Understanding Gravity and Mass

This science article discusses the meaning of ‘gravity’.Gravity is another ‘force from a distance’- just like the force of magnetism which we talked about in Science Experiments-Magnetic Attraction

The force of gravity – an introduction

Things very often come in two’s…..

You don’t lay a table without a salt and pepper pot!

salt and pepper pot

Humans have two eyes!

Famous picture of Sharbat Gula

Walruses have two tusks!

walruses crowding together showing their tusks

Tandem bicycles have two wheels and two riders.

tandem bicycle ridden by two gentlemen

Gravity and mass act together as a pair – You need to understand the meaning of ‘mass’ before you can understand the meaning of ‘force of gravity’.

Science projects – so what is mass?

We will start by saying what mass is not.’Mass’ is not the same as weight. Describing how heavy or light an object is on planet earth is not the same as describing the ‘mass’ of that object on planet earth.

Angry kid saying , How many times have I told you? Mass is not the same as weight!

So what is mass?

Mass is the amount, or quantity, of ‘stuff’ inside something.We can say that the bigger the ‘thing’ the more ‘stuff’ that ‘thing’ will have inside it.

We can also say that the smaller the ‘thing’, the less ‘stuff’ that ‘thing’ will have inside it. A big car has more ‘stuff’ inside it than a small car.

A big car has a greater mass than a small car.

1930's big car and small car next to each other

An elephant has more ‘stuff’ inside it than a mouse. An elephant has a greater mass than a mouse.


The moon has less ‘stuff’ inside it that the earth. The moon has a smaller mass than the earth.

Moon and Earth from space showing the contrast in sizes

The earth has less ‘stuff’ inside it than the sun. The earth has a smaller mass than the sun.

Earth shown next to the sun demonstrating vast difference in sizes

Sometimes the ‘stuff’ inside an object is packed very close together… like the stuff inside these bar bells lifted by this Olympic athlete.

On planet earth, if ‘stuff’ is packed very close together it can feel very heavy.

Li Hongli of China competes in the men's 77kg weightlifting event at the Beijing 2008 Olympics

At other times the ‘stuff’ inside an object is packed wide apart. The ‘stuff’ inside this imitation mammoth bone is made of styrofoam.

On planet earth, if stuff is packed wide apart and made of styrofoam it can feel very light.

mammoth bone made of styrofoam

Science projects -what do we mean by ‘stuff’?

We mean the very, very, very small atoms that are found in everything.

Atoms are so small that in the width of one of the hairs on your head or in one grain of sand there are one million atoms!!

You can find out a little more about atoms in Static Electricity Advanced Version.

In this science article we do not look at atoms any further…we just need to know that inside an object with more ‘stuff’ there will be more atoms. Inside an object with less ‘stuff’ there will be fewer atoms!

Sometimes atoms are packed very close together and sometimes they are packed wide apart!

atom with electrons in orbit

Science projects- an introduction to gravity

  •  Look at this diagram of a boy standing in different places around the world. He is holding a ball. What will happen to his ball if he drops it at each of the places he is standing in? Why does this happen?

boy dropping football in different locations around the world

  • Now look at this diagram of a boy standing on top of the world. Imagine a hole has opened up which passes right through the center of the earth. The boy drops the ball into the hole. What now happens to that ball?**

boy dropping ball in hole through centre of the world

Science projects – so what do we mean by ‘the force of gravity’?

We mean a ‘force of attraction’ between two objects. Each of two objects has a different mass. For you to notice any gravitational force there has to be a very big difference between the ‘mass’ of the two objects.

sky diver free falling upside down in the sky

The mass of planet earth is far greater than the mass of this skydiver. The skydiver is being pulled towards the center of the earth. Of course it is 100% certain that the skydiver will never reach the center of the earth, but that is where the force of gravity is pulling him!

With the ‘force of gravity’ there has to be some ‘motion’ or ‘movement’- an object with a small mass being attracted towards the center of  another object with a far greater mass.

Even when people are just walking along a street or sitting down, they are still being pulled downwards to the center of the earth!

silver human statue sitting on a cube watched by a crowdlondondailypicture.com

So what does the earth’s gravitational field ‘look like’? Just like the force of magnetism, the force of gravity is invisible! This diagram gives you an idea of what the earth’s gravitational field might appear.

Key stage 2 science-The Earth, a space satellite and a lattice simulating gravity in 4 dimensions

The closer you get to the earth the stronger the force of gravity. The further away from earth, the weaker the force of gravity. You can see an example of how much stronger the force of gravity is closer to earth here at spaceplace.nasa.gov.

Us humans have a smaller mass than the earth, so the force of gravity is pulling us to the center of the Earth which is 3959 miles (or 6371 km) from the surface of the earth.

  • What stops us from being pulled to the center of the earth?

two people hiking in the Tyrol Alps , Austria

On planet earth the force of gravity changes depending upon where you are.

Earths gravitational field-strongest gravitational force shown red, weakest in blue

The strongest gravitational ‘pull’ is shown in bright red and the weakest in dark blue. You can see the how the earth’s force of gravity changes here at BBC Science.

The gravitational force of the earth changes because the mass of the rocks inside the earth changes.

Where the ‘stuff’ inside the rocks is packed close together and the mass is greater, the force of gravity is stronger. Where the ‘stuff’ inside the rocks is loosely packed and the mass is smaller, the force of gravity is weaker.

  • Where on earth is the force of gravity strong and where is it weak?

Science projects- how gravity affects us

Gravity affects us humans in everything we do……from the very dangerous

Time delay sequence of a person diving into the sea near dangerous rocks

to the fun…..

Two sportsmen throwing and catching an American footballImage Credit:wikihow
to the illegal…..

hand throws litter out of a car

Gravity affects nature…

Key stage 2 science leaves falling from tree in fall

…and animals!

polar bear diving into water surrounded by ice

.. however an object has to be truely MASS-IVE before it is able to pull another much smaller object towards it. If you were standing at the front (on the bow) of one of these tugs the mass of the supertanker would still be far too small to pull you towards it so you stick to its hull!

Knock Nevis supertanker alongside tugs

Force of gravity -earth’s gravitational attraction of the moon

The force of the earth’s gravitational field keeps the moon orbiting the earth.

moon taken from space showing upper atmosphere and curvature of the earth

It is amazing to think that the moon is 238,900 miles (384,400 km) away!

Bryce Canyon National Park Utah with image of full moon against backdrop of mountains

The moon, with its smaller mass, has an effect on the earth. From a distance of 238,400 miles the gravitational force of the moon creates the tides…

Fishing port at Bay of Fundy, Alma, New Brunswick showing contrast between high tide and low tide

The force of the moon’s gravity pulls the earth’s seas towards the moon, as you can see in this diagram;

Diagram showing how tide on part of earth facing the moon

Force of gravity- the sun’s gravitational attraction of the planets

Objects with a large mass can attract objects with a smaller mass across huge distances. The earth is 93 million miles (150 million kilometers ) away from the sun, yet the sun’s gravitational force keeps our planet orbiting the sun!

Key stage 2 science- All the planets in the Solar System

The force of the sun’s gravity is even known to affect the Oort Cloud- a mass of icy particles and rocks which is 6 trillion miles (10 trillion kilometers) from the sun!

The sun , outer planets and the Oort cloud

Science games and science experiments

  • Drop ‘meteorites’ of different masses onto the ‘moon’. Check the diameters of different sized impact craters at sciencebuddies.org
  • Balance a pencil on your finger to find the ‘center of gravity at lovemyscience.com

** After the boy drops the ball into the hole it falls to the center of the earth where it stops.With gravity, the object with the smaller mass is attracted to the center of the object with the bigger mass.

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