Definition of force
Push a stalled car, pull open a stuck drawer, kick a football, or catch one thrown at you — in every one of these you are doing the same physical thing: applying a force. A force is nothing exotic; it is simply a push or a pull, and it is always the result of two objects acting on each other. This article pins down that definition precisely, shows why a force can never belong to a single object acting alone, introduces the newton as its SI unit and the spring balance as the instrument that measures it, and works through everyday scenes — a trolley, a resting book, a magnet and a nail — to train you to spot the two interacting objects behind every force you see.
A force is a push or a pull that arises only from the interaction of two objects, is measured in newtons (N) using a spring balance, and cannot exist with just one object alone.
The core idea
Force as a push or a pull. Whenever you lift a schoolbag, stretch a rubber band, shut a door, or throw a stone, you are applying a force to something. There is no deeper trick to the word — in everyday physics, force just means a push or a pull that one object exerts on another.
A force always needs two objects. This is the single most important idea in the whole topic: a force never appears out of nowhere. It is the result of an interaction between at least two objects — one object does the pushing or pulling, and a second object experiences it. A hand pushes a trolley; a magnet pulls an iron pin; the Earth pulls a falling mango toward itself. Notice that in every single case you can point to two separate things: the one applying the force and the one receiving it. Take either one away and there is nothing left to call a force.
Contact is not required. Many students assume a force needs touching, but that is not part of the definition. The magnet pulling the nail, or the Earth pulling the mango, are both forces even though nothing is in physical contact. What matters is the interaction between two objects, not whether they touch.
Measuring force. Force is measured with a spring balance, an instrument with a spring inside and a scale marked in newton (N), the SI unit of force. When you hang an object on the hook of a spring balance, the spring stretches by an amount that depends on how large the pull (the object's weight) is, and a pointer on the marked scale reads off the force. The smallest division marked on that scale is the instrument's least count, and reading it carefully — noting exactly where the pointer sits between two marked lines — is a basic practical skill you will use again with other measuring instruments.
What force does. Because a force is something you can feel and observe, its presence shows up through the effects it has: it can start something moving, stop it, speed it up, slow it down, change its direction, or change its shape. Those effects are the subject of the next article in this chapter — here the job is only to nail down what a force is and where it comes from.
Why it works
A force is defined as an interaction, so its very existence is tied to two objects — and this is not just a rule to memorise, it follows from how we notice forces at all. We never detect a force directly; we detect it through what it does — a change of motion or a change of shape in some object. But a change needs a cause, and a cause needs a source: something outside the object doing the pushing or pulling. If a single object were floating in an empty universe with nothing else present, there would be no second object to interact with it, so nothing could push or pull it, and it could neither exert nor experience any force. This is exactly why the CBSE definition insists on the phrase "interaction between two objects" rather than just "a push or a pull" — the interaction wording is what rules out a force belonging to one object alone. The same reasoning explains why contact is not a requirement. What makes something a force is the interaction — one object affecting another — not whether the two objects are touching. A magnet's pull on a nail across an air gap, or the Earth's pull on a falling object across empty space, still satisfies the definition perfectly: there are two objects, and one is pulling the other. Later chapters will sort forces into those that need contact and those that act at a distance, but at this stage the test for "is this a force" is always the same one: can I name the two objects involved?
Worked example
Question: For each situation below, state (a) whether a force is being applied, and (b) which two objects are interacting to produce it: (i) a girl pushing a shopping trolley, (ii) a book resting quietly on a table with nothing else touching it, (iii) a magnet pulling an iron nail toward it without touching it.
Step 1. Situation (i): the girl's hands are in direct contact with the trolley and she is exerting a push on it, so a force is being applied. The two interacting objects are the girl (her hands) and the trolley.
Step 2. Situation (ii): the book is simply resting; no new push or pull is being described here. The table does support the book — an interaction studied later as a contact force — but the situation as stated is not asking us to analyse that resting/support action, only to notice that nothing extra is happening.
Step 3. Situation (iii): the magnet is exerting a pull on the nail, drawing it closer, even though the two are not touching. The two interacting objects are the magnet and the iron nail, and this counts as a force because non-contact interactions are still pushes or pulls.
Answer: situations (i) and (iii) each involve a clear force with two named interacting objects — girl and trolley in (i), magnet and nail in (iii) — while (ii) describes no new applied force.
Competitive stretch
Question: A student claims, "A force can exist even if there is only one object in the entire universe." Is the student correct?
Step 1. Recall the definition: a force is a push or a pull arising from the interaction of two objects.
Step 2. With only a single object anywhere, there is nothing else for it to interact with — no second object to push it, pull it, or be pushed or pulled by it.
Step 3. Since an interaction by definition needs at least two participants, a lone object cannot exert or experience any force.
The student's claim is incorrect — a force always needs at least two objects, one applying it and one experiencing it.
Common traps
- Thinking a force can exist with just one object involved — always ask "what two objects are interacting here?"
- Confusing force with the motion it produces — a force is the push or pull that can cause a change in motion, but force and motion are not the same thing.
- Assuming a force always needs direct touching — non-contact forces such as magnetic, electrostatic, and gravitational force are still pushes or pulls, even without contact.
- Mixing up the newton (N), the SI unit of force, with units of mass (kg) or length (m).
Ask the tutor: Want to test yourself on more everyday scenes and spot the two interacting objects behind each one?
Can a force exist without any contact between the objects
Yes. The definition of force only requires an interaction between two objects — it never requires them to touch. A magnet pulling an iron nail from a distance, or the Earth pulling a falling ball toward itself, are both genuine forces even though nothing is touching. You will later learn to separate forces into contact forces and non-contact (distance) forces, but both kinds satisfy the same basic definition: a push or a pull between two objects.
If I stand still and push against a wall but nothing moves, am I still applying a force
Yes. A force is the push or pull itself, not the motion it may or may not cause. You are clearly pushing on the wall — your muscles exert a push, and the wall is the second object receiving it — so a force is present even though the wall does not move. Whether that force produces a visible change of motion depends on other factors you will study later, such as opposing forces acting on the wall.
Why can't a single object in empty space have a force acting on it
Because a force is defined as arising from an interaction, and an interaction needs two participants — one to apply the push or pull and one to receive it. With only one object anywhere, there is nothing else to interact with it, so there is no source for any push or pull to come from. This is exactly why definitions of force always mention two objects rather than describing force as a property that one object can simply "have" on its own.