Apples and moons
According to myth, Isaac Newton was sitting under an apple tree
when, upon being hit on the head by a falling apple, he discovered
the law of gravity—implying maybe that the knock on the head
produced the flash of insight. (Apple falls—boink—light bulb
lights up above head and, hmm, it appears that the ground is
exerting a force on the apple pulling it downwards.) Of course it
was not that simple. Newton was not the first human to ever notice
that things fall down! His insight was much more impressive than
that.
It may well be a myth that an apple actually fell on Newton’s
head, but by Newton’s own account it was contemplating a falling
apple on his mother’s farm (along with other things such as
why the Moon goes round the Earth) that led him to his famous
universal law of gravitation. What was it about the falling apple
that Newton could see that all others before him could not? Stated
as simply as possible, he saw beyond the obvious—that all objects
had a tendency to want to move downwards towards the Earth—
and realized that there was a force of attraction between the apple
and the Earth that not only caused the apple to fall downwards
towards the Earth but the Earth to fall upwards towards the apple. In
fact, it is better not to think in terms of objects falling but rather
that the Earth and the apple are attracted towards each other.Outgoing, friendly, popular, family man. All these traits were
quite alien to Isaac Newton. Born inWoolsthorpe in Lincolnshire,
UKon Christmas day 16421, he was a loner who never married and
whodid not have many friends. Hebecame, later in life, embroiled
in lengthy and bitter disputes with other scientists about who had
reached certain discoveries first. However, despite the negative
image of scientists in general in today’s popular media which
sadly puts so many teenagers off the subject, Newton was most
certainly not a typical scientist. What he lacked in social skills he
made up for by being, in many people’s view, the greatest scientist
who ever lived. He made so many important contributions to
so many fields that most of the physics taught at school today is
known as Newtonian physics. This is to distinguish it from the
modern physics of the twentieth century that will be discussed in
this book. Newton also invented the mathematical technique of
calculus which is the standard tool for studying most of physics
today. The discovery of calculus, however, was the cause of a
long-running controversy. The dispute was whether Newton or
the German mathematician Gottfried Leibnitz could lay claim to it.
In the scientific circles of the time the dispute, in which the English
and the Germans each claimed that the other man had stolen their
man’s ideas, took on a patriotic fervour akin to the modern-day
rivalry of the two countries’ footballing encounters. However,
unlike the modern all too frequent penalty shoot-out resolutions,
in the battle of calculus there was no clear winner. Each had, it
seems, developed the technique independently. In any case, most
of the ground work had been laid down half a century earlier by
the great French mathematician Fermat.
Back to gravity. Long before Newton, it was realized that
the reason objects fall is because the Earth exerts a force on all
things that pulls them towards it. It was also known that the
Moon orbits the Earth because the Earth exerts some mysterious
force on it stopping it from floating off into space. Newton made the connection between these two phenomena. Attributing the
motion of the Moon and the falling apple to one and the same force
(gravity) was a bold stroke of genius. Until then it was believed
that entirely different laws of nature governed the behaviour of
earthly objects (apples) and heavenly bodies (the Moon).
Newton’s law of gravity states that any two objects in the
Universe will be attracted towards each other by an invisible force.
The Earth and each and every object on its surface, the Earth and
the Moon, the Sun and the planets, even the Sun and the rest of
our galaxy, are all being pulled towards each other. Thus it is not
just the Earth that keeps us stuck to its surface; in a sense, we are
keeping the Earth stuck to our feet since we are pulling the Earth
towards us with as much force as it is exerting on us. When I
said earlier that the Earth falls upwards towards the falling apple,
I meant it quite literally. It is just that, being stuck to the surface
of the Earth, we see the apple moving towards the Earth. But the
apple has just as much right in claiming (inasmuch as apples have
rights) that it is not moving at all and that it is the Earth that moves
towards it.
Likewise, a man and a woman floating close together out
in empty space will be physically attracted towards each other—
even if they are not ‘physically attracted’ to each other!—by a
gravitational force that will cause them to slowly drift even closer
together. This force will, however, be very weak (equivalent to
the tiny force needed to pick up a single grain of sugar if they had
started off a few centimetres apart). The force of gravity is very
weak when the masses involved are small.
How is it that the same force of gravity that causes the apple
to fall does not pull the Moon down to Earth too? The difference
between the two cases is that, despite the Moon’s much greater
mass, it is in orbit around the Earth and at any moment is moving
in a direction that is a tangent to its orbital path, whereas the apple
is moving towards the Earth’s centre. This is actually a rather
bad way of putting it. A better definition of ‘in orbit’ is to say
that the Moon is falling towards the Earth in a curve that forms a
circular path around the Earth so that it never manages to get any
closer. WhenNewton first calculated this during the plague year of 1666 he thought he had got the wrong answer and, disappointed,
refrained frompublishing his results. It was only many years later,
when discussing the problem with his friend Edmund Halley (he
of comet fame) that he realized the importance of his discovery.
Newton’s law of gravity has been tremendously successful for
over three hundred years. Note that it is known as a law of gravity,
since scientists were so sure it was the last word on the subject
they elevated it above a mere theory that could be dismissed if
and when something better came along. But that is precisely what
did happen in 1915. The name was Einstein. Albert Einstein.
According to myth, Isaac Newton was sitting under an apple tree
when, upon being hit on the head by a falling apple, he discovered
the law of gravity—implying maybe that the knock on the head
produced the flash of insight. (Apple falls—boink—light bulb
lights up above head and, hmm, it appears that the ground is
exerting a force on the apple pulling it downwards.) Of course it
was not that simple. Newton was not the first human to ever notice
that things fall down! His insight was much more impressive than
that.
It may well be a myth that an apple actually fell on Newton’s
head, but by Newton’s own account it was contemplating a falling
apple on his mother’s farm (along with other things such as
why the Moon goes round the Earth) that led him to his famous
universal law of gravitation. What was it about the falling apple
that Newton could see that all others before him could not? Stated
as simply as possible, he saw beyond the obvious—that all objects
had a tendency to want to move downwards towards the Earth—
and realized that there was a force of attraction between the apple
and the Earth that not only caused the apple to fall downwards
towards the Earth but the Earth to fall upwards towards the apple. In
fact, it is better not to think in terms of objects falling but rather
that the Earth and the apple are attracted towards each other.Outgoing, friendly, popular, family man. All these traits were
quite alien to Isaac Newton. Born inWoolsthorpe in Lincolnshire,
UKon Christmas day 16421, he was a loner who never married and
whodid not have many friends. Hebecame, later in life, embroiled
in lengthy and bitter disputes with other scientists about who had
reached certain discoveries first. However, despite the negative
image of scientists in general in today’s popular media which
sadly puts so many teenagers off the subject, Newton was most
certainly not a typical scientist. What he lacked in social skills he
made up for by being, in many people’s view, the greatest scientist
who ever lived. He made so many important contributions to
so many fields that most of the physics taught at school today is
known as Newtonian physics. This is to distinguish it from the
modern physics of the twentieth century that will be discussed in
this book. Newton also invented the mathematical technique of
calculus which is the standard tool for studying most of physics
today. The discovery of calculus, however, was the cause of a
long-running controversy. The dispute was whether Newton or
the German mathematician Gottfried Leibnitz could lay claim to it.
In the scientific circles of the time the dispute, in which the English
and the Germans each claimed that the other man had stolen their
man’s ideas, took on a patriotic fervour akin to the modern-day
rivalry of the two countries’ footballing encounters. However,
unlike the modern all too frequent penalty shoot-out resolutions,
in the battle of calculus there was no clear winner. Each had, it
seems, developed the technique independently. In any case, most
of the ground work had been laid down half a century earlier by
the great French mathematician Fermat.
Back to gravity. Long before Newton, it was realized that
the reason objects fall is because the Earth exerts a force on all
things that pulls them towards it. It was also known that the
Moon orbits the Earth because the Earth exerts some mysterious
force on it stopping it from floating off into space. Newton made the connection between these two phenomena. Attributing the
motion of the Moon and the falling apple to one and the same force
(gravity) was a bold stroke of genius. Until then it was believed
that entirely different laws of nature governed the behaviour of
earthly objects (apples) and heavenly bodies (the Moon).
Newton’s law of gravity states that any two objects in the
Universe will be attracted towards each other by an invisible force.
The Earth and each and every object on its surface, the Earth and
the Moon, the Sun and the planets, even the Sun and the rest of
our galaxy, are all being pulled towards each other. Thus it is not
just the Earth that keeps us stuck to its surface; in a sense, we are
keeping the Earth stuck to our feet since we are pulling the Earth
towards us with as much force as it is exerting on us. When I
said earlier that the Earth falls upwards towards the falling apple,
I meant it quite literally. It is just that, being stuck to the surface
of the Earth, we see the apple moving towards the Earth. But the
apple has just as much right in claiming (inasmuch as apples have
rights) that it is not moving at all and that it is the Earth that moves
towards it.
Likewise, a man and a woman floating close together out
in empty space will be physically attracted towards each other—
even if they are not ‘physically attracted’ to each other!—by a
gravitational force that will cause them to slowly drift even closer
together. This force will, however, be very weak (equivalent to
the tiny force needed to pick up a single grain of sugar if they had
started off a few centimetres apart). The force of gravity is very
weak when the masses involved are small.
How is it that the same force of gravity that causes the apple
to fall does not pull the Moon down to Earth too? The difference
between the two cases is that, despite the Moon’s much greater
mass, it is in orbit around the Earth and at any moment is moving
in a direction that is a tangent to its orbital path, whereas the apple
is moving towards the Earth’s centre. This is actually a rather
bad way of putting it. A better definition of ‘in orbit’ is to say
that the Moon is falling towards the Earth in a curve that forms a
circular path around the Earth so that it never manages to get any
closer. WhenNewton first calculated this during the plague year of 1666 he thought he had got the wrong answer and, disappointed,
refrained frompublishing his results. It was only many years later,
when discussing the problem with his friend Edmund Halley (he
of comet fame) that he realized the importance of his discovery.
Newton’s law of gravity has been tremendously successful for
over three hundred years. Note that it is known as a law of gravity,
since scientists were so sure it was the last word on the subject
they elevated it above a mere theory that could be dismissed if
and when something better came along. But that is precisely what
did happen in 1915. The name was Einstein. Albert Einstein.
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