Forces and Equilibrium

In mechanics, understanding forces is essential. Before you can apply Newton's laws to solve a problem, you must be able to identify every force acting on an object, draw an accurate free-body diagram, and determine whether the object is in equilibrium. This lesson covers the main types of force and the conditions for equilibrium, with particular attention to inclined plane problems and friction.

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Types of Force

Weight (W)

Weight is the gravitational force acting on an object. It always acts vertically downward towards the centre of the Earth.

W = mg

where m is the mass in kg and g = 9.81 m/s² (near Earth's surface). Weight is measured in newtons (N).

Normal Contact Force (N or R)

When two surfaces are in contact, they exert a force on each other perpendicular to the contact surface. This is the normal contact force (sometimes called the normal reaction).

On a horizontal surface, the normal force acts vertically upward. On a slope, it acts perpendicular to the slope surface — not vertically.

Friction (F)