Mass and weight

This section explains the concept of weight as a force, the equation to calculate weight, and its relationship with mass and gravitational field strength.

What is weight

Weight is the force acting on an object due to gravity.

• Near the Earth's surface, this force is caused by the gravitational field around the Earth.

• Weight depends on:

  • The mass of the object.

  • The gravitational field strength at the object’s location.

Key Points:

• The gravitational field strength (g) varies between planets.

  • Example: g = 9.8 N/kg on Earth but g = 1.6 N/kg on the Moon.

• Weight is measured in newtons (N), as it is a force.

Equation for weight

Equation for Weight

The weight of an object can be calculated using the formula:

W=m×gW = m \times gW=m×g

Where:

• W = Weight (N)

• m = Mass (kg)

• g = Gravitational field strength (N/kg)

Example Calculation:

A person with a mass of 70 kg standing on Earth (g = 9.8 N/kg) has a weight of:

W = 70 kg × 9.8 N/kg = 686 N

Weight vs Mass

• Mass:

  • The amount of matter in an object.

  • Measured in kilograms (kg).

  • Does not change with location.

• Weight:

  • A force due to gravity acting on the mass of the object.

  • Measured in newtons (N).

  • Changes depending on the gravitational field strength.

Key Concept:

Weight and mass are directly proportional:

W ∝ m

This means that doubling the mass will double the weight.

Centre of mass

• • Centre of Mass

  • The weight of an object acts at a single point, called the centre of mass.

  • This is the point where the object's mass is evenly distributed.

Measuring weight

Weight is measured using a calibrated spring-balance, also known as a newtonmeter.

• Example:

• If an object stretches the spring-balance to read 5 N, the object’s weight is 5 N.

Key skills and application