CIE IGCSE Physics

Revision Notes

1.3.1 Mass & Weight

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Mass

  • Mass is a measure of the quantity of matter in an object at rest relative to the observer
    • Mass is a scalar quantity
    • The SI unit for mass is the kilogram (kg)

  • Consequently, mass is the property of an object that resists change in motion
    • The greater the mass of an object, the more difficult it is to speed it up, slow it down, or change its direction
  • A mass may sometimes be given in grams (g)
    • 1000 g = 1 kg
    • 1 g = 0.001 kg

Worked example

An object has a mass of 2.7 kg.

State the number of grams in 2.7 kg.

1. State the conversion between g and kg

    • 1 kg = 1000 g

2. Convert 2.7 kg into g

    • 2.7 kg = 2.7 × 1000 = 2700 g

Weight

  • Weight is a gravitational force on an object with mass
    • Weight is a force, so it is a vector quantity
    • The SI units for force are newtons (N)

EXTENDED

  • Weight is the effect of a gravitational field on a mass
  • 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

  • The weight of a body is equal to the product of its mass (m) and the acceleration of free fall (g)

 

Weight equation, downloadable AS & A Level Physics revision notes

  • You can rearrange this equation with the help of the formula triangle:

mg-triangle, IGCSE & GCSE Physics revision notes

Use the formula triangle to help you rearrange the equation

 

Gravitational Field Strength

  • Gravitational field strength is defined as:

The force per unit mass acting on an object in a gravitational field

  • On Earth, this is equal to 9.8 N/kg 

  • Gravitational field strength is also known as acceleration of free fall, or acceleration due to gravity
    • In this context the units are m/s2
  • 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

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, 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

Exam Tip

You won't be expected the learn the exact value of g (9.81 N/kg), but you will be expected to remember that g = 9.8 N/kg and use it in calculations

Using a Balance

  • The weight of two objects can be compared using a balance
    • Because the gravitational field strength is constant everywhere on Earth, this also allows us to measure the mass of an object

 m space equals space W over g

Types of balance, IGCSE & GCSE Physics revision notes

A balance can be used to compare two different weights

 

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