What a force is
A force is a push or a pull. It can start an object moving, stop it, speed it up, slow it down or change its direction. It can also squeeze, stretch or bend things without moving them much at all. Forces have both a size and a direction, which is why physicists draw them as arrows.
Forces are measured in newtons, named after Isaac Newton. One newton is roughly the force needed to hold up a small apple against gravity. Several forces usually act on an object at the same time, and what matters for its motion is their combined effect, called the net force. A book resting on a table has gravity pulling it down and the table pushing it up. These balance, so the net force is zero and the book stays put.
Newton described how forces and motion fit together in his 1687 book, usually known as the Principia. His three laws became the foundation of classical mechanics and a landmark of the Scientific Revolution, the period when careful measurement and mathematics became central to explaining nature.
The first law: inertia
The first law says that an object keeps doing what it is already doing, either staying at rest or moving in a straight line at constant speed, unless a net force acts on it. This tendency is called inertia.
Most people’s everyday experience seems to contradict this, because a ball rolling across a floor slows down and stops. The reason is that forces are acting: friction and air resistance. On ice, a puck slides much farther because friction is smaller. In the near-vacuum of space, where almost nothing pushes back, a spacecraft can coast for years with its engines off.
Inertia is also why you lurch forward when a car brakes suddenly. The car slows down, but your body tends to keep moving at its earlier speed until the seat belt applies a force. Seat belts exist largely because of this law.
The second law: force, mass and acceleration
The second law links the net force on an object to its acceleration, meaning any change in speed or direction. In its usual form it is written as:
force = mass × acceleration
For a given push, a heavier object accelerates less than a lighter one. A worked example: a car with a mass of 1,200 kilograms that accelerates at 2 meters per second squared needs a net force of 1,200 × 2 = 2,400 newtons. If the same force acted on a 600-kilogram vehicle, the acceleration would be 2,400 ÷ 600 = 4 meters per second squared, twice as large.
This law explains why an empty shopping cart is easy to get moving and a full one is not. It also helps separate mass from weight. Mass is how much matter an object has, and it is the same everywhere. Weight is the force of gravity on that mass. Near Earth’s surface, a 1-kilogram mass weighs about 9.8 newtons, since gravity gives falling objects an acceleration of roughly 9.8 meters per second squared. On the Moon the same mass would weigh about one sixth as much, though its mass would not change.
The third law: action and reaction
The third law says that when one object exerts a force on a second object, the second exerts an equal and opposite force on the first. These forces come in pairs, and each pair acts on two different objects.
Everyday examples are easy to find:
- When you walk, your foot pushes backward on the ground and the ground pushes forward on you.
- When you jump off a small boat, you move one way and the boat drifts the other.
- A swimmer pushes water backward and is pushed forward by the water.
- A balloon releasing air pushes the air out and is pushed in the opposite direction.
A common confusion is to ask why equal and opposite forces do not simply cancel. They do not cancel because they act on different objects. For cancellation, forces would need to act on the same object. The principle is also behind how a rocket works. It throws exhaust gas backward at high speed and is pushed forward in return. It does not need air to push against, which is one reason rocket orbit launches work in the vacuum above the atmosphere, a topic covered in more depth elsewhere.
Putting the laws together
Consider a skateboarder rolling along a flat street. Because of the first law, the skateboard keeps rolling unless something slows it, and friction and air resistance gradually do. To speed up, the rider pushes backward on the ground. By the third law, the ground pushes forward on the rider. By the second law, that net forward force produces acceleration, and a lighter rider on the same board would accelerate more for the same push.
Falling objects follow the same logic. Gravity pulls with a force proportional to mass, and by the second law a larger mass needs a larger force for the same acceleration. In a vacuum, objects of different masses therefore fall together. In air, a feather lags behind a stone because air resistance matters much more for it. The nature of gravity itself, and how the picture changed from a pull at a distance to curved spacetime, is a separate story told in the history of the theory of gravity.
Where the laws stop working
Newton’s laws are extremely accurate for everyday speeds and sizes, and engineers still use them to design bridges, vehicles and machines. They are approximations, though. Near the speed of light, the laws need correction, and Albert Einstein’s relativity gives a better account. For atoms and subatomic scales, quantum physics is needed. Neither refutes Newton’s work in ordinary situations; rather, they extend physics into regimes Newton had no way to observe.
Newton’s laws also describe how forces change motion, but they do not by themselves say where forces come from. That question leads into gravity, electric and magnetic forces and the forces inside atoms, which are explored in the other articles in this cluster.