Contact vs Non-Contact Forces
Contact forces act only when objects touch, such as friction, tension, the normal force and air resistance, while non-contact forces act across a distance through a field, such as gravity, electric force and magnetic force. In brief: contact forces ultimately arise from electromagnetic interactions between atoms at the surfaces. Non-contact forces are described with fields: a mass creates a gravitational field, a charge an electric field, and a magnet a magnetic field.
A hand pushes a shopping cart, but the Earth also pulls on the cart without touching it. Dividing forces into these two groups helps to identify all of the forces on an object. This guide describes both groups with examples, explains the field idea, compares them in a table, works through an example of listing forces and corrects common mistakes.
What are contact forces?
- Normal force: the push of a surface on an object resting on it, perpendicular to the surface.
- Friction: a force parallel to the surfaces that resists sliding.
- Tension: the pull transmitted along a rope, string or cable.
- Applied force: a push or pull by a person or machine.
- Air resistance and drag: the force of a fluid opposing the motion through it.
- Spring (elastic) force: the push or pull of a deformed spring or elastic material.
- Buoyancy and pressure forces of fluids on objects.
At the microscopic level, “touching” means that the electron clouds of atoms in two surfaces come close enough that they repel each other, so contact forces are electromagnetic in origin.
What are non-contact forces?
- Gravitational force: the attraction between masses. The weight of an object is the gravitational force of the Earth on it.
- Electric force: attraction between opposite charges and repulsion between like charges, as when a charged balloon sticks to a wall.
- Magnetic force: forces between magnets, and on moving charges and current-carrying wires.
- Nuclear forces: the strong force holds protons and neutrons together in a nucleus over very short ranges, and the weak force is involved in radioactive decay.
These forces are explained by fields: a region of space where a force acts on a suitable object. The Earth creates a gravitational field around it, and an object in the field feels a force. The strength of the field decreases with distance.
Contact and non-contact forces compared
| Feature | Contact forces | Non-contact forces |
|---|---|---|
| Need touching? | Yes | No |
| Explained by | Interactions of atoms at surfaces | Fields |
| Examples | Friction, tension, normal force, drag | Gravity, electric, magnetic |
| Range | Zero distance | Large distances (gravity, electromagnetism) |
Worked example: listing the forces
A 2.0 kg metal block rests on a wooden table, and a magnet is held above it, pulling it upward with 5.0 N. A string pulls the block sideways with 3.0 N, but the block does not move.
- Non-contact forces: weight W = mg = 2.0 × 9.8 = 19.6 N downward (gravity); magnetic force 5.0 N upward.
- Contact forces: tension 3.0 N sideways (string); normal force N upward from the table; static friction f sideways, opposing the string.
- Vertical balance: the block is at rest, so N + 5.0 = 19.6, giving N = 14.6 N.
- Horizontal balance: f = 3.0 N in the direction opposite to the tension.
Without the magnet, the normal force would be 19.6 N. The magnet reduces the pressing force of the block on the table.
How was action at a distance explained?
Newton found “action at a distance” by gravity puzzling, and said he would not make hypotheses about its cause. Michael Faraday introduced the idea of fields in the 1830s to 1840s to describe electric and magnetic forces, and James Clerk Maxwell developed the mathematical theory in the 1860s. Einstein’s general relativity (1915) describes gravity as the curvature of spacetime. In modern physics, forces are exchanged between particles by force carriers.
Which mistakes are common?
Mistake 1: Forgetting non-contact forces
Weight acts on every object near the Earth, even when nothing seems to touch it.
Mistake 2: Thinking that a force needs a medium
Gravity and magnetic forces also act in a vacuum.
Mistake 3: Listing “inertia” or “motion” as a force
Inertia is a property of matter. Only interactions are forces.
Mistake 4: Assuming that normal force always equals weight
It equals the weight only if no other vertical force acts and the object is not accelerating vertically.