Force

In physics, a force is an action that can cause an object to change its velocity or its shape, or to resist other forces, or to cause changes of pressure in a fluid. It helps us understand how things move or stay still. For example, when you push a swing or pull a toy, you are using force. Because both the strength and direction of a force matter, it is described as a vector quantity, meaning 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 how much force is acting on an object. For instance, if you weigh 50 newtons, that means the force of gravity pulling you down is equal to 50 newtons.

Force is very important in classical mechanics, the study of how objects move under the influence of different forces. It is central to all three of Newton's laws of motion. Common types of forces include elastic forces, like the springboard of a pogo stick; frictional forces, like the grip of your shoes on the ground; contact or "normal" forces, like the support of a table under a book; and gravitational forces, like the pull of the Earth that keeps us from floating away. Even in modern physics, which includes relativity and quantum mechanics, the idea of force remains important, though it is understood through more fundamental interactions.

Development of the concept

Philosophers in antiquity studied force in objects and machines, but they made mistakes, like thinking a force was needed to keep something moving. These ideas were corrected later by Galileo Galilei and Sir Isaac Newton, who created clear rules about how things move.

Later, Einstein explained how forces work on very fast objects, and modern science uses a Standard Model to describe forces between tiny particles. We now 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 is important for making things move. Early thinkers like Archimedes studied how forces work in liquids, called buoyant forces.

Aristotle thought that objects move to their "natural places" on Earth, like rocks falling to the ground. He believed that a force was needed to keep things moving, such as pushing a cart. However, 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 lead to the modern understanding of force.

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 published his famous book, Philosophiæ Naturalis Principia Mathematica. In this book, Newton explained three laws of motion that help us understand forces today.

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 that way unless something changes. This happens because the laws of physics work 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. The bigger the force, the more the object will speed up or change direction. 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 specific directions and have different strengths. Because both direction and strength matter, forces are called "vector quantities." This means they follow special math rules. To find out 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 are consequences of more fundamental forces. In these situations, we can use simple models to better understand how forces work. For example, we can think of solid objects as unchanging shapes to study their movement.

Gravitational force or Gravity

The main article about this is Gravity.

What we call gravity was first recognized as a universal force by Isaac Newton. Before Newton, people didn’t understand that the reason objects fall to the ground was connected to the movement of planets and stars. Galileo helped by showing that all objects fall at the same rate, no matter their weight. Today, we know the pull of gravity on Earth is about 9.81 meters per second squared, and it always points toward the center of the Earth. This means the force of gravity on an object depends on its mass — heavier objects feel a stronger pull.

Newton showed that the same force that makes apples fall also keeps the Moon orbiting Earth. He discovered that gravity gets weaker with distance, following an inverse square law. This means that the force of gravity between two objects depends on both their masses 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 either push or pull depending on the type of charge. Later, James Clerk Maxwell showed that electric and magnetic forces are related and can create waves that travel at the speed of light.

Normal

The main article about this is Normal force.

When two objects touch, the force between them that pushes them apart is called the normal force. This force keeps tables firm and floors strong. It also explains why objects 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 prevents motion before it starts, and kinetic friction, which slows motion once it’s happening. Friction depends on how hard the surfaces push together and the materials they’re made of.

Tension

The main article about this is Tension (physics).

Tension is the force that pulls along a rope or string. Imagining a massless, unbreakable string helps us understand how forces travel through ropes and pulleys. Pulleys can change the direction of a force and make it easier to lift heavy objects by spreading the force over multiple ropes.

Spring

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

A spring pulls back toward its normal length when stretched or compressed. This straight-line relationship between how much a spring is stretched and the force it creates was first described by Robert Hooke. The stronger the spring, the more force it produces for the same amount of stretch.

Centripetal

The main article about this is Centripetal force.

When an object moves in a circle, it needs a force to keep it on that curved path — this is called centripetal force. This force points toward the center of the circle and doesn’t change the object’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 objects made of many atoms, forces can affect different parts differently. In fluids like water or air, differences in pressure create forces that push objects. These forces explain why things float, how airplanes fly, and how wind moves. In solid objects, forces can stretch, squeeze, or shear the material, changing its shape in various ways.

Concepts derived from force

Main article: Torque

Forces can make objects spin around, 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 is useful in studying 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, which is the force needed to accelerate a one-kilogram mass by one meter per second squared. Another unit, used in the CGS system, is the dyne, which measures the force to accelerate a one-gram mass by one centimeter per second squared.

In English units, the pound-force is the force exerted by gravity on a pound-mass. There is also a metric unit called the kilogram-force, though 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.