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

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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

Weight Force, downloadable IGCSE & GCSE Physics revision notes

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

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

Weight equation, downloadable AS & A Level Physics revision notes

  • g is known as the acceleration due to gravity or the gravitational field strength
    • On Earth, this is equal to 9.81 m/s2 (or N/kg)

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

  • Mass (measured in kilograms, kg) is related to the amount of matter in an object
  • Weight (measured in newtons, N) is the force of gravity on a mass
    • The weight of an object and the mass of an object are directly proportional
    • The size of this force depends on the gravitational field strength (often called gravity, g, for short)

Examiner Tip

It is a common misconception that mass and weight are the same, but they are in fact very different

  • Since weight is a force - it is a vector quantity
  • Since mass is an amount - it is a scalar quantity

Measuring Weight

  • Mass is commonly measured using a top pan balance
    • The weight can then be indirectly found through calculation using:

W = mg

  • Where g on Earth is ≈ 10 N/kg
  • Weight can be measured directly using a calibrated spring-balance, also known as a newton-meter
    • This device is a type of weighing scale which measures force in Newtons
    • It consists of a spring fixed at one end with a hook to attach an object at the other

Newtonmeter, downloadable IGCSE & GCSE Physics revision notes

Weight can be measured using a top-pan balance or a newton-meter

Examiner Tip

Since mass is measured in kilograms in Physics, if it is given in grams make sure to convert to kg by dividing the value by 1000!

Weight & 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

Mass vs weight, downloadable AS & A Level Physics revision notes

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 diagram, downloadable AS & A Level Physics revision notes

Gravitational field strength of the planets in our solar system

Worked example

A student estimates she would have a weight of 190 N on Mars. Calculate the weight of the student on Earth. The gravitational field strength on Earth is 9.8 N/kgThe gravitational field strength on Mars is 3.8 N/kg.

Step 1: List the known quantities

    • Weight on Mars, WM = 190 N
    • Gravitational field strength on Mars, gM = 3.8 N/kg
    • Gravitational field strength on Earth, gE = 9.8 N/kg

Step 2: Write out the equation relating mass and weight and rearrange for mass

W = mg

    • Divide both sides by g:

Step 3: Calculate the student’s mass

    • The student’s mass is the same anywhere in the universe

Step 4: Calculate the student’s weight on Earth

WE = m × gE = 50 × 9.8 = 490 N

Examiner Tip

You do not have to remember the gravitational field strength g on other planets, but just that it increases with the mass of the planet. The value of will be given in your exam questions.

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.