A contact force only acts when two objects are physically touching. There are four you’ll meet again and again — friction, drag, tension and the normal force — and knowing the symbol and direction of each makes free-body diagrams much faster to draw.
📘 What you need to know
A contact force acts between objects that are physically touching.
Surface friction (Ff) — opposes sliding between two surfaces in contact.
Drag / fluid resistance (Fd) — opposes motion through a fluid (a liquid or gas); air resistance is a type of drag.
Tension (FT) — the force in a rope, string or spring being pulled at both ends.
Normal force (FN) — the support from a surface, acting perpendicular to that surface.
What “contact force” means
Definition
A contact force acts between objects that are physically touching.
If the two objects aren’t touching, any force between them is a non-contact force (like gravity or magnetism) — we cover those separately. The four contact forces below are the ones that show up on almost every mechanics diagram.
The four everyday contact forces, with the direction each one acts.
The four contact forces
Surface friction (Ff)
Friction acts between two solid surfaces that are touching and opposes the relative sliding between them — for example, a car’s tyres on the road. It always acts parallel to the surface, in the direction that resists motion.
Drag / fluid resistance (Fd)
Drag is a friction-type force that appears when an object moves through a fluid — a liquid or a gas. Air resistance is a type of drag. It always opposes the object’s motion through the fluid and grows with speed.
Tension (FT)
Tension is the force that builds up inside a rope, string, cable or spring when it is pulled at both ends — like a mass hanging from a spring fixed to a clamp. It acts along the rope, away from the object it’s attached to.
Normal force (FN)
The normal (reaction) force is the support a surface gives to an object resting on or pressing against it, such as a book on a table. It is the part of the contact force that acts perpendicular to the surface.
Quick reference: friction Ff → along surface, opposes sliding • drag Fd → opposes motion through a fluid • tension FT → along the rope, away from the object • normal FN → perpendicular to the surface.
Contact forces in action
Most real situations involve several contact forces at once. A block being pulled across a rough floor by a rope feels three contact forces — tension, friction and the normal force — plus its weight, which is a non-contact force.
Tension, friction and the normal force are contact forces; weight (Fg) acts at a distance, so it is non-contact.
Worked examples
WE 1
Naming the forces on a sledge
A child pulls a sledge across snow by a rope. Name the contact forces acting on the sledge and say what each one is.
Tension, FTthe rope pulls the sledge alongFriction, Ffbetween the sledge and the snow, opposing motionNormal force, FNthe snow pushes up on the sledgeDrag, Fdair resistance as it moves through the air
WE 2
Contact or non-contact?
Sort these forces into contact and non-contact: tension, weight, the magnetic force, the normal force, friction.
Contact (objects must touch)tension, normal force, frictionNon-contact (act at a distance)weight, magnetic force
💡 Top tips
If nothing is touching, it isn’t a contact force. Weight, magnetism and electrostatic forces act at a distance.
The normal force is perpendicular to the surface, so on a slope it tilts — it isn’t always vertical.
Friction and drag always oppose motion — draw them pointing against the velocity.
Tension points away from the object, along the rope or string.
Air resistance = fluid resistance = drag. The IB uses “fluid resistance” and the symbol Fd.
⚠ Common mistakes
Calling weight a contact force. Gravity acts without contact, so weight is non-contact.
Drawing the normal force straight up on a slope. It’s perpendicular to the surface, not to the ground.
Pointing friction the wrong way. It opposes the direction of (attempted) motion.
Forgetting drag exists. Anything moving through air or water feels it, even if the question doesn’t name it.
Up next: Non-Contact Forces — gravity, electrostatic and magnetic forces, which act through a field without the objects ever touching.
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