Forces. Theory and practice with simulations

04/06/2026

The online force simulations on this page allow us to improve our understanding of the concept of forces in physics. We will discover how they act, how we can classify forces, what the main types of forces are, and why forces are so important.

This Thematic Unit is part of our Physics collection

Contact Force

Type of interaction that requires bodies to physically touch to be exerted, such as pushing or friction.

Distance Force

Interaction that acts between bodies without physical contact, mediated by fields such as gravitational or electric.

Elastic Force

Force exerted by objects such as springs that have been deformed and seek to return to their original shape.

Force

Interaction capable of modifying the state of rest or motion of a body, or of producing a deformation in it.

Newton (N)

Unit of force measurement in the International System, defined as the amount of force needed to accelerate one kilogram at one meter per second squared.

Normal Force

Perpendicular force exerted by a surface on an object resting upon it, counteracting its weight.

Resultant Force

Single force obtained through the vector sum of all forces acting simultaneously on an object.

Tensile Force

Force transmitted through a stretched flexible object, such as a rope, cable, or chain, pulling on objects at its ends.

What are forces in physics

In physics, a force is defined as an interaction that can change the state of motion or deformation of an object. Force is a vector magnitude. Its unit in the International System is the newton (N). The study of forces is fundamental to understanding physical phenomena and describing the behavior of systems in the universe.

Classification of forces in physics

Forces can be classified in different ways depending on the criteria we use. One of the most common ways in physics is to divide them into contact forces and distance forces:

Contact forces

These act when two bodies are in physical contact. Examples: friction, tension in a rope, or the normal force exerted by a surface.

Distance forces

These are exerted without direct contact between bodies. Examples: gravitational force, electric force, and magnetic force.

Another way of classifying forces is according to their origin:

Fundamental forces

These are the four basic interactions of nature: gravitational, electromagnetic, strong nuclear, and weak nuclear.

Derived forces

These arise as a result of the combination of fundamental forces, such as friction or tension.

This classification of forces in physics is key to understanding how objects interact and explaining everyday phenomena such as walking, throwing a ball, or the orbit of the planets.

Types of forces in physics

In the following, we will explore different types of forces and their main characteristics.

Gravitational force

This is the force of attraction between two objects with mass. The gravitational force always acts towards the center of mass of the objects and depends on the mass of the objects and the distance between them, following Newton’s law of universal gravitation.

Electromagnetic force

It is responsible for the interactions between electric charges. This force can be attractive or repulsive and acts through electric and magnetic fields. The electromagnetic force is the basis of phenomena such as friction, magnetic force and interactions between atoms and molecules.

Normal force

It acts perpendicular to the contact surface between two objects and is equal and opposite to the force exerted by the object on the surface.

Frictional force

Arises when there is resistance to relative motion between two surfaces in contact.

Tensile force

Occurs in stretched objects, such as ropes or cables, and acts along them.

Elastic force

This occurs when an elastic body, such as a spring, is deformed and tends to return to its original shape.

The importance of forces in science, technology, and everyday life

The concept of force is one of the pillars of physics because it allows us to explain and predict the movement of bodies. Thanks to its study, humanity has been able to develop everything from Newton’s laws to advanced technologies such as airplanes, automobiles, satellites, and energy systems. In everyday life, forces are present in every action: walking, writing, opening a door, or simply holding an object. Understanding how forces work not only helps us comprehend the natural world, but also facilitates innovations in engineering, medicine, transportation, and virtually all areas of knowledge. Therefore, the importance of forces goes far beyond theory: they are the foundation that connects science with everyday life and the driving force behind much of technological progress.

Contact Force

Type of interaction that requires bodies to physically touch to be exerted, such as pushing or friction.

Distance Force

Interaction that acts between bodies without physical contact, mediated by fields such as gravitational or electric.

Elastic Force

Force exerted by objects such as springs that have been deformed and seek to return to their original shape.

Force

Interaction capable of modifying the state of rest or motion of a body, or of producing a deformation in it.

Newton (N)

Unit of force measurement in the International System, defined as the amount of force needed to accelerate one kilogram at one meter per second squared.

Normal Force

Perpendicular force exerted by a surface on an object resting upon it, counteracting its weight.

Resultant Force

Single force obtained through the vector sum of all forces acting simultaneously on an object.

Tensile Force

Force transmitted through a stretched flexible object, such as a rope, cable, or chain, pulling on objects at its ends.

Explore the exciting STEM world with our free, online, simulations and accompanying companion courses! With them you’ll be able to experience and learn hands-on. Take this opportunity to immerse yourself in virtual experiences while advancing your education – awaken your scientific curiosity and discover all that the STEM world has to offer!

Force simulations

Addition of forces I

Force is a vector quantity. When several forces act on an object, the resultant force is the vector sum of those forces. Change the modulus and direction of the forces and see how the resultant force changes.

Addition of forces II

Force is a vector quantity. When several forces act on an object, the resultant force is the vector sum of those forces. See what happens when you move the arms of the “strongman” holding the book.

Giants of science

“If I have seen further, it is by standing on the shoulders of giants”

Isaac Newton

René Descartes

1596

–

1650

René Descartes developed analytic geometry and contributed to classical mechanics and the study of optics

“Pienso, luego existo”

Gottfried Wilhelm Leibniz

1646

–

1716

Gottfried Wilhelm Leibniz was one of the founders of infinitesimal calculus, created modern mathematical notation, and contributed to mechanics and optics with groundbreaking theories

“Music is the hidden exercise of arithmetic of the soul, which does not know it is calculating.”

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Joseph-Louis Lagrange

1736

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1813

Joseph-Louis Lagrange formulated the Lagrangian equations, key in classical mechanics to describe motion. He also founded variational calculus, applying it to algebra and mathematical physics

“As soon as a mathematical truth is established, it always serves as a starting point for new truth”

Léon Foucault

1819

–

1868

Léon Foucault demonstrated Earth’s rotation with his famous pendulum and measured the speed of light with great precision, revolutionizing experimental optics

“The pendulum does not lie: the Earth moves beneath our feet”

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Test your knowledge

How are the forces acting on a block decomposed, and why is this decomposition essential for analyzing its motion?

When several forces act on a block, each force can be broken down into components along perpendicular axes—typically horizontal and vertical. This decomposition is crucial because it reveals which parts of each force contribute to motion and which parts counteract it. For instance, the weight always points downward, while the normal force acts perpendicular to the surface. On an inclined plane, the weight splits into a component parallel to the surface, which tends to pull the block downward, and a perpendicular component, which determines the magnitude of the normal force. By working with these components instead of the original oblique forces, the analysis becomes far more manageable. It allows us to determine whether the block will accelerate, remain in equilibrium, or experience a change in motion. In short, decomposition transforms a complex force diagram into a set of simpler, solvable relationships.

What roles do the normal force and friction play in the behavior of a block on a surface, and how are these two forces related?

The normal force is the surface’s reaction to the block’s weight; it acts perpendicular to the surface and prevents the block from sinking into it. Friction, on the other hand, acts parallel to the surface and opposes motion or the tendency to move. These two forces are closely linked because the maximum static friction depends directly on the magnitude of the normal force. A larger normal force means the surfaces press together more strongly, increasing the maximum friction that can be generated. This explains why a block on a horizontal surface experiences more friction than the same block on an inclined plane: the normal force decreases as the plane tilts. Understanding this relationship is essential for analyzing real‑world situations such as braking, dragging objects, or studying motion on ramps.

Why doesn’t a block move even if I push it a little? Does it make sense that it “refuses” to start sliding at first?

Yes, it makes perfect sense. Before the block begins to move, static friction acts to oppose your push. This friction can adjust itself up to a certain maximum value. As long as your applied force is smaller than that maximum, the block stays still. Only when your push exceeds the maximum static friction does the block finally break free and start sliding. It’s as if the surface offers a flexible resistance that grows until it reaches its limit.

What happens if a block is placed on an inclined plane? How come it starts sliding even without anyone pushing it?

On an inclined plane, the weight of the block no longer acts purely perpendicular to the surface. Part of the weight points down the slope, creating a natural tendency for the block to slide. If this downhill component is greater than the maximum static friction, the block will start moving on its own. As the incline becomes steeper, the downhill component increases while the normal force decreases, reducing friction. That’s why even a small increase in angle can suddenly make the block slip.

How come a block can be in equilibrium even when several forces are acting on it? Shouldn’t it move if so many things are pulling on it?

Not necessarily. A block is in equilibrium when the vector sum of all forces acting on it is zero. This can happen even if many forces are present, as long as they balance each other perfectly. For example, the weight can be balanced by the normal force, and a horizontal push can be balanced by friction. What matters is not the number of forces but how they combine. If every force is countered by another, the block remains at rest or moves with constant velocity.