# 3.1.1 Mass & Weight

### What is Mass?

• Mass is the measure of the amount of matter in an object
• Consequently, this is the property of an object that resists change in motion
• The greater the mass of a body, the smaller the change produced by an applied force
• The SI unit for mass is the kilogram (kg)

#### Exam Tip

• Since mass is measured in kilograms in Physics, if it is given in grams make sure to convert to kg by dividing the value by 1000
• It is a common misconception that mass and weight are the same, but they are in fact very different
• Weight is the force of gravity acting upon an object
• Weight is a vector quantity
• Mass is the amount of matter contained in the object
• Mass is a scalar quantity

### Weight

• Weight is the effect of a gravitational field on a mass
• Since it is a force on an object due to the pull of gravity, it is measured in Newtons (N) and is a vector quantity
• The weight of a body is equal to the product of its mass (m) and the acceleration of free fall (g)

Weight equation

• g is the acceleration due to gravity or the gravitational field strength
• On Earth, this is 9.81 m s−2 (or N kg−1)

#### Free fall

• An object in free fall is falling solely under the influence of gravity
• On Earth, all free-falling objects accelerate towards Earth at a rate of 9.81 m s−2
• In the absence of air resistance, all bodies near the Earth fall with the same acceleration regardless of their mass

#### Mass v Weight

•  An object’s mass always remains the same, however, its weight will differ depending on the strength of the gravitational field on different planets
• For example, the gravitational field strength on the Moon is 1.63 N kg-1, meaning an object’s weight will be about 6 times less than on Earth

On the moon, your mass will stay the same but your weight will be much lower

• Although you only need to memorise g on Earth, its value on other planets in our solar system is given in the diagram below. Notice how much this varies according to the size of the planet

Gravitational field strength of the planets in our solar system

#### Worked example

Calculating mass of a space probe

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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