Introduction
In Mechanics problems across 9709, almost every question involving forces on objects requires you to calculate the weight of that object — the gravitational force pulling it downward. Understanding the precise relationship between an object's mass (a scalar quantity, measured in kilograms) and its weight (a vector force, measured in newtons) is therefore a fundamental building block for the entire Mechanics unit.
The syllabus requires you to use with the approximation throughout your calculations. This single formula connects every Mechanics topic — from particles on inclined planes to connected particles over pulleys — and you will apply it in virtually every exam question.
Core Concept
Mass is the amount of matter in an object. It is a scalar quantity, measured in kilograms (kg), and it does not change regardless of where the object is located.
Weight is the gravitational force acting on an object due to the Earth's gravitational field. It is a vector quantity, measured in newtons (N), and it always acts vertically downward.
The two quantities are related by Newton's Second Law applied to free-fall: a mass in a gravitational field of strength experiences a downward force . This is the object's weight.
In 9709 Mechanics, the gravitational acceleration is taken as:
This is an approximation (the precise value near Earth's surface is approximately ), but the syllabus explicitly requires the use of in all numerical calculations.
Direction: Weight always acts vertically downward. In free-body diagrams, it is drawn as a downward arrow from the object's centre of mass, typically labelled or .
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