What Is a Force?

In physics, a force is simply a push or a pull. It is what makes things start moving, stop moving, speed up, slow down or change direction. Forces are everywhere. Gravity pulls you to the ground. Friction stops a sliding box. A magnet pulls a paper clip. Your foot pushes a football. Even when you are sitting still, forces are at work: gravity is pulling you down, the chair is pushing you up. Forces are measured in newtons (N), named after the English scientist Sir Isaac Newton, who first worked out how they behave.

  • What it isA push or a pullMeasured in newtons (N)
  • DirectionForces have directionAnd a strength (magnitude)
  • Balanced forcesNo change in motionObject stays still or coasts
  • Unbalanced forcesChange motionSpeeds up, slows down or turns
  • Four fundamental forcesGravity, electromagnetic, strong, weakGovern the whole universe
  • 1 newtonForce to lift an appleRoughly 100 grams

Different kinds of force

Forces come in many varieties. Some you experience every day:

  • Gravity: pulls all objects with mass towards each other. Earths gravity holds you on the ground.
  • Friction: resists motion between surfaces sliding past each other.
  • Normal force: the push that a surface (like a chair or floor) gives back when you press on it.
  • Applied force: any force you apply with your hand, foot or muscles.
  • Magnetic force: between magnets and certain metals.
  • Electrical force: between positive and negative electric charges.
  • Tension: the pull through a stretched string or rope.
  • Spring force: a stretched or squashed spring pulls or pushes back.
  • Air resistance: the friction of moving through air.
  • Buoyancy: the upward push of a fluid (like water) on a submerged object.

Forces have size and direction

Every force has two parts: a strength (how big the push or pull is) and a direction (which way it points). For example, "10 newtons downward" is a complete description of a force; just saying "10 newtons" is not enough.

Scientists usually draw forces as arrows. The length of the arrow shows the strength; the direction of the arrow shows the direction. This makes force diagrams easy to draw and understand.

Fact The newton is the official unit of force, defined as the force needed to accelerate a 1-kilogram object at 1 metre per second per second. It is roughly the weight of a small apple. Sir Isaac Newton (1642 to 1727) discovered the laws of motion that bear his name. The unit was named after him in 1948.

Balanced and unbalanced forces

Usually more than one force acts on an object at any given time. Whether the object moves (or changes its motion) depends on whether the forces are balanced or unbalanced.

  • Balanced: all forces cancel out. The object stays still, or keeps moving steadily in a straight line. Example: a book sitting still on a table. Gravity pulls it down; the table pushes it up. The two cancel.
  • Unbalanced: forces do not cancel. The object accelerates (speeds up, slows down or changes direction) in the direction of the net force. Example: kicking a stationary football. The kick is much bigger than the friction holding the ball, so the ball flies off.

Measuring force

The simplest way to measure force is with a spring balance (or "newton meter"): a stretchy spring with a scale. The amount the spring stretches is proportional to the force pulling on it. Calibrated correctly, the scale shows the force in newtons.

You probably use this every time you weigh yourself. Bathroom scales measure how hard you push down on them due to Earths gravity. They usually display your weight in kilograms (mass), but inside, they are really measuring a force.

The four fundamental forces

At the deepest level, physicists believe all the forces in the universe come from just four basic kinds:

  • Gravity: weakest of the four, but acts on everything with mass and has unlimited range. Holds planets, stars and galaxies together.
  • Electromagnetic: between electric charges and magnets. Responsible for almost all everyday forces (friction, springs, chemical bonds, light).
  • Strong nuclear: holds protons and neutrons together inside atomic nuclei. Extremely strong but only over tiny distances.
  • Weak nuclear: causes some kinds of radioactive decay. Also short-range.

Almost all everyday pushes and pulls you experience (touching, walking, magnets, electricity) come from the electromagnetic force. Gravity is the other one you feel constantly. The nuclear forces only matter inside atoms.

Did you know? When you push on a wall, the wall pushes back on you with an equal and opposite force. This is one of Newtons three laws of motion. The push you feel from the wall is the electromagnetic force between the atoms in your hand and the atoms in the wall: they cannot get closer to each other because of electron repulsion. You never actually "touch" anything: your atoms hover just above the surface atoms, repelled by electrical force.

What forces do

Forces can:

  • Start motion: kicking a stationary ball.
  • Stop motion: applying brakes.
  • Speed up motion: an engine accelerating a car.
  • Slow down motion: air resistance on a falling parachute.
  • Change direction: a tennis racket hitting a ball back.
  • Change shape: squeezing a balloon, stretching a rubber band.
Try this Stand on a set of bathroom scales and watch the reading. Without bending or moving your weight elsewhere, push down hard with your feet. The reading shoots up. Now relax suddenly: the reading dips briefly below your normal weight. By changing the force between you and the scales, you change what the scales measure (even though your mass is the same). This is why a lift starting upwards feels like you weigh more, and a lift dropping down feels like you weigh less.
Deeper dive: Newtons revolution

Before Isaac Newton, people had many vague ideas about why things move. The ancient Greek philosopher Aristotle thought that heavier objects fall faster (wrong) and that moving things naturally slow down (also wrong, sort of). The medieval scholar Philoponus made some progress. Galileo (1564 to 1642) cleared up many misconceptions with careful experiments rolling balls down ramps.

But it was Newton who finally pulled it all together. In his masterpiece, the Philosophiae Naturalis Principia Mathematica (1687), Newton set out:

  • Three laws of motion, describing how forces and motion relate. (See Newtons three laws.)
  • The law of universal gravitation, the same force that pulls apples down to the ground keeps the Moon in orbit around Earth and the planets in orbit around the Sun.
  • The mathematical tools to predict any motion under any force.

The Principia is often called the most important scientific book ever written. It launched the modern era of physics. For 200 years, Newtons mechanics explained essentially every observed motion in the universe. Even today, almost every everyday engineering calculation (cars, bridges, rockets, footballs, factories) uses Newtons rules.

Albert Einsteins theory of relativity in the early 1900s showed that Newtons laws are not quite perfect at very high speeds or near very strong gravity. Quantum mechanics later showed they break down at the scale of atoms. But for everyday situations, Newtons laws are essentially exact, and they remain the foundation that every student of physics starts with.

For more, see Newtons three laws of motion and friction.