A force is a push or pull

1. A force does not appear on its own; it is caused by an interaction between objects.
2. For example, a boot pushes a football because the boot and ball interact while touching; the Earth pulls the ball because they interact gravitationally, even when they are not touching.
3. Forces can change an object’s speed, direction or shape; here the key skill is to identify which objects interact and what type of force results.
4. Force is measured in newtons ($N$).

Contact forces

1. Friction: a force between surfaces that opposes motion, or attempted motion, between them.
2. Air resistance: a force from the air on an object moving through it, acting against the motion.
3. Tension: a pulling force in a stretched rope, string or cable.
4. Normal contact force: a force from a surface on an object touching it, acting at right angles to the surface.
5. For example, a book resting on a desk feels a normal contact force from the desk, because the two are touching.

Non-contact forces

1. Gravitational force: the attractive force between masses, such as the Earth pulling an object downwards.
2. Electrostatic force: the force between electrically charged objects, which can attract or repel.
3. Magnetic force: the force between magnets, magnetic materials or current-carrying wires, which can attract or repel.
4. For example, a magnet attracts a steel paper clip before it touches it: this is a non-contact magnetic force.

Force is a vector quantity

1. Force is a vector because it has a size (in newtons, $N$) and a direction.
2. In diagrams, a force is drawn as an arrow that starts on the object the force acts on.
3. The arrow points in the direction of the force, and a longer arrow can represent a larger force.
4. Label each arrow with the type of force, or with the objects involved.
5. For example, for a falling ball the gravitational force acts downwards, so the arrow is drawn on the ball pointing down.

Interactions between pairs of objects

1. When two objects interact, the interaction produces a force on each object.
2. Name both objects and say which object each force acts on.
3. When a person pushes a wall, the person exerts a contact force on the wall, and the wall exerts a contact force on the person.
4. For non-contact forces the same idea applies: the Earth exerts a gravitational force on the ball, and the ball exerts a gravitational force on the Earth.
5. Represent each of these forces as a vector, using arrows to show their directions.

Describing an interaction clearly

1. Use clear object-pair language, such as “the desk exerts a normal contact force on the book”.
2. Avoid vague statements like “there is a push”; always say what is pushing what.
3. On a force diagram, put each arrow on the object experiencing the force, point it the correct way, and label it clearly.