Mass and Weight

Introduction to Mass and Weight

• Mass is a scalar property of matter that measures the amount of matter in an object. It is constant everywhere and is measured in kilograms (kg).
• Weight is a force due to gravity. It measures how strongly gravity pulls an object towards the centre of the Earth.
• Unlike mass, weight is a vector quantity and it varies depending on the strength of gravitational field.

Differences Between Mass and Weight

• Mass does not change as it is the amount of matter in an object. Whether an object is on Earth, the Moon, or in deep space, its mass stays the same.
• Weight, on the other hand, changes with location. An object weighs less on the Moon than it does on Earth because the Moon’s gravitational field strength is less than the Earth’s.
• Mass is measured in kilograms (kg), while weight is measured in newtons (N) since it is a force.

Calculating Weight

• The weight of an object can be calculated by multiplying its mass by the acceleration due to gravity (g) using the formula: Weight = Mass x gravity or W = mg.
• On the Earth’s surface, the acceleration due to gravity is approximately 9.8 m/s². This means that each kg of mass experiences a force due to gravity of 9.8 N.

Weight as a Resultant Force

• When an object is in free fall, the force due to gravity (its weight) is the only force acting upon it. Therefore, the weight of the object is known as a resultant force.
• An object in free fall accelerates at the same rate as the acceleration due to gravity, assuming negligible air resistance.

Effect of Mass and Weight on Motion

• The mass of an object affects how much it accelerates under a given force, according to Newton’s Second Law (F = ma). The more mass an object has, the less it accelerates under the same force.
• The weight of an object affects how strongly it is pulled downwards. Heavier objects experience a greater force of gravity, but this doesn’t affect how fast they fall, as the acceleration due to gravity is the same for all objects.