Weight (Edexcel IGCSE Physics)

• Weight is defined as:

The force experienced by an object with mass when placed in a gravitational field

Weight and mass

• Weight and mass are different in physics
• Mass is a measure of how much matter there is in an object
  • Mass has magnitude but not direction
  • Therefore, mass is a scalar quantity
• Weight is a force 
  • Forces have magnitude and direction
  • Therefore, weight is a vector quantity

Gravitational field strength

• Planets have strong gravitational fields
  • Hence, they attract nearby masses with a strong gravitational force
• Different planets have different gravitational field strengths
  •  This depends on the mass of the planet
  • More massive planets have stronger gravitational field

Weight, mass and gravitational field strength

• 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

• Weight, mass and gravitational field strength are related using the equation:

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

• The gravitational field strength on Earth is 10 N/kg 
• is also used to describe the acceleration of an object in freefall in a gravitational field
  • The acceleration of freefall on Earth is 10 m/s²
  • These quantities are two ways of describing the thing 

• The weight that an object experiences depends on:
  • The object's mass
  • The mass of the planet attracting it

• The mass of an object and the weight acting on it are directly proportional
  • If one doubles, the other also doubles
  • If one is halved, the other is also halved
• The magnitude of the weight force depends on the gravitational field strength