Force

In physics, a force is something that can make an object move, change shape, or stay still. It helps us understand why things move or don’t move. For example, when you push a swing or pull a toy, you are using force. Because both how strong the force is and which way it goes matter, it is called a vector. This means it has both size and direction.

The SI unit of force is the newton (N), named after the scientist Sir Isaac Newton. We use this unit to measure force. For example, if you weigh 50 newtons, it means the force of gravity pulling you down is 50 newtons.

Force is very important in classical mechanics, the study of how objects move when forces act on them. It is key to all three of Newton's laws of motion. Some common forces are elastic forces, like a pogo stick’s springboard; frictional forces, like your shoes gripping the ground; contact or "normal" forces, like a table holding up a book; and gravitational forces, like Earth’s pull that keeps us from floating away. Even in modern physics, which includes relativity and quantum mechanics, the idea of force is still important, though it is seen through more fundamental interactions.

Development of the concept

Long ago, thinkers studied force in objects and machines. They thought a force was needed to keep something moving, but that was wrong. Later, Galileo Galilei and Sir Isaac Newton made clear rules about how things move.

After that, Einstein explained how forces work on objects that move very fast. Today, science uses a Standard Model to describe forces between tiny particles. We know four main forces: strong, electromagnetic, weak, and gravitational.

Pre-Newtonian concepts

See also: Aristotelian physics and Theory of impetus

Since ancient times, people have known that force helps things move. Early thinkers like Archimedes looked at how forces work in liquids, called buoyant forces.

Aristotle thought objects move to special places on Earth, like rocks falling down. He believed a force was needed to keep things moving, such as pushing a cart. But this idea couldn't explain why arrows keep flying after being shot. Later, Galileo Galilei showed that objects keep moving unless something, like friction, stops them. This helped us understand force better today.

Newtonian mechanics

Main article: Newton's laws of motion

Sir Isaac Newton described how objects move using the ideas of inertia and force. In 1687, he wrote a book called Philosophiæ Naturalis Principia Mathematica. In this book, Newton explained three laws of motion that help us understand forces.

Newton's first law says that an object at rest will stay at rest, and an object moving at a steady speed in a straight line will keep moving unless something changes. This happens because physics works the same for everyone who is not moving themselves.

Newton's second law tells us that a force is needed to change how an object moves. A bigger force makes the object speed up or change direction more. The law also says that bigger objects need more force to move the same amount as smaller objects.

Newton's third law explains that for every action, there is an equal and opposite reaction. If you push on a wall, the wall pushes back on you with the same strength. This means forces always come in pairs and affect two objects at the same time.

Combining forces

Forces act in different directions and have different strengths. Because both direction and strength matter, forces are called "vector quantities." This means they follow special math rules.

To see what happens when two forces act on the same object, you need to know both the strength and direction of each force.

When forces balance each other out, the object is in equilibrium. This means the forces add up to zero, so the object either stays still or moves at a steady speed in a straight line. For example, a book resting on a table has gravity pulling it down, but the table pushes up with an equal force, keeping the book in place.

Examples of forces in classical mechanics

Some forces come from bigger, more basic forces. We can use simple ideas to learn how forces work. For example, we can think of solid things as fixed shapes to study how they move.

Gravitational force or Gravity

The main article about this is Gravity.

We call gravity a force that pulls things down. Isaac Newton was the first to say that gravity is a force that works everywhere. Before Newton, people didn’t know why things fall to the ground or why planets move. Galileo helped by showing that all things fall at the same speed, no matter how heavy they are. Today, we know gravity on Earth pulls about 9.81 meters every second, every second, and it always points to the center of the Earth. This means bigger things feel a stronger pull.

Newton showed that the same force that makes apples fall also keeps the Moon moving around Earth. He found that gravity gets weaker the farther away things are, following an inverse square law. This means the force of gravity depends on how heavy the objects are and how far apart they are.

Electromagnetic

The main article about this is Electromagnetic force.

The force between electric charges was first described in 1784 by Coulomb. He found that this force depends on how much charge each object has and how far apart they are. It can push or pull depending on the type of charge. Later, James Clerk Maxwell showed that electric and magnetic forces are related and can make waves that move at the speed of light.

Normal

The main article about this is Normal force.

When two things touch, the force that pushes them apart is called the normal force. This force keeps tables firm and floors strong. It also explains why things don’t fall through the ground — the ground pushes back with an equal force.

Friction

The main article about this is Friction.

Friction is the force that slows down or stops motion between two touching surfaces. There are two types: static friction, which stops motion before it starts, and kinetic friction, which slows motion once it’s happening. Friction depends on how hard the surfaces push together and what they’re made of.

Tension

The main article about this is Tension (physics).

Tension is the force that pulls along a rope or string. Thinking of a rope that has no weight and can’t break helps us understand how forces move through ropes and pulleys. Pulleys can change the direction of a force and make it easier to lift heavy things by sharing the force across several ropes.

Spring

The main articles are Elasticity (physics) and Hooke's law.

A spring pulls back to its normal size when stretched or squeezed. This idea that how much a spring is stretched matches the force it makes was first described by Robert Hooke. The stronger the spring, the more force it makes for the same stretch.

Centripetal

The main article about this is Centripetal force.

When something moves in a circle, it needs a force to keep it on that curved path — this is called centripetal force. This force points to the center of the circle and doesn’t change the thing’s speed, only its direction. It’s what makes cars turn on curves and planets orbit the Sun.

Continuum mechanics

The main articles are Pressure, Drag (physics), and Stress (mechanics).

In real things made of many tiny parts, forces can affect different parts in different ways. In fluids like water or air, differences in pressure make forces that push on objects. These forces explain why things float, how airplanes fly, and how wind moves. In solid things, forces can stretch, squeeze, or bend the material, changing its shape in many ways.

Concepts derived from force

Main article: Torque

Forces can make objects spin, and this spinning motion is called rotation. The force that causes rotation is called torque. Torque is connected to force the way spinning is connected to regular movement. Just like force has a strength and direction, torque does too.

Another idea related to force is called "yank," which is how quickly a force changes over time. This idea helps us study how animals move and how robots work.

Forces also help us understand concepts like impulse (the effect of a force acting over time), work (the effect of a force moving something), and power (how fast work is done). These ideas come from math that looks at how forces add up over time or distance.

Main articles: Impulse, Mechanical work, and Power (physics)

Main article: Potential energy

Sometimes, instead of thinking about forces directly, scientists think about something called potential energy. This is stored energy that can turn into movement. For example, gravity pulls objects down, and this pull can be thought of as coming from gravitational potential energy.

Main article: Conservative force

Certain forces, like gravity and the force from springs, are called conservative forces. These forces help keep the total energy of a system the same. When only conservative forces act on a system, the energy can change between moving energy (kinetic) and stored energy (potential), but the total amount stays constant.

Units

The SI unit of force is the newton. It is the force needed to move a one-kilogram mass by one meter each second.

Another unit is the dyne. It measures the force to move a one-gram mass by one centimeter each second.

In English units, the pound-force is the force from gravity on a pound-mass. There is also a unit called the kilogram-force, but it is not part of the modern SI system and is used less often.

Revisions of the force concept

At the start of the 20th century, new ideas in physics helped explain things we see in space and with very tiny particles. These ideas changed how we think about force.

Special theory of relativity

Main article: Relativistic mechanics § Force

The special theory of relativity shows that mass and energy are linked. As an object moves faster, it gets harder to speed it up more because it gains energy. Even though Newton’s idea of force still works in math, we have to think differently about momentum when things move close to the speed of light. This means we need more force to make an object speed up the same amount when it is already moving very fast.

Quantum mechanics

Main articles: Quantum mechanics and Pauli exclusion principle

Quantum mechanics looks at the world of very small particles, like atoms and the pieces inside them. In this world, we can’t always know exactly where a particle is or how fast it is going. Instead, we use probabilities. Quantum mechanics often talks about energy instead of force, but it still connects to Newton’s ideas in some ways. It also helps explain why atoms stay together, even though the pieces inside should pull apart.

Fundamental interactions

Main article: Fundamental interaction

There are four main forces in the universe. The strong force and weak force work only over very short distances and affect tiny particles, like those inside atoms. The electromagnetic force acts between electric charges, like the pull or push you feel from a magnet. The gravitational force pulls masses toward each other, keeping planets in orbit around the Sun.

These forces come from different ways tiny particles interact. For example, friction — the force that keeps a book from sliding off a table — comes from the electromagnetic force between atoms. Scientists have developed theories to explain how these forces work, showing that some forces are related. For instance, they discovered that electric and magnetic forces are two sides of the same coin, called electromagnetism.