Definition · Plain-language
Force
A force is a push or a pull that can change the motion, direction or shape of an object.
The step most authors miss
Doing CRediT right? Don’t stop at the statement.
A CRediT statement credits you inside one paper. The recognition CRediT was built for happens when those roles are tied to you, persistently. Sign in with your ORCID — free — and claim your CRediT contributions on casrai.org, the home of the standard. They become a verified, portable part of your identity, not a line that disappears into one PDF.
Free: claim your contributions, then export a journal-ready CRediT statement, schema.org structured data, JATS XML, CSV or BibTeX — and preview your public profile. A membership publishes that profile publicly and verifies the journals you serve.
A push or a pull
A force is an interaction that pushes or pulls on an object. It can make a stationary object move, speed up or slow down a moving one, change its direction, or change its shape by stretching, squashing or bending it. Because a force has both a size and a direction, it is a vector quantity, and its SI unit is the newton — the force needed to accelerate a one-kilogram mass at one metre per second squared. Forces are everywhere, from the gravity holding you down to the friction that lets you walk.
Balanced and unbalanced forces
Usually several forces act on an object at once, and what matters is the net (resultant) force — their combined effect once direction is taken into account. If the forces are balanced, the net force is zero and the object’s motion does not change: it stays still or keeps moving at constant velocity. If the forces are unbalanced, the net force is non-zero and the object accelerates in the direction of that net force. This is why a parked car stays put until an unbalanced force, from the engine, sets it moving.
Types of force
Forces come in two broad families. Contact forces act where objects touch, including friction, the normal (support) force, tension in ropes, and air resistance. Non-contact forces act at a distance through fields, including gravity, the electromagnetic force between charges and magnets, and the strong and weak nuclear forces inside atoms. At the deepest level, physicists recognise four fundamental forces — gravitational, electromagnetic, strong and weak — from which all the everyday forces ultimately arise.
Key facts
At a glance
- Definition: a push or pull that can change motion or shape
- Quantity type: a vector — has magnitude and direction
- SI unit: the newton (N)
- Effect: a net force causes acceleration (force = mass × acceleration)
- Balanced forces: net force zero — motion unchanged
- Fundamental forces: gravitational, electromagnetic, strong, weak
Common misconceptions
What people often get wrong
Often heard: A moving object must have a force pushing it along.
Actually: An object keeps moving at constant velocity with no net force, by the law of inertia. A force is needed to change motion, not to maintain steady motion in the absence of friction.
Often heard: Force and energy are the same thing.
Actually: A force is a push or pull, measured in newtons; energy is the capacity to do work, measured in joules. A force does work, and transfers energy, only when it moves an object through a distance.
Often heard: If forces are balanced, the object must be stationary.
Actually: Balanced forces mean zero net force and therefore no change in motion — but the object can be moving at a constant velocity, not only at rest.
Going deeper
