Mass, Weight and Gravity

Understanding Mass and Weight

Mass is a measure of the amount of matter in an object, it remains constant regardless of where the object is in the universe.
Weight is the force acting on an object due to gravity. It is dependent on the object’s mass and the strength of the gravitational field it is in.
Mass is a scalar quantity, meaning it only has magnitude (size) and no direction.
Weight is a vector quantity, meaning it has both magnitude and direction. The direction is usually towards the centre of the massive body causing the gravitational attraction (downwards on Earth).
The SI unit of mass is the kilogram (kg), while the unit of weight is the newton (N).

Distinguishing Between Mass and Weight

Although mass and weight are related, they are not the same. Mass is a property of an object, whereas weight is a force acting upon it.
Mass is constant everywhere in the universe but the weight of an object can change depending on where it is. For example, your mass would be the same on the Moon as it is on Earth, but your weight would be less on the Moon due to its weaker gravitational field.

Calculating Weight

Weight is calculated using the following formula: W = m * g, where ‘W’ is weight, ‘m’ is mass, and ‘g’ is the gravitational field strength.
If mass is given in kilograms and the gravitational field strength in newtons per kilogram (N/kg), the result will be in newtons.
The value of gravitational field strength on Earth is approximately 9.8 N/kg.

Gravity

Gravity is a force of attraction that exists between any two masses.
It is the force that gives weight to physical objects and causes them to fall towards the ground when dropped.
The strength of gravity on a celestial body’s surface is known as gravitational field strength. It varies from one celestial body to another.

Falling in a Gravitational Field

When an object is in free fall under the effect of gravity alone (ignoring air resistance), it falls at a constant acceleration. This acceleration is the same as the gravitational field strength (9.8 m/s² on Earth).
All objects in free fall near the Earth’s surface, regardless of their mass, fall with the same acceleration. This was famously demonstrated by Galileo dropping different weights from the Leaning Tower of Pisa.