 4
A free-body diagram shows all the forces on one object as arrows from a point or from the outline of that object. Do not draw the forces that object exerts on something else.
- 5
The resultant force is the vector sum. If forces are in a straight line, subtract opposites. A non-zero resultant changes speed or direction (or both).
- 6
Equilibrium means resultant force is zero (and, when moments are in play, resultant moment is zero). The object is stationary or moving at constant velocity.
- 7
Friction and drag usually oppose relative motion. Driving force from an engine is a contact force via the road; it can balance drag at a steady speed.
- 8
Resolve only when the question needs a component. At GCSE, many problems stay on a line: forwards minus backwards, or up minus down.
Quotations worth analysing
Short evidence. Real method.
“A force is a push or a pull that acts on an object due to the interaction with another object.”
Every force is an interaction. If you draw a force on a free-body diagram, you should be able to name the other object that exerts it.
“Contact forces need the objects to be physically touching; non-contact forces act at a distance.”
Gravity still acts on an apple in the air. Friction does not, until surfaces meet. That classification is a common one-mark starter.
Go deeper
Free-body diagrams are the whole forces topic in one sketch
Pick one object. Draw it. Draw every force that acts on it, not the forces it applies to the floor or the rope. A book on a table: weight down (Earth on book), normal contact up (table on book). If those arrows are equal, resultant is zero and the book stays put. A skydiver: weight down, drag up. If drag is smaller, there is a downward resultant and they accelerate. If the arrows are equal, they are at terminal velocity. Students add a “force of motion” forwards as if movement needed a dedicated arrow. It does not. Motion is what you conclude after you have found the resultant. Label each arrow with the type of force and a rough size. Equal lengths mean equal magnitudes. That picture earns method marks before any calculation.
Go deeper
Resultant force is a subtraction, then a sentence about motion
Two forces, 12 N right and 5 N left, give a resultant of 7 N right. Then say what that does: the object accelerates to the right if it is free to move. If the question also gives mass, F = ma is the next line, with F as 7 N, not 12 N. Using the driving force alone is the classic error. Vectors at right angles need Pythagoras for the magnitude and a sketch for the direction; Combined papers often keep forces on one line. Non-contact examples belong in the same diagrams: weight is always there near Earth; electrostatic and magnetic arrows appear when charges or magnets are in the story. Name the type in the first three words of the explain sentence: “The non-contact gravitational force…”
See the idea in action
A 8.0 N driving force and 3.0 N of drag act on a 2.5 kg trolley, plus 25 N weight down and 25 N normal contact up. Vertical resultant = 0, so no vertical acceleration. Horizontal resultant = 8.0 − 3.0 = 5.0 N forwards. The trolley accelerates. (If mass is used: a = F/m = 5.0 / 2.5 = 2.0 m/s² forwards.) The weight and the normal contact are a contact / non-contact pair that cancel on this object; they are not a Newton-third-law pair with each other. Driving force and drag are both contact forces.
Exam technique
Turn knowledge into marks
Classify each force as contact or non-contact, draw it on one object, then find the resultant before you talk about motion. Equal opposite arrows mean equilibrium, not “no forces”.
Common mistakes
Do not give these marks away
- 01
Drawing both sides of a Newton-third-law pair on the same free-body diagram.
- 02
Inventing a “force of motion” or saying a moving object must have a forwards force even at constant velocity.
- 03
Calling weight a contact force, or calling friction a non-contact force.
Which of these is a non-contact force?
ATension in a rope
BFriction on a brake pad
CGravitational attraction on a satellite
DNormal contact from a desk
Show the answer
Gravitational attraction on a satellite. Gravity acts at a distance. Tension, friction and normal contact all need touching. A satellite still has weight even in orbit.
Quick questions
If this is the bit you searched
What is the difference between a contact and a non-contact force?
Contact forces need physical touching (friction, tension, normal contact, drag). Non-contact forces act at a distance (gravity, electrostatic, magnetic).
What is a resultant force?
The single force that has the same effect as all the forces acting together: the vector sum. Zero resultant means constant velocity (including rest).
What should a free-body diagram include?
Only the forces on one chosen object, drawn as labelled arrows. Do not include forces that object exerts on other things.
Is air resistance a contact force?
Yes. Air particles collide with the moving object. It is a contact force that usually increases with speed.