IB Physics HLTopic 1 — Motion, Forces & EnergyPaper 1 & 2Forces from touching~8 min read
Contact Forces
Some forces only exist when two objects are actually touching. A book presses on a table, a rope pulls on a load, the road grips a tyre, the air slows a falling ball. These are contact forces — and once you can name the four main ones and say which way each points, filling in a free-body diagram becomes second nature.
📘 What you need to know
A contact force acts between objects that are physically touching
Surface friction (Ff) opposes the motion of an object sliding over a surface
Viscous drag (Fd) is friction from a fluid — it opposes motion through a liquid or gas (air resistance is one type)
Tension (FT) acts within a rope or spring when it is pulled at both ends
Normal force (FN) is the support from a surface, acting perpendicular to it
The opposite is a non-contact force, which acts at a distance through a field (covered next)
What counts as a contact force
The definition is exactly what it sounds like:
Contact force
A force that acts between objects that are physically touching
If the two objects aren’t touching, it isn’t a contact force — that would be a non-contact force like gravity or magnetism, which reach across empty space. There are four contact forces you need to know, and each one has a fixed direction relative to what’s happening.
The four contact forces and the direction each one points. Friction and drag both oppose motion; tension pulls along a rope; the normal force pushes straight out of a surface.
Surface friction
Surface friction (Ff) is a force that opposes motion. It shows up whenever the surfaces of two solid objects rub against each other — a car’s tyres gripping the road, a box dragged across the floor, a sledge on snow. It always points against the direction the object is trying to move (or is moving).
Viscous drag
Viscous drag (Fd), also called fluid resistance, is a type of friction that acts when an object moves through a fluid — a liquid or a gas. Air resistance is the everyday example: it’s just drag from the air. Like surface friction, it always opposes the object’s motion, and it grows as the object speeds up.
Tension
Tension (FT) is the force that builds up inside a rope, string, cable, or spring when it’s pulled from both ends. Think of a mass hanging from a clamp on a spring: the spring is stretched, and the tension in it pulls the mass up while pulling the clamp down. Tension always acts along the rope, pulling away from the object at each end.
Normal force
The normal force (FN) is the support a surface gives to anything resting on it. “Normal” here means perpendicular — the force always points at 90° straight out of the surface. A book on a table feels an upward normal force from the table that stops it falling through; a block on a ramp feels a normal force perpendicular to the ramp’s slope, not straight up.
Watch the word “normal.” In everyday English it means “ordinary,” but in physics it means “perpendicular.” The normal force is perpendicular to the surface — so on flat ground it points straight up, but on a slope it tilts with the slope. It is not always vertical, and it is not always equal to the weight.
Contact forces in action
Here are three everyday situations, each showing a different contact force at work — friction and drag opposing motion, and the normal force holding an object up.
Friction and drag both act against the direction of motion. The normal reaction pushes up on the resting block, balancing its weight so it stays put.
WE 1
A book of mass 4.0 kg rests on a flat, horizontal table. Taking g = 9.81 m s−2, find the normal force the table exerts on the book.
Step 1 — the book is at rest, so forces balance
normal force up = weight down
Step 2 — find the weightFg = mg = 4.0 × 9.81 = 39.2 NStep 3 — the normal force matches itnormal force = 39.2 N upwardOn flat ground the normal force equals the weight — but only because the surface is horizontal. On a slope it would be smaller.
💡 Top tips
Friction and drag always oppose motion. Draw them pointing backward along the direction of travel, never forward.
“Normal” means perpendicular — to the surface, not to the ground. It tilts on a slope.
Air resistance is drag. The IB syllabus calls it fluid resistance and uses the drag symbol Fd.
Normal force isn’t always the weight. They’re only equal on flat ground with nothing else pushing vertically.
WE 2
A load of mass 6.0 kg hangs motionless from a single vertical rope. Identify the contact force in the rope and calculate its size. Take g = 9.81 m s−2.
Step 1 — name the contact force
the rope is pulled at both ends → tension
Step 2 — the load is at rest, so tension = weightFT = mg = 6.0 × 9.81 = 58.9 Ntension = 58.9 NThe tension acts up the rope on the load, and down the rope on the clamp — the same force felt at both ends.
Quick recap: contact forces act only between touching objects. Friction opposes sliding; drag opposes motion through a fluid; tension pulls along a rope; the normal force pushes perpendicular to a surface. “Normal” means perpendicular, and the normal force isn’t always equal to the weight.
⚠ Common mistakes
Drawing friction or drag along the direction of motion — they always oppose it
Assuming the normal force is always vertical — on a slope it’s perpendicular to the slope
Assuming the normal force always equals the weight — only true on flat ground with no other vertical forces
Treating air resistance as separate from drag — air resistance is fluid drag
Forgetting that tension is felt at both ends of a rope, pulling each object toward the other
Contact forces need touching — but some forces reach across empty space with nothing in between. Gravity holds the Moon in orbit; magnets attract without touching. Those are non-contact forces, and they’re next: the gravitational, electrostatic, and magnetic forces that act at a distance through a field.
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