What Is a Force?
A force is always one object acting on another, and it never comes alone.
Lesson at a glanceShow the key points
- A force is an interaction: always exerted by one body on another. Ask who exerts it and on what.
- The SI unit of force is the newton (N), roughly the force needed to hold a 102 g mass near Earth's surface.
- Forces are vectors; several forces on a particle combine by vector addition into a resultant.
- Newton's third law: if A exerts F on B, then B exerts −F on A, along the line joining them.
- All forces in nature reduce to four kinds: gravitational, electromagnetic, nuclear (strong) and weak.
Push a door, hang a bag on a hook, feel a gust of wind on your face. Every one of these is a force, and every one of them involves two objects, not one.
Pushes and pulls
In everyday language a force is a push or a pull. Push a trolley and the force points away from you; pull a drawer and it points toward you. Hold a heavy suitcase and you exert a large force; hold an empty cup and the force is small.
You do not need muscles to exert a force. A strong wind leans on you. A rope holds a hanging lantern exactly as a hand could. A plastic ruler rubbed on dry hair lifts scraps of paper off a desk. A beach ball pushed under water shoots back up: the water is pushing it. Non-living things exert forces all the time.
A book lying on a table, for instance, feels an upward force exerted by the table on the book. Saying just “there is a force on the book” leaves out half the story.
Measuring force: the newton
The SI unit of force is the newton (N). To get a feel for it: holding a 102 g mass (a small apple) steady near the Earth’s surface takes about 1 N. A precise definition comes from Newton’s second law in the next chapter: 1 N is the force that gives 1 kg an acceleration of 1 m/s².
Force has both size and direction and adds by the triangle rule, so it is a vector. When several forces act on a particle we combine them with vector addition into a single resultant force, exactly as in the chapter on vectors.
Forces come in pairs
Look again at the examples above. The table pushes the book up, and the book presses down on the table. The rope holds the lantern up, and the lantern stretches the rope downward. Whenever body A exerts a force on body B, body B exerts a force on body A too. This observation is Newton’s third law:
The two forces form an action–reaction pair. Which one you call “action” is arbitrary. The crucial point, explored below, is that they act on different bodies.
Three small charged dust grains float near one another, far from anything else. If we count only the electric forces between them, how many forces act in total, and how many action–reaction pairs are there?
Only four kinds of force
Friction, tension, the push of a spring, the lift on a wing, the force that holds an atom together: the variety looks endless. Yet physicists have found that every force in nature is one of just four fundamental interactions:
- Gravitational: every mass attracts every other mass.
- Electromagnetic: forces between electric charges, which secretly account for almost every everyday push and pull.
- Nuclear (the strong force): holds protons and neutrons together inside nuclei.
- Weak: responsible for certain radioactive decays.
The rest of this chapter visits each of them in turn.
- 1A force is an interaction: always by one body on another. Name both.
- 2SI unit: the newton (N); 1 N ≈ the force needed to hold 102 g near Earth's surface.
- 3Forces are vectors and combine by vector addition.
- 4Third law: A on B is equal and opposite to B on A, and the two act on different bodies.
- 5Every force is gravitational, electromagnetic, nuclear or weak.
Check yourself
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