Identify common forces such as weight, normal contact force, friction, and tension, and draw free-body diagrams

What this dot point is asking

SEAB wants you to recognise the common forces that act on everyday objects, to know the newton as the unit of force, and to draw a free-body diagram that shows every force acting on one chosen object as a labelled arrow. The big idea is that forces are pushes or pulls, they are vectors, and a clear diagram is the first step in almost every dynamics question.

The answer

Force is a push or a pull

A force is a push or a pull that can change an object's speed, direction, or shape. Force is a vector, so it has size and direction, and its unit is the newton ($\text{N}$). One newton is the force that gives a $1\ \text{kg}$ mass an acceleration of $1\ \text{m s}^{-2}$.

Common forces at O-Level

Weight and the normal contact force

Weight is the pull of gravity, $W = mg$, always pointing down. When an object rests on a surface, the surface pushes back with a normal contact force perpendicular to the surface. On a flat table these two are vertical and, for a still object, equal in size.

Free-body diagrams

A free-body diagram shows just one object (often as a dot or a box) with an arrow for every force acting on it. The rules:

1. Choose one object only.
2. Draw an arrow for each force, starting from the object and pointing the way the force acts.
3. Label each arrow with the name of the force.
4. Make stronger forces longer if a scale is asked for.

Only forces acting on the object go on the diagram, never forces the object exerts on something else.

Examples in context

Example 1. A child on a slide. On a sloping slide the weight still points straight down, but the normal contact force points perpendicular to the slide's surface, and friction acts up the slope opposing the slide downward. Drawing these three correctly is the key to explaining why the child accelerates.

Example 2. A car cruising at steady speed. A car moving at constant speed has its forward driving force balanced by air resistance and friction, and its weight balanced by the normal contact force from the road. The free-body diagram has four arrows that cancel in pairs, which is exactly what constant velocity requires.

Try this

Q1. Name the force that always acts on an object because of gravity, and state its direction. [2 marks]

• Cue. Weight, acting vertically downward.

Q2. A box rests on a horizontal floor. Draw, in words, the free-body diagram of the box. [2 marks]

• Cue. The box with one arrow down (weight) and one arrow up (normal contact force from the floor), equal in size.

Q3. State what a free-body diagram should and should not show. [2 marks]

• Cue. It shows every force acting on the one chosen object; it does not show forces that object exerts on anything else.