A force is a push or pull that can change an object’s motion, direction, or shape. In physics, forces explain why a ball rolls, why a car slows down when its brakes are applied, why objects fall toward Earth, and why a magnet attracts a paper clip. Forces are fundamental to understanding how objects interact, from tiny particles to planets and stars.
A force does not necessarily make an object move. It can keep a stationary object at rest, maintain an object’s motion, or change its speed or direction. What matters is how all the forces acting on an object combine. When those forces are unbalanced, the object’s motion changes. When they balance, the object’s acceleration is zero.
Understanding force begins with Newton’s laws of motion, but it also involves several distinct types of forces, including gravity, friction, tension, and electromagnetic forces. These principles provide a practical way to explain many familiar experiences and the physical processes behind them.
What is a force in physics?
In physics, a force is an interaction that can change an object’s velocity or deform it. Velocity describes both how fast an object moves and the direction in which it travels. A change in either speed or direction means that the object’s velocity has changed.
For example, kicking a stationary soccer ball applies a force that sets it in motion. Kicking a moving ball from the side can change its direction. Squeezing a rubber ball can change its shape even if its overall motion remains unchanged.
Force is a vector quantity, meaning it has both magnitude and direction. Magnitude describes how strong the force is, while direction describes where it acts. Two forces of equal strength can produce different effects if they act in different directions.
In classical mechanics, forces are commonly described as either contact forces or non-contact forces. Contact forces occur when objects physically interact, such as when a hand pushes a door. Non-contact forces act across a distance, such as gravity pulling an apple toward Earth or a magnet attracting an iron object.
Force is not a substance stored inside an object. It describes an interaction between objects or, in the case of fields, between an object and a physical field. This distinction helps explain why an object can continue moving even when no continuing push is applied.
How is force measured?
The standard unit of force in the International System of Units is the newton, abbreviated N. One newton is the amount of net force required to accelerate a mass of one kilogram at one meter per second squared.
Newton’s second law expresses the relationship among force, mass, and acceleration:
F = ma
Here, F represents the net force acting on an object, m represents its mass, and a represents its acceleration.
For example, if a 2-kilogram object experiences a net force of 6 newtons, its acceleration is:
a = F ÷ m = 6 N ÷ 2 kg = 3 m/s²
This means its velocity changes by 3 meters per second each second, assuming the net force remains constant.
The word net is important. An object can experience several forces simultaneously, and the equation uses their combined effect rather than the strength of just one force. If two people push a box in opposite directions with equal strength, their forces cancel. If one person pushes harder, the box accelerates in the direction of the stronger push, provided other forces do not change the result.
Mass also matters. For the same net force, an object with greater mass experiences less acceleration than an object with smaller mass. This relationship explains why a lightly loaded shopping cart is easier to accelerate than a heavily loaded one.
What are the main types of forces?
Forces can be grouped in different ways, depending on how they act. In everyday mechanics, it is useful to distinguish contact forces from forces that act at a distance. At a more fundamental level, modern physics describes interactions through four fundamental forces.
Contact forces
Contact forces arise when objects or materials physically interact. Although surfaces may look smooth, their microscopic structures and electromagnetic interactions produce the forces we experience when touching, pushing, pulling, or supporting something.
Applied force is a force exerted on an object by a person or another object. Pushing a shopping cart, pulling a suitcase, and pressing a button are examples. The resulting motion depends on the applied force and all the other forces acting on the object.
Normal force is the support force a surface exerts on an object in contact with it. A book resting on a table experiences a downward gravitational force and an upward normal force. If the book remains at rest on a horizontal table and no other vertical forces act on it, these forces balance. The normal force acts perpendicular to the surface, and its magnitude is not always equal to the object’s weight.
Friction resists relative sliding between surfaces or the tendency of surfaces to slide. It allows shoes to grip the ground, car tires to gain traction, and hands to hold objects securely. Static friction acts when surfaces are not sliding relative to each other, while kinetic friction acts during sliding. Friction can also convert mechanical energy into thermal energy, which is why rubbing your hands together makes them warmer.
Tension is a pulling force transmitted through a stretched rope, string, cable, or similar connector. When a bucket hangs from a rope, the rope exerts an upward tension force on the bucket. Tension allows cranes to lift loads, elevators to move, and suspension bridges to support weight.
Spring force arises when an elastic object is stretched or compressed. A spring generally pushes or pulls toward its equilibrium length. For an ideal spring within its elastic range, Hooke’s law describes the relationship:
F = −kx
Here, k is the spring constant, x is the displacement from equilibrium, and the minus sign indicates that the force acts opposite the displacement. Springs appear in mattresses, vehicle suspensions, mechanical scales, and many other devices.
Air resistance is a drag force exerted by air on a moving object. It acts opposite the object’s motion relative to the air. Its strength depends on factors such as speed, shape, surface area, and air density. A parachute increases drag substantially, allowing a falling person to descend more slowly.
Buoyant force is the upward force a fluid exerts on an immersed object. Fluids include liquids and gases. According to Archimedes’ principle, the buoyant force equals the weight of the fluid displaced by the object. This force helps explain why ships float, why an object feels lighter underwater, and why helium-filled balloons can rise in air.
These forces can act together. A book sliding across a table, for instance, may experience gravity downward, a normal force upward, an applied push forward, and friction backward. Its motion depends on the combined effect of those forces.
Non-contact forces
Non-contact forces act without direct physical contact between the interacting objects. Gravity and electromagnetic interactions are familiar examples. They are often described using fields, which assign a physical influence to locations in space.
Gravitational force is the attraction between objects with mass. On Earth, gravity pulls objects toward the planet’s center and gives them weight. It also keeps the Moon in orbit around Earth and the planets in orbit around the Sun.
Near Earth’s surface, the magnitude of an object’s weight can be approximated by:
W = mg
Here, W is weight, m is mass, and g is the local acceleration due to gravity, approximately 9.8 m/s² near Earth’s surface.
Mass and weight are not the same. Mass measures an object’s inertia, or resistance to changes in motion, and is measured in kilograms. Weight is the gravitational force acting on that mass and is measured in newtons. An astronaut has nearly the same mass on Earth and on the Moon but weighs less on the Moon because lunar gravity is weaker.
Electric force acts between electrically charged objects. Opposite charges attract, while like charges repel. When you rub a balloon against your hair, electric charges can become separated, allowing the balloon to attract hair or cling to a wall. Electric forces are also central to the behavior of atoms and molecules.
Magnetic force acts on magnets, magnetic materials, and moving electric charges. A refrigerator magnet can hold a note against a metal door, while an electromagnet can lift steel objects in a scrapyard. Magnetic and electric forces are closely related aspects of electromagnetism, rather than completely separate fundamental interactions.
What are the four fundamental forces of nature?
The contact and non-contact forces commonly discussed in everyday physics are useful descriptions of interactions at the scale of ordinary objects. At a deeper level, physicists recognize four fundamental interactions: gravity, electromagnetism, the strong nuclear interaction, and the weak nuclear interaction.
Gravity governs the attraction between masses and plays a major role in the structure and evolution of planets, stars, galaxies, and the universe on large scales. In everyday circumstances, it is responsible for falling objects and the weight of things on Earth’s surface.
Electromagnetism governs interactions between electric charges and magnets. It underlies electricity, magnetism, light, chemical bonding, and most contact forces encountered in ordinary life. The normal force that supports a book and the friction that allows a shoe to grip a floor ultimately arise from electromagnetic interactions between matter.
The strong nuclear interaction binds quarks together to form particles such as protons and neutrons. A related effect of the strong interaction helps bind protons and neutrons within atomic nuclei, overcoming the electric repulsion between positively charged protons at the relevant distances.
The weak nuclear interaction is involved in certain forms of radioactive decay and in nuclear processes that help power stars. It can change one type of fundamental particle into another, making it essential to several processes that influence the composition and evolution of matter.
These four interactions differ greatly in their properties, ranges, and roles. Gravity dominates many large-scale astronomical processes, while electromagnetism explains most familiar interactions between ordinary objects. The strong and weak interactions are especially important in the subatomic world.
What effects can a force have on an object?
A force can produce several effects, depending on its direction, magnitude, point of application, and the other forces present.
A force can start or stop motion
A stationary object begins accelerating when a nonzero net force acts on it. Pushing a parked shopping cart makes it start moving. Applying the brakes to a moving bicycle produces forces that slow it down until it stops.
Stopping an object does not mean eliminating force altogether. Braking works by creating forces that oppose motion, while the overall change in velocity depends on the net force.
A force can change speed
A force acting in the direction of motion generally increases an object’s speed, while a force acting against its motion generally decreases it. A cyclist speeds up by pedaling, and a cyclist slows down by applying the brakes.
The effect depends on the net force, not simply on whether a force points forward or backward. A moving object can experience several forces that partly or completely cancel one another.
A force can change direction
An object can change direction even when its speed remains constant. When a car travels around a bend, the inward net force causes its velocity to change direction. Similarly, gravity continually changes the direction of the Moon’s velocity as it orbits Earth.
This is why an object moving in a circle is accelerating even if its speed does not change. Acceleration describes any change in velocity, including a change in direction.
A force can change shape
Forces can deform materials by stretching, compressing, bending, or twisting them. Pressing a sponge compresses it, pulling a rubber band stretches it, and bending a ruler changes its shape.
Some materials return to their original shape when the force is removed. This is elastic deformation. Other materials retain some or all of the change, a process known as plastic deformation. If the force exceeds a material’s strength, the material may crack or break.
A force can produce rotation
A force applied away from an object’s axis of rotation can create a turning effect called torque. Opening a door by pushing near its handle is easier than pushing close to its hinges because the greater distance from the axis produces more torque for the same perpendicular force.
Torque depends on the force, the distance from the axis, and the angle at which the force acts. This principle explains why longer wrenches can make stubborn bolts easier to turn.
How do balanced and unbalanced forces work?
An object may experience several forces at once. The combined effect is called the net force, found by adding the forces as vectors. Their directions matter as much as their magnitudes.
Balanced forces produce zero net force. They do not cause acceleration, although they do not necessarily mean that an object is motionless. A book resting on a table has balanced vertical forces. A car moving at a constant velocity along a straight road also has zero net force when its forward driving force balances the resistive forces acting backward.
Unbalanced forces produce a nonzero net force and therefore cause acceleration. If a person pushes a box harder than friction resists its motion, the box accelerates. If a soccer ball is kicked, the force from the foot changes the ball’s velocity during the brief contact.
Consider a box being pushed horizontally with a force of 20 N while friction acts in the opposite direction with a force of 8 N. If these are the only horizontal forces, the net horizontal force is:
Fₙₑₜ = 20 N − 8 N = 12 N
The box accelerates in the direction of the push. Its acceleration depends on its mass, so the same 12 N net force produces a greater acceleration for a lighter box than for a heavier one.
Balanced forces also help explain why objects do not fall through tables or floors. A stationary object on a supporting surface experiences gravity downward and a normal force upward. When the forces balance, the object has no vertical acceleration.
Newton’s three laws of motion and force
Sir Isaac Newton’s three laws of motion provide a foundation for understanding how forces affect objects in classical mechanics. They describe inertia, the relationship between net force and acceleration, and the interactions between objects.
Newton’s first law: The law of inertia
Newton’s first law states that an object remains at rest or continues moving at constant velocity unless acted on by a nonzero net external force.
This principle is known as inertia: an object’s tendency to resist changes in its motion. A heavy cart is generally harder to accelerate than a light cart because it has greater mass and therefore greater inertia.
The law also explains why objects do not require a continuous force to keep moving in an ideal situation. On a nearly frictionless surface, a puck can glide for a long time after being pushed. In ordinary environments, friction and air resistance gradually slow moving objects, creating the impression that motion naturally dies away.
Newton’s second law: Force, mass, and acceleration
Newton’s second law states that an object’s acceleration depends on the net force acting on it and its mass. For an object with constant mass, the relationship is expressed as:
Fₙₑₜ = ma
Greater net force produces greater acceleration for the same mass. Greater mass produces less acceleration for the same net force.
This law is useful for predicting motion. Engineers use it when designing vehicles, elevators, aircraft, and machines. It also explains familiar experiences, such as why a fully loaded shopping cart requires more force than an empty one to achieve the same acceleration.
Newton’s third law: Action and reaction
Newton’s third law states that when one object exerts a force on another, the second object simultaneously exerts an equal-magnitude force in the opposite direction on the first.
When you walk, your foot pushes backward against the ground. The ground exerts a forward force on your foot, helping move you ahead. A swimmer pushes water backward, and the water pushes the swimmer forward.
These forces do not cancel each other on a single object because they act on different objects. The force exerted by your foot acts on the ground, while the force exerted by the ground acts on you. This distinction is essential for understanding how interactions produce motion.
Everyday examples of force in action
Forces are involved in nearly every ordinary movement, even when their effects are not obvious.
Walking and running: Your feet push backward against the ground, and static friction from the ground helps propel you forward. Without sufficient friction, as on an icy surface, your feet may slip and walking becomes difficult.
Driving a car: The engine supplies power to the wheels, and friction between the tires and road provides the external force that accelerates the vehicle. Braking increases forces that oppose motion, while steering changes the direction of the forces between the tires and road to guide the car around a turn.
Throwing a ball: Your hand applies a force that accelerates the ball. Once the ball leaves your hand, the hand’s force no longer acts on it. Gravity and air resistance then influence its trajectory, causing its direction and speed to change over time.
Riding a bicycle: Pedaling helps the wheels exert forces on the road, and the road pushes the bicycle forward. Friction in moving parts and air resistance oppose motion. When a cyclist turns, forces from the road help redirect the bicycle and rider.
Opening a door: Pushing the handle creates torque around the hinges. Applying the same force farther from the hinges generally makes the door easier to rotate because the turning effect is greater.
Using a magnet: A magnet can attract a paper clip without touching it. The magnetic interaction acts across the gap, illustrating that physical contact is not necessary for every force.
Floating in water: Water exerts an upward buoyant force on an immersed object. An object floats when its weight can be balanced by the buoyant force before it becomes fully submerged. Whether an object floats depends on its average density relative to the fluid and on the amount of fluid it displaces.
Jumping: Your legs push against the ground, and the ground exerts an upward force on your body. Once you leave the ground, that contact force disappears. Gravity then pulls you downward, bringing you back to the surface.
A falling parachutist: Gravity pulls the parachutist downward, while air resistance acts upward during the descent. As speed increases, drag generally grows. Eventually, drag can balance weight, producing terminal velocity: a constant falling speed with zero net force. Opening a parachute greatly increases drag and reduces the terminal speed.
These examples show that the motion of an object cannot be understood by looking at just one force. The interaction of all relevant forces determines whether the object speeds up, slows down, changes direction, rotates, or remains in equilibrium.
What is the difference between force, mass, weight, and energy?
Force, mass, weight, and energy are related concepts, but each describes something different.
Mass measures how much inertia an object has. It is measured in kilograms and remains essentially unchanged when an object moves from Earth to the Moon.
Weight is the gravitational force acting on an object. It is measured in newtons and depends on the local gravitational field. An object’s weight can change when the gravitational environment changes, even if its mass stays the same.
Energy is the capacity of a physical system to produce changes or transfer energy through interactions. It is measured in joules. Kinetic energy is associated with motion, while gravitational potential energy depends on position in a gravitational field.
Force can transfer energy when it acts through a displacement. For a constant force acting in the direction of motion, the work done is:
W = Fd
Here, W represents work, F is the force, and d is the displacement in the direction of the force. More generally, work depends on the component of force along the displacement. A force perpendicular to the displacement does no work on a point-like object, although it can still change the direction of motion.
For example, carrying a bag horizontally at a constant height involves an upward force from your hand, but that force does no mechanical work on the bag in the idealized horizontal-motion model because the force is perpendicular to the displacement. Walking up stairs, by contrast, involves doing work against gravity as you raise your body.
The distinction helps explain why force and energy should not be treated as interchangeable. Force describes an interaction that can change motion or shape, while energy describes a measurable property of a system that can be transferred or transformed.
Why force matters in science and technology
The study of force is central to engineering, transportation, construction, sports, and space exploration. Engineers calculate forces to ensure that bridges support their loads, buildings withstand wind, and vehicle components operate safely. They also study friction, impacts, and material deformation to improve performance and reduce damage.
In sports, force explains how athletes accelerate, jump, throw, and change direction. Coaches and sports scientists use the relationships among force, mass, and acceleration to analyze technique and performance. Protective equipment is designed in part to manage the forces experienced during collisions.
In transportation, understanding force helps engineers develop braking systems, improve tire traction, design aircraft wings, and predict vehicle motion. In space exploration, gravitational forces determine orbital paths, while rocket propulsion relies on the interaction between expelled gases and the rocket.
At microscopic scales, the study of fundamental interactions explains the stability of atoms, chemical reactions, and nuclear processes. At astronomical scales, gravity shapes the motion of planets, stars, and galaxies.
The concept of force connects these very different situations through a common physical framework. By identifying the forces acting on an object, determining their directions and magnitudes, and calculating their combined effect, it becomes possible to explain and predict how the physical world behaves.