Types of Forces

A force is a push, pull, or twist that can:

• Change the speed of an object (make it faster or slower)
• Change the direction of an object's motion
• Change the shape of an object (stretch, compress, or bend it)

Forces are measured in newtons (N), named after Sir Isaac Newton.

$\text{Force is a vector quantity — it has both magnitude and direction.}$

A scalar quantity has magnitude only (e.g., mass, temperature), while a vector quantity has both magnitude and direction (e.g., force, velocity).

Contact forces occur when two objects are physically touching. The force acts at the point of contact between the objects.

2.1 Friction

Friction is a force that opposes motion between two surfaces that are sliding (or trying to slide) across each other.

• Acts in the opposite direction to the movement
• Converts kinetic energy into thermal (heat) energy
• Can be useful (e.g., grip between shoes and the ground) or a nuisance (e.g., wear on moving parts)

Example: When you push a book across a table, friction between the book and the table surface slows the book down.

2.2 Normal Contact Force (Reaction Force)

The normal contact force acts perpendicular (at right angles) to the surface when an object rests on it.

• A book on a table: the table pushes upwards on the book with a normal contact force equal to the book's weight
• Also called the reaction force

2.3 Tension

Tension is the pulling force transmitted through a string, rope, cable, or wire when it is pulled at both ends.

• Acts along the length of the string or rope
• The tension is the same throughout the string (if the string is massless and there is no friction)

Example: A child on a swing — the ropes are under tension, supporting the child's weight.

2.4 Air Resistance (Drag)

Air resistance is a type of friction that acts on objects moving through the air.

• Increases with the speed of the object
• Increases with the surface area of the object
• Acts in the opposite direction to the movement

Example: A parachute increases air resistance to slow down a skydiver.

2.5 Upthrust (Buoyancy)

Upthrust is the upward force exerted by a fluid (liquid or gas) on an object placed in it.

• Depends on the volume of fluid displaced
• If upthrust equals weight, the object floats
• If weight exceeds upthrust, the object sinks

Example: A boat floats because the upthrust from the water equals the boat's weight.

2.6 Applied Force

An applied force is any force applied to an object by a person or another object.

• Pushing a trolley
• Kicking a football
• Lifting a bag

Non-contact forces act between objects that are not touching. The force acts over a distance through a field.

3.1 Gravitational Force (Gravity)

Gravitational force is the attraction between any two objects that have mass.

• Every object with mass exerts a gravitational pull on every other object
• The more massive the object, the stronger the gravitational pull
• On Earth, gravity pulls everything towards the centre of the planet

$W = m \times g$

Where:

• $W$ = weight (in N)
• $m$ = mass (in kg)
• $g$ = gravitational field strength (on Earth, $g \approx 9.8 \text{ N/kg}$)

Key point: Mass and weight are different!

• Mass is the amount of matter in an object (measured in kg) — it stays the same everywhere
• Weight is the force of gravity acting on the mass (measured in N) — it changes depending on the gravitational field strength

3.2 Electrostatic Force

Electrostatic forces act between objects that are electrically charged.

• Like charges repel (positive repels positive; negative repels negative)
• Opposite charges attract (positive attracts negative)
• The force gets weaker as the distance between the charges increases

Example: After rubbing a balloon on your hair, the balloon can stick to a wall due to electrostatic attraction.

3.3 Magnetic Force

Magnetic forces act between magnets or between a magnet and a magnetic material (iron, steel, nickel, cobalt).

• Like poles repel (north repels north; south repels south)
• Opposite poles attract (north attracts south)
• The force gets weaker as the distance increases
• Magnetic forces act through a magnetic field

Example: A compass needle is deflected by the Earth's magnetic field, always pointing towards magnetic north.

A field is the region around an object where a non-contact force acts on other objects.

Fields are represented using field lines:

• The direction of the field line shows the direction of the force
• The closer together the field lines, the stronger the field

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force back on the first. These are called interaction pairs.

• A book resting on a table: the book pushes down on the table (weight), and the table pushes up on the book (normal contact force)
• The Earth pulls you down (gravity), and you pull the Earth up with the same force (though the Earth is so massive you don't notice it moving!)

Important: Interaction pairs always:

• Act on two different objects
• Are equal in magnitude
• Act in opposite directions
• Are the same type of force

Free Body Diagrams

A free body diagram shows all the forces acting on a single object. Each force is drawn as an arrow:

• The length of the arrow represents the magnitude (size) of the force
• The direction of the arrow shows the direction of the force

Example: A ball falling through air:

• Weight (downwards) — long arrow
• Air resistance (upwards) — shorter arrow
• The ball accelerates downwards because weight > air resistance

• Forces are pushes or pulls measured in newtons (N)
• Contact forces require touching: friction, tension, normal force, air resistance, upthrust
• Non-contact forces act at a distance through fields: gravity, electrostatic, magnetic
• Forces are vectors — they have magnitude and direction
• $W = m \times g$ relates weight, mass, and gravitational field strength
• Free body diagrams show all forces acting on a single object
• Balanced forces → constant velocity; unbalanced forces → acceleration