Weight, Mass & Gravity (OCR Gateway GCSE Physics)

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Katie M

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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)

What is 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)

Measuring Mass and Weight

  • The weight (and hence mass) of two objects can be compared using a balance

 

Types of balance, IGCSE & GCSE Physics revision notes

A balance can be used to compare two different weights

 

Exam Tip

  • 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

 

Defining Weight

  • Weight is defined as:

The force acting on an object due to gravitational attraction

  • Planets have strong gravitational fields
    • Hence, they attract nearby masses with a strong gravitational force

  • Because of weight:
    • Objects stay firmly on the ground
    • Objects will always fall to the ground
    • Satellites are kept in orbit

5-1-4-weight-force

Some of the phenomena associated with gravitational attraction and the weight force

Acceleration due to Gravity

  • 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, meaning an object’s weight will be about 6 times less than on Earth

Mass vs weight, downloadable AS & A Level Physics revision notes

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

  • The value of g (gravitational field strength) varies from planet to planet depending on their mass and radius
  • A few examples of varying gravitational field strength are shown below:

Gravitational field strength diagram, downloadable AS & A Level Physics revision notes

Gravitational field strength of the planets in our solar system

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

Exam Tip

You do not have to remember the gravitational field strength g on other planets, but just that it increases with the mass of the planet. The value of will be given in your exam questions.

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.