Forces come from interactions between objects

1. A force is a push or a pull that one object exerts on another, and it is measured in newtons, $N$.
2. A force never appears on its own, because it is produced by an interaction, so every force has an object that exerts it and an object that experiences it.
3. A force has both a size and a direction, so it is drawn as an arrow whose length shows the size and whose point shows the direction.
4. Interactions fall into two groups: those that act by contact, where surfaces touch, and those that act at a distance, where the objects are separated by a gap.
5. Naming the correct force is the first step in almost every forces question, because the mark is reserved for the precise name rather than for the word force on its own.

Non-contact forces act through fields

1. In a non-contact interaction the two objects are separated by a gap, and the force is carried across that gap by a field.
2. Three non-contact interactions are needed at this level: gravitational, electrostatic and magnetic.
3. A gravitational force acts between any two objects that have mass.
4. A tennis ball falling from a window is inside the Earth's gravitational field, so the ball is pulled towards the centre of the Earth while the ball pulls the Earth upwards with a force of the same size.
5. Gravitational forces are always attractive, and they become weaker as the two masses move further apart.
6. An electrostatic force acts between objects that carry an electric charge.
7. A polythene rod rubbed with a dry cloth gains electrons and becomes negatively charged, so it sits in its own electrostatic field and attracts small pieces of paper without touching them.
8. Electrostatic forces can attract or repel, because like charges repel and unlike charges attract.
9. A magnetic force acts between two magnets, or between a magnet and a magnetic material such as iron, steel, nickel or cobalt.
10. A bar magnet held a few millimetres above a steel paperclip lifts it through the magnetic field in the gap, and the paperclip pulls the magnet downwards with a force of the same size.
11. Magnetic forces can also attract or repel, because like poles repel and unlike poles attract.
12. To describe any non-contact interaction, name the field, name the property that puts the object into that field, which is mass, charge or magnetism, and then give the direction of the force.

Contact forces act where surfaces touch

1. A contact force acts only while two surfaces are touching, and it disappears the instant they separate.
2. The normal contact force is the push that a surface gives to an object resting on it, and the word normal means at $90^{\circ }$ to that surface.
3. A textbook lying on a desk is pulled down by its weight and pushed up by the normal contact force from the desk, so two arrows are drawn on the book pointing in opposite directions.
4. On a slope, such as a ramp, the normal contact force is still perpendicular to the surface, so it is no longer vertical.
5. Friction acts along the surfaces, in the direction that opposes sliding or the tendency to slide.
6. Pushing a heavy box across a hall floor to the right produces a friction force on the box that acts to the left.
7. Friction acts even when nothing is moving, so a box that will not budge has a friction force equal in size and opposite in direction to your push.
8. Friction is useful when a tyre grips a wet road or a brake pad grips a disc, and unwanted when it heats a machine and wears its moving parts away.
9. Pushes and pulls from a hand, a rope or a spring are also contact forces, because they stop the moment contact is lost.

Interactions produce pairs of forces

1. Whenever two objects interact, each one exerts a force on the other, so forces are always produced in pairs.
2. The two forces in a pair are equal in size, opposite in direction, of the same type, and they act on two different objects.
3. When a swimmer pushes backwards on the wall of a pool, the wall pushes forwards on the swimmer with a force of the same size, and that forward force on the swimmer is what starts the swim.
4. The two forces in a pair never cancel each other out, because they act on different objects, so only one of them is ever drawn on the object being studied.
5. Balanced forces are a different idea, because those act on the same object and add to a resultant of zero.
6. Each force in a pair is represented as a vector arrow drawn on the object it acts on, starting at that object and pointing along the direction of the force.
7. Every arrow is labelled with the name of the force, and arrow lengths are kept in proportion so that a longer arrow means a larger force.