## Sunday, 21 July 2013

### Science: The Gravity Test

What is Gravity?
It is the force that attracts any object with a mass to the centre of Earth.  What causes gravity is still unknown.

The Earth has Gravity, it is what holds everything down on the planet: trees, water, animals, buildings and even the air we breathe are all held here by the invisible force of gravity.

Another amazing fact is that all the planets, the stars and the moons in our universe have gravity.  Even our own bodies have gravity but we don't notice our own gravity as the Earth's gravity is much, much stronger.

So is gravity affected by the mass of an object?

When we say mass we do not mean its weight.  Mass is really the amount of material or 'stuff' an object is made from.  Weight is actually the result of gravity pulling on the mass.  Mass is measured in grams and weight is measured in ounces and pounds.

Your mass never changes, it stays the same; you could travel from planet to planet and your mass would remain the same. Your weight would change depending on how the gravity of the planet pulls on you.

So let's experiment...

Take two objects the same size but with a different density.  We chose a block of balsa wood and a block of play dough...

Use a Metre rule and measure and mark (we used Blu-tack and paper arrows) 2 metres, 1 metre and 50cm high up a wall, like this...

Use a Metre rule and measure and mark (we used Blu-tack and paper arrows) 2 metres, 1 metre and 50cm high up a wall, like this...

In your science journal you might want to draw out a table like this...

and then write in your hypothesis (guess what you think will happen) before you physically perform your experiment.  Are you right?

Write your conclusion:  You will discover that the proximity (the closeness) of the object is a factor on how gravity affects an object that is falling or to put it another way the closer the object is to the ground, the stronger the gravitational pull is.  Even though then there is hardly any difference between the two items even though one seems heavier than the other.

This makes sense as the Earth and the moon have a stronger  pull on each other than the Earth and the planet Mars because the earth and the moon are closer to one another.

This was the topic of a famous story about scientist Galileo Galilei who is said to have tried dropping two objects from the Leaning Tower of Pisa to find the answer. Surprisingly the two objects hit the ground at almost the same time, a bit like in our gravity test. Gravity works this way on all objects unless wind resistance gets in the way, which is what slowed the balsa wood down. So if one of those balls had been attached to a parachute, it would have slowed the ball down.