Weight (Edexcel IGCSE Physics (Modular))

Weight

• 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