What Is a Force?
A force is an external push or pull on an object. Forces are responsible for interactions between objects, and their strength is measured in newtons (N).
Forces act on everything, all the time. Right now, gravity is pulling you downward and the ground is pushing you upward. You are surrounded by forces!
There are two main categories of forces:
- Contact forces — forces that act when objects physically touch each other (e.g. friction, the push of the ground on your feet)
- Non-contact forces — forces that act between objects without any physical contact (e.g. gravity, magnetism)
Magnetism as a non-contact force
A bar magnet has a north pole and a south pole. If you bring the north pole of one magnet near the south pole of another, they attract — pulling toward each other — even without touching. Bring two north poles together and they repel — pushing away from each other.
The unit of force is the newton (N), named after the scientist Sir Isaac Newton.
(!image: Diagram showing examples of contact forces (a hand pushing a box, friction on a sliding object) and non-contact forces (gravity pulling an apple downward, two magnets attracting each other), labelled clearly.)
Friction — A Key Contact Force
Friction is a contact force that acts between two surfaces in contact, opposing the relative motion between them. It always acts in the direction that resists movement.
Friction is caused by the microscopic roughness of surfaces — even surfaces that look smooth have tiny bumps and ridges that catch on each other. The rougher the surfaces, the greater the friction.
Friction plays a vital role in everyday life:
- Helpful friction: The grip between your shoes and the floor stops you slipping. Brakes on a bicycle use friction to slow the wheels.
- Unhelpful friction: Friction between moving parts in an engine wastes energy as heat. This is why machines are lubricated with oil.
Friction on a sliding book
You give a book a push across a table. Even after you stop pushing, the book slows down and stops. Why? Friction between the book's cover and the table surface acts backward against the direction of motion, slowing the book down.
If the table were perfectly frictionless (like very smooth ice), the book would keep sliding at the same speed — balanced forces!
Friction is a resistive force — it always acts in the opposite direction to motion. You will see friction appear again when we discuss balanced forces at constant speed.
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