Introduction

Whenever an object moves, stops, changes direction, or changes shape, one or more forces are acting on it. Forces are responsible for everything from a football flying through the air to planets orbiting the Sun. Some forces require objects to touch, while others act over great distances without any physical contact.

Understanding the different types of forces is one of the most important foundations of mechanics. By identifying the forces acting on an object and representing them correctly, we can explain and predict how objects will behave.

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What Is a Force?

A force is a push or pull resulting from an interaction between two objects.

A force can:

• Start motion.
• Stop motion.
• Speed up an object.
• Slow down an object.
• Change direction.
• Change shape.

The SI unit of force is the newton (N).

Forces are always the result of interactions between objects.

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Figure 1. Forces are pushes or pulls caused by interactions between objects.

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Contact and Non-Contact Forces

Forces are divided into two main categories.

Contact Forces

A contact force acts only when two objects are touching.

Examples include:

• Normal force.
• Friction.
• Tension.
• Air resistance.
• Applied force.
• Spring force.

Without contact, these forces cannot act.

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Non-Contact Forces

A non-contact force acts without physical contact.

Examples include:

• Gravitational force.
• Magnetic force.
• Electrostatic force.

These forces can act across empty space.

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Figure 2. Contact forces require physical contact, while non-contact forces act at a distance.

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Common Contact Forces

Normal Force

The normal force is the support force exerted by a surface.

Examples:

• A table supporting a book.
• The ground supporting a person.

The normal force acts perpendicular (at 90°) to the surface.

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Friction

Friction opposes motion between surfaces in contact.

Examples:

• Shoes gripping the ground.
• Bicycle brakes.
• Sliding a box across the floor.

Friction usually acts opposite to the direction of motion or attempted motion.

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Tension

Tension is the force transmitted through a stretched rope, string, or cable.

Examples:

• A hanging lamp.
• A tug-of-war rope.
• A suspended bridge.

Tension always pulls along the rope.

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Air Resistance (Drag)

Air resistance opposes the motion of objects moving through air.

Examples:

• A falling skydiver.
• A moving cyclist.
• A speeding car.

Air resistance increases as speed increases.

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Applied Force

An applied force is a force exerted directly by a person or another object.

Examples:

• Pushing a shopping trolley.
• Pulling a suitcase.
• Kicking a football.

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Spring Force

A spring force is exerted when a spring or elastic object is stretched or compressed.

Examples:

• A stretched rubber band.
• A compressed spring.
• A spring balance.

The spring force acts to return the object to its original shape.

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Figure 3. Common contact forces include normal force, friction, tension, air resistance, applied force, and spring force.

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Common Non-Contact Forces

Gravitational Force

Gravity is the attractive force between objects with mass.

Examples:

• Objects falling to Earth.
• The Moon orbiting Earth.
• Planets orbiting the Sun.

Gravity acts toward the centre of the attracting object.

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Magnetic Force

Magnetic forces act between magnets and magnetic materials.

Magnets can:

• Attract.
• Repel.

Magnetic forces are important in motors, generators, and compasses.

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Electrostatic Force

Electrostatic forces act between electrically charged objects.

Unlike charges:

• Attract.

Like charges:

• Repel.

Examples:

• A rubbed balloon sticking to a wall.
• Static electricity after walking across carpet.

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Figure 4. The three main non-contact forces are gravitational, magnetic, and electrostatic forces.

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Force Interactions

Forces always involve two interacting objects.

Examples:

• The Earth pulls on an apple, and the apple pulls on the Earth.
• A person pushes a wall, and the wall pushes back.
• A magnet attracts a paper clip, and the paper clip attracts the magnet.

These interactions follow Newton's Third Law, which states that forces occur in equal and opposite pairs.

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Representing Forces Using Vectors

A vector has both:

• Magnitude (size).
• Direction.

Forces are represented by arrows.

The arrow shows:

• The direction of the force.
• The length represents the size of the force.

For example:

• → 5 N (5 newtons to the right)
• ↑ 10 N (10 newtons upward)

Longer arrows represent larger forces.

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Figure 5. Forces are vectors, so they are represented by arrows showing both size and direction.

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Classifying Forces by Their Origin

Classifying forces helps identify which interactions are affecting an object's motion.

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Why Understanding Forces Is Important

Understanding forces allows scientists and engineers to:

• Predict motion.
• Design vehicles.
• Build bridges.
• Launch rockets.
• Improve sports performance.
• Develop safer buildings and machines.

Every moving object is influenced by one or more forces.

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Worked Example

Question

A book is resting on a table.

Identify the forces acting on the book.

Solution

Two forces act on the book:

• Gravitational force (weight) acting downward.
• Normal force from the table acting upward.

These forces are equal in magnitude and opposite in direction, so they are balanced and the book remains at rest.

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Real-World Connection

When a skydiver jumps from an aircraft, several forces act at the same time. Gravity pulls the skydiver downward, while air resistance pushes upward against the motion. As the skydiver speeds up, air resistance increases until it becomes equal to the weight. At this point, the forces are balanced, and the skydiver falls at a constant speed called terminal velocity.

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Did You Know?

Although gravity is the weakest of the four fundamental forces of nature, it dominates the motion of planets, stars, and galaxies because it always acts as an attractive force and operates over enormous distances. Even the tiny gravitational attraction between everyday objects exists—it is simply far too small to notice.

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Key Terms

Air resistance (drag) – A contact force that opposes the motion of an object through air.

Applied force – A force exerted directly by a person or another object.

Contact force – A force that acts only when two objects are touching.

Electrostatic force – A non-contact force between electrically charged objects.

Force – A push or pull resulting from an interaction between objects.

Friction – A contact force that opposes motion between surfaces in contact.

Gravitational force – The attractive force between objects with mass.

Magnetic force – A non-contact force between magnets or magnetic materials.

Normal force – The support force exerted by a surface, acting perpendicular to it.

Non-contact force – A force that acts without physical contact between objects.

Spring force – A force exerted by a stretched or compressed elastic object.

Tension – The pulling force transmitted through a rope, string, or cable.

Vector – A quantity with both magnitude and direction.

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Key Takeaways

• Forces are pushes or pulls that arise from interactions between objects.
• Contact forces require physical contact, while non-contact forces act at a distance.
• Common contact forces include normal force, friction, tension, air resistance, applied force, and spring force.
• The main non-contact forces are gravitational, magnetic, and electrostatic forces.
• Forces are represented by vectors, which show both magnitude and direction.
• Identifying and classifying forces is essential for explaining and predicting the motion of objects.