IB Physics HLTopic 1 — Motion, Forces & EnergyPaper 1 & 2Forces through a field~7 min read
Forces at a Distance
Contact forces need objects to be touching. But some forces reach right across empty space with nothing in between: the Earth holds the Moon in orbit from 380,000 km away, and a magnet tugs a paperclip before they ever meet. These are non-contact forces — they act at a distance through a field. There are three you need to know, and gravity is the one behind an object’s weight.
📘 What you need to know
A non-contact force acts at a distance, without the objects touching, through the action of a field
The gravitational force (Fg) is the attraction between any two objects with mass
The electrostatic force (Fe) acts between charged objects — it can attract or repel
The magnetic force (Fm) acts between magnetic poles — it can attract or repel
Gravity is always attractive; the electrostatic and magnetic forces can go either way
Weight is the gravitational force on an object, found from Fg = mg
What counts as a non-contact force
The definition:
Non-contact force
A force that acts at a distance, without physical contact, through the action of a field
The key idea is the field. Every mass creates a gravitational field around it, every charge an electric field, every magnet a magnetic field. Another object placed in that field feels a force — even across a vacuum, with nothing touching. There are three of these forces on the syllabus.
The three non-contact forces. Gravity always pulls masses together; the electrostatic and magnetic forces can attract or repel depending on the charges or poles involved.
Gravitational force
The gravitational force (Fg) is the attractive force between any two objects that have mass, felt through a gravitational field. It’s what holds a planet and a comet, or the Earth and the Moon, in their orbits. Gravity is always attractive — there’s no such thing as gravitational repulsion.
On Earth, the gravitational force acting on an object is what we call its weight. It’s found from:
WeightFg = mg
where m is the mass in kg and g is the gravitational field strength (about 9.81 N kg−1 on Earth). Weight is a force, measured in newtons — not the same thing as mass.
Mass and weight get muddled all the time. Mass is how much matter something contains — it never changes, whether you’re on Earth, the Moon, or floating in space. Weight is the gravitational force on that mass, so it depends on where you are: the same object weighs about six times less on the Moon, because the Moon’s g is roughly six times smaller. Same mass, different weight.
Electrostatic force
The electrostatic force (Fe) acts between charged objects sitting in an electric field. Unlike gravity, it can go both ways: like charges repel (two positives push apart) while opposite charges attract (a positive and a negative pull together). The attraction between a proton and an electron in an atom is an everyday example.
Magnetic force
The magnetic force (Fm) acts between magnetic poles in a magnetic field, and it can also attract or repel. Like poles repel (north pushes north away) while opposite poles attract (north pulls toward south). It’s the force you feel resisting you when you try to push two bar magnets together the wrong way round.
Gravitational always attracts
Electrostatic attract or repel
Magnetic attract or repel
WE 1
An object has a mass of 12 kg. Calculate its weight on Earth, where g = 9.81 N kg−1, and on the Moon, where g = 1.62 N kg−1. What does this tell you about the object’s mass?
Step 1 — weight on Earth using Fg = mgFg = 12 × 9.81 = 118 NStep 2 — weight on the MoonFg = 12 × 1.62 = 19.4 NEarth: 118 N | Moon: 19.4 NThe weight changes because g changes — but the mass stays 12 kg everywhere. Mass is fixed; weight depends on the field.
💡 Top tips
Gravity only attracts. If a question hints at gravitational repulsion, something’s wrong — masses always pull together.
Weight is a force (in newtons), found from Fg = mg. Mass (in kg) is separate and never changes.
Like repels, opposite attracts — for both charges and magnetic poles.
No touching required. Non-contact forces act through a field, so they still work across a vacuum.
Quick recap: non-contact forces act at a distance through a field. There are three — gravitational, electrostatic, and magnetic. Gravity is always attractive; the electric and magnetic forces attract or repel. An object’s weight is just the gravitational force on it, Fg = mg, and it changes with g while the mass stays fixed.
⚠ Common mistakes
Confusing mass and weight — mass is fixed and in kg; weight is a force in N and depends on g
Thinking gravity can repel — the gravitational force is always attractive
Saying like charges or like poles attract — like repels, opposite attracts
Assuming a force needs contact — these three act through a field across empty space
Using the wrong g — remember it’s smaller on the Moon and other planets
That’s the full cast of forces — contact and non-contact. From here we start looking at particular forces in depth and putting real numbers on them. Next up is Frictional Forces: how friction actually arises, the difference between static and dynamic friction, and the equations that predict it.
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