Article

1. Gravity For Kids
Gravity is the force that keeps us from floating off into space. It literally keeps our
feet on the ground! We feel the effects of gravity every second of our lives
(unless we’re astronauts), but most of us don’t give it a second thought. This
article sets out to explain gravity for kids – and anyone else who has ever
wondered why they fall down, rather than up.
(See more Science Articles.)
Before we get started, here’s a question:
If I had a 1kg weight in one hand, and a 100g weight in the other hand, and
dropped them at exactly the same time, which would fall the fastest and therefore
land first?
You’ll find the answer lower down the page!
If you dropped these weights at the same time, which of them would reach the
ground first?
Gravity For Kids: Introduction

2. Let’s start at the beginning.
If you drop a stone, it falls to the floor. Why is this?
Everyone knows that the answer is ‘Gravity’!
But what is gravity? It’s more than just a word!
Gravity is more than just a word – it’s what’s pulling these skydivers towards the
Earth!
The real answer is that a force – a force we call gravity – is acting on the stone,
and pulling it towards the Earth.
Gravity is, quite simply, a force of attraction between objects. The stone is
‘attracted’ to – pulled towards – the Earth.
Thanks to Isaac Newton, we know that:
The more mass an object has, the bigger the force of attraction.
And …
The closer something is, the bigger the force of attraction.

3. The Earth, having a very large mass, and being very close, exerts a strong pull on
anything in its vicinity, including:
a stone when you drop it,
your body when you fall over,
and something large and round flying above your head …
… The moon!
But why doesn’t the moon come crashing into the Earth? And why didn’t Neil
Armstrong come falling towards the Earth when he took his ‘small step for a man’?
We’ll get to that. But first we’re going to take a look at how we came to
understand gravity.
Gravity for kids – it’s what keeps your feet on the ground!
The Discovery of Gravity

4. Back to the question from the top of the page:
If I had a 1kg weight in one hand, and a 100g weight in the other hand, and
dropped them at exactly the same time, which would fall the fastest … and
therefore land first?
The answer (which I hope you got right) is that they would both fall at the same
speed (or, more specifically, with the same acceleration), and both would hit the
ground at the same time.
Don’t worry if you got it wrong. Even Aristotle (who lived from 384 to 322 BC)
believed that heavier objects fall faster than lighter objects, so you’re in good
company!
In fact, most people believed this until around 1638, when Italian astronomer
Galileo Galilei proved that objects fall at the same speed – whatever their weight
(or, strictly speaking, their mass. See this article to find out the difference).

5. Galileo’s experiments helped us to
understand gravity.
Note: An astronomer is someone who studies ‘celestial bodies’, such as moons,
stars, planets, comets and galaxies.
Another note: We refer to Galileo Galilei by his first name, for the very good
reason that that was how he referred to himself, as was the custom in Italy at the
time.
The story goes that Galileo proved his theory by dropping different-sized balls
from the leaning tower of Pisa.

6. He found that the speed at which an object falls is determined not by how heavy
it is, but by how long it has been falling.
Leaning Tower Of
Pisa – Where Galileo is said to have performed his gravity experiments.
Galileo’s Experiment Repeated … On The Moon!
In 1971 astronaut Dave Scott performed a similar experiment to Galileo’s. Not
from the leaning tower of Pisa, but on the moon. Scott dropped a feather and a
hammer, and – surprise, surprise –both reached the surface of the moon at the
same time.
Galileo calculated the rate at which objects accelerate while falling to Earth. That
rate is now known to be 9.81 m/s² – although this figure changes slightly
depending on whereabouts on Earth it is measured.
The figure also ignores the effects of air resistance and any other factors which
may create anything other than a perfect, frictionless fall.
Galileo increased our understanding of gravity, but even he couldn’t explain why
it was that things fell to the ground in the first place.

7. Isaac Newton
Cambridge University, England, where Isaac Newton studied.
It was up to Englishman Isaac Newton – arguably the greatest scientist who ever
lived – to realise what gravity actually was.
Like all scientists, Newton used the work of other scientists as a basis for his own
theories.
German astronomer Johannes Kepler had worked out the rotation of the planets
round the sun (using observations made by Danish astronomer Tycho Brahe).

8. Using Kepler’s laws of planetary motion, the location of any planet can be
calculated.
Using Kepler’s laws of planetary motion, Newton realised that gravity was a force
of attraction, and the size of the force depended both on the mass of the objects
involved, and the distance they were from each other.
Newton’s Law Of Universal Gravitation
Newton’s law of universal gravitation, published in 1687 states that ‘any two
bodies in the universe attract each other with a force that is directly
proportional to the product of their masses and inversely proportional to
the square of the distance between them’.
The law can be written as an equation:
F = G ((m1xm2) / r²).
The force (F) between two objects of masses m1 and m2 is equal to the product
of their masses divided by the square of the distance (r) between them.
G is the gravitational constant. It remains the same wherever it is applied in
the Universe.

9. Using Newton’s laws, we can find out why two objects of different mass fall with
the same acceleration.
Basically, if the mass of an object is doubled, the gravitational force is doubled,
but the rate of acceleration remains the same.
If the mass of an object is halved, then the gravitational force is halved, but again
the rate of acceleration remains the same.
Using Maths to Find Planets
Scientists realised that Neptune had to exist – before it was even discovered!
Using Newton’s Law Of Universal Gravitation, scientists were able to calculate the
presence of Neptune before it was ‘properly’ discovered.
In the 1820s it was found that Uranus (then the most distant discovered planet)
was not where Newton’s laws suggested it should be.

10. Over the next decades, astronomers in England and France independently
concluded that Uranus’s movement was being affected by another, unknown,
planet, and calculated where this other planet would appear. This planet was
Neptune, and it was found in 1846, proving that Newson’s law was correct.
It was Neptune’s gravitational pull that was affecting the orbit of Uranus.
Gravity, The Moon, And Tides
Tides are caused by the Moon’s gravitational pull.
We’ve found that gravity is the force of attraction between two objects.
Every particle of matter is attracted to every other particle in the galaxy, but the
force exerted between individual particles is very small, because their mass is very
small.
But with large objects, such as the moon and the Earth, the force of attraction is
very strong.

11. So strong, in fact, that it’s enough to keep the moon spinning around the Earth,
rather than flying off into space.
(The mass of the Earth is around 6 million million million million kg.)
Because the moon is smaller, and less dense, than Earth, it has a significantly
smaller mass. Therefore the force of gravity on the moon is much smaller. Around
17% of the Earth’s, in fact.
Neil Armstrong’s weight on the moon was only 17% of what it was on Earth. This
enabled him to jump higher than he could on Earth, but was still enough to bring
him back to the Moon’s surface.
Although the mass of the moon is much less than that of Earth’s, it still affects the
Earth. The force it exerts as it orbits the Earth pulls the oceans towards it, causing
tides.
Gravity keeps this satellite in orbit above the Earth.

12. Gravity for Kids Conclusion
We’ve covered a lot in this article. Thanks to Galileo you’ve found that all objects
(a stone, you, a grand piano) fall at the same rate, no matter what their mass.
Again, thanks to Galileo, you’ve found that on Earth, objects accelerate at 9.81
m/s² (give or take) when dropped, and that this figure varies ever so slightly
depending on where you are on the Earth.
Newton’s work showed us that gravity is a force of attraction between objects,
and is dependent both on the masses of the objects involved, and on the
distance they are from each other.
From this you know that the force of attraction increases for an object with more
mass, and decreases for an object with less mass, meaning that their acceleration
is a constant, G.
We’ve found that the moon is responsible for pulling the seas towards it, making
the tides go out and come back in again.
Why not read more about the lives and work of the scientists in the article? Many
of them made other incredible discoveries that were not just related to gravity.