Mass & Weight (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

Mass

• Mass is defined as:

A measure of the quantity of matter in an object 

• 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

• Mass is a scalar quantity that has magnitude but no direction
• Mass is measured in kilograms (kg)
  • Sometimes mass may be given in grams (g) but this will need to be converted to kilograms when used in calculations
    • 1000 g = 1 kg
    • 1 g = 0.001 kg
  • To convert g to kg, divide the mass in g by 1000
  • To convert kg to g, multiply the mass in g by 1000

Weight

• Weight is defined as:

A gravitational force on an object with mass

• Since weight is a force, it is a vector quantity with both magnitude and direction
• Weight is measured in newtons (N)

Extended tier only

Weight and gravitational field strength

• Weight is the effect of a gravitational field on a mass
• Weight is defined as:

The force acting on an object with mass when placed in a gravitational field

• 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

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

• 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 

• Where:
  • = gravitational field strength, measured in newtons per kilogram (N/kg)
  • = force of weight, measured in newtons (N)
  • = mass of object, measured in kilograms (kg)

Mass vs. weight

• An object’s mass always remains the same, regardless of its location in the Universe
• The weight force exerted on the object will differ depending on the strength of the gravitational field in its location
• 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

On the Moon, a person's mass will stay the same but their weight will be much lower

• You can find more information about the gravitational field strength on different planets in the revision note Gravitational field strength

Extended tier only

• An object in free fall in a vacuum, in a uniform gravitational field, will accelerate at a rate also known as
  • Where  = acceleration of free fall
  • In this context,  = 9.8 m/s²
  • Gravitational field strength and acceleration of free fall are equivalent quantities