Some forces don’t need the objects to touch at all — they act through a field. These non-contact forces come in three types: gravitational, electrostatic and magnetic. The most important one for this topic is gravity, because that’s where weight comes from.
📘 What you need to know
A non-contact force acts at a distance, without the objects touching, through the action of a field.
Gravitational force (Fg) — the attraction between any two masses; always attractive.
Electrostatic force (Fe) — the force between charged objects; can attract or repel.
Magnetic force (Fm) — the force between magnetic poles; can attract or repel.
Weight is a gravitational force: the pull of the Earth on an object, Fg = mg.
What “force at a distance” means
Definition
A non-contact force acts at a distance, without physical contact, through the action of a field.
Unlike contact forces, these don’t need two surfaces pressing together. Each object sits in a field created by the other, and the field is what transmits the force across the gap.
The three non-contact forces. Gravity only ever pulls; electrostatic and magnetic forces can pull or push.
The three forces at a distance
Gravitational force (Fg)
Any two objects with mass attract each other through a gravitational field — for example, a planet and a passing comet. Gravity is always attractive. On Earth, the gravitational pull of the planet on an object is what we call its weight.
Electrostatic force (Fe)
Charged objects feel a force in an electric field. The force is attractive between opposite charges (like a proton and an electron) and repulsive between like charges.
Magnetic force (Fm)
Magnetic poles feel a force in a magnetic field. Opposite poles attract (north–south) and like poles repel (north–north or south–south).
Weight: the gravitational force you meet most
Of the three, gravity is the one that turns up in nearly every mechanics question — as the object’s weight. Weight is the gravitational pull of the Earth on a mass, and it always points straight down, towards the centre of the Earth.
WeightFg = mg
Weight acts through the gravitational field, with no contact — the object doesn’t need to be touching the Earth to feel it.
Quick reference: gravitational Fg → always attractive • electrostatic Fe → attract or repel • magnetic Fm → attract or repel. Weight = mg, the Earth’s gravitational pull.
Worked examples
WE 1
Sorting a sledge’s forces
A child drags a sledge up a hill by a rope. Identify the contact and non-contact forces acting on the child and sledge.
Contact forcestension (rope), normal force (ground), friction (sledge–ground and shoes–ground), drag (air)Non-contact forceweight — the Earth’s gravitational pull
WE 2
Weight on the Moon
An astronaut has a mass of 80 kg. On the Moon the gravitational field strength is g = 1.6 N kg⁻¹. Find the astronaut’s weight there.
Use Fg = mgFg = 80 × 1.6weight = 128 Nmass stays 80 kg — only the weight changes
💡 Top tips
No contact needed. If a force acts across a gap, it’s gravitational, electrostatic or magnetic.
Gravity only pulls. There’s no such thing as a repulsive gravitational force at this level.
Mass vs weight. Mass (kg) is the same everywhere; weight (N) depends on the local g.
Symbols. Weight is often written W or mg; if a question gives a symbol, use it, otherwise use the syllabus symbol Fg.
⚠ Common mistakes
Treating weight as a contact force. It acts through a field, so it’s non-contact.
Saying mass changes on the Moon. Mass is constant — it’s the weight that’s smaller.
Thinking like charges or like poles attract. Like charges repel; like poles repel.
Forgetting electrostatic and magnetic forces can push. Unlike gravity, they can be repulsive.
Notice that the same sledge problem mixes contact and non-contact forces — most real situations do. Weight is the non-contact force you’ll add to almost every free-body diagram. Up next: Frictional Forces, a closer look at how friction and drag actually behave.
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