Weight, Mass & Gravity (OCR Gateway GCSE Physics: Combined Science)

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

 

• 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

• 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