Weight, Mass & Gravity (OCR GCSE Physics A (Gateway))

Mass & Weight

What is mass? 

• Mass is the measure of the amount of matter in an object

• Consequently, this is the property of an object that resists change in motion

• The greater the mass of a body, the smaller the change produced by an applied force

• The SI unit for mass is the kilogram (kg)

What is weight?

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

• Since it is a force on an object due to the pull of gravity, it is measured in Newtons (N) and is a vector quantity

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

Measuring Mass and Weight

• The weight (and hence mass) of two objects can be compared using a balance

A balance can be used to compare two different weights

Defining Weight

• 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

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

Acceleration due to Gravity

• 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

On the moon, your mass will stay the same but your weight will be much lower

• 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 of the planets in our solar system

Free Fall

• An object in free fall is falling solely under the influence of gravity

• On Earth, all free-falling objects accelerate towards Earth at a rate of 9.81 m s⁻²

• In the absence of air resistance, all bodies near the Earth fall with the same acceleration regardless of their mass