Things very often come in two’s…..
You don’t lay a table without a salt and pepper pot!
Humans have two eyes!
Walruses have two tusks!
Tandem bicycles have two wheels and two riders.
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’.
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.
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.
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.
The earth has less ‘stuff’ inside it than the sun. The earth has a smaller mass than the sun.
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.
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.
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!
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.
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!
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.
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.
On planet earth the force of gravity changes depending upon where you are. In this image the strongest gravitational ‘pull’ is shown in bright red and the weakest in dark blue.
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.
Gravity affects us humans in everything we do……from the very dangerous
to the fun…..
Gravity affects nature…
.. 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!
The force of the earth’s gravitational field keeps the moon orbiting the earth.
It is amazing to think that the moon is 238,900 miles (384,400 km) away!
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…
The force of the moon’s gravity pulls the earth’s seas towards the moon, as you can see in this diagram;
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!
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!
** 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.