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School-college Physics Notes: Forces Section 2.1 Introduction to gravity

GCSE level physics exam revision notes Forces 2:

2.1 Introduction to the non-contact force of gravity

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INDEX of physics notes: FORCES section 2 on mass, weight, gravity

2.1 Introduction to the non-contact force of gravity

This page will answer questions such as ... Why does gravity vary from planet to planet?

A gravitational attractive forces acts between all objects of any mass, no matter how close or far apart they are.

Gravity is universal and a non-contact force that exists wherever there is mass.

All objects have a gravitational field around them.

Therefore, there is always a gravitational force of attraction between all objects all the time, and it doesn't matter whether they are in contact or not. - even though technically gravity is a non-contact force, it acts through all materials even if they are touching.

The space affect by the gravity effect of the object's mass is called the gravitational field.

 

You experience the pull of a gravitational field it as you jump up vertically against it's force and are then pulled back down to earth by the same force.

It was Isaac Newton who first realised that objects fell to Earth due to gravitational field attraction.

He not only recognised that gravity was a universal law of nature, but realised you could not only apply it to 'falling objects', but also to the motion of the planets around our Sun and our Moon orbiting the Earth.

BUT, remember, you are attracting the Earth at the same time as it attracts you. You don't notice this because your mass is so much smaller than the Earth. You only consciously experience yourself being attracted to the Earth.

 

Gravity makes everything fall towards the surface of an object e.g. like a planet, and it is gravity that gives everything 'weight' (explained below).

The force of gravitational attraction between two masses increases by two factors:

(i) The bigger the two masses involved

With its bigger mass, an elephant is more strongly attracted to the earth than you!

The greater a planet's mass, the greater the strength of the gravitational field around it.

(ii) The closer the two objects are together

The further you go above the Earth's surface the weaker the gravitational attraction between you and your planet.

Note: Although not needed for GCSE: attractive force F m1 x m2/d2

F = force of attraction between the two objects,

m = the masses of the two attracted objects,

d = distance from centres of gravity of the two objects

So you can relate this arithmetical proportionality equation to the two rules above)

 

You don't really notice gravitational attraction between objects around you because the gravitational fields are too weak.

However, because the huge mass of the Earth and its very strong gravitational field, you definitely notice objects falling towards it..

BUT, with gravity, we are always dealing with mutual attraction, so, what you don't notice is that your body is attracting the Earth towards it - think about it!

 

INDEX physics notes FORCES section 2 on mass, weight and gravity

Key points about mass, weight and gravity - introducing the force of gravity

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

A structured set of summary revision notes on gravity tailored to the GCSE/IGCSE Physics specifications across major UK exam boards (AQA, Edexcel, OCR, WJEC, CCEA, CIE):

Gravity: The Non-Contact Force

Core Concepts

  • Gravity is a non-contact force that acts at a distance between any two masses.
  • It causes objects to attract each other, with the Earth’s gravity pulling objects towards its centre.
  • Weight is the force due to gravity acting on a mass.

Key Definitions

Term Definition
Gravity A force of attraction between masses
Gravitational Field A region where a mass experiences a force due to gravity
Gravitational Field Strength (g) Force per unit mass (N/kg); on Earth, approx. 9.8 N/kg
Mass Amount of matter in an object (kg); does not change with location
Weight Force due to gravity (N); depends on gravitational field strength

Essential Equation for weight

Weight (W) = Mass (m) × Gravitational Field Strength (g)
W = m × g
  • Units:
    • Weight: Newtons (N)
    • Mass: Kilograms (kg)
    • g: Newtons per kilogram (N/kg)

Examples of Gravitational Field Strength Variations

  • Earth: ~9.8 N/kg
  • Moon: ~1.6 N/kg
  • Jupiter: ~24.8 N/kg
  • Implication: Same object has different weight on different planets, but mass remains constant.

Centre of Mass and Gravity

  • Centre of mass: Point where the mass of an object is considered to act.
  • For regular shapes, it's at the geometric centre.
  • For irregular shapes, it can be found using the plumb line method.

Required Practical for most exam boards

Finding the centre of gravity of an irregular object:

  1. Suspend the object from a point.
  2. Hang a plumb line and draw a vertical line.
  3. Repeat from another point.
  4. The intersection of lines = centre of gravity.

Student Tips for Exam questions on gravity

Conceptual Clarity

  • Understand the difference between mass and weight.
  • Know that gravity is universal—it acts between all masses, not just Earth.

Calculation Practice

  • Use W = m × g confidently.
  • Be ready to rearrange the formula to find mass or g.

Graph Interpretation

  • Be able to interpret or sketch weight versus. mass graphs.
  • Recognise that the graph is a straight line through the origin (directly proportional).

Common Misconceptions

  • Gravity only acts on Earth →  It acts everywhere in the universe.
  • Weight and mass are the same →  Mass is constant; weight varies with g.

Keywords, phrases and learning objectives for the force of gravity

Know that a gravitational force field exists around any object.

Know the bigger the mass of the objects the stronger the gravitational field and the greater the force of attraction between objects.

Know the shorter the distance between objects, the stronger the gravitational attractive force between the objects.

(Strictly speaking the distance is taken as the shortest distance between the two centres of mass of the two objects mutually attracting each other)

(See Part 2.4 The centre of mass of an object)

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INDEX physics notes FORCES section 2 on mass, weight and gravity

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