The Key People

Comprehending the physics of movement took mankind a long time. The reason for this is that our own senses can be confused by different sorts of motion, all happening at the same time. Unravelling the reality from the intutive perception needed careful observation by some special people, who now have their place in history. Following the scientific reasoning through history will help us understand the true nature of movement and what that tells us about our Universe.

From the Aristotle (384-322 BC) to Nicholas Copernicus (1473-1543)

The ancient Greeks, largely as a result of Aristotle's teaching, thought the Earth was stationary at the centre of the Universe, and the heavens rotated around the Earth. This was the view that was accepted by most people until a Polish priest called Copernicus published a book that suggested that the Sun was at the centre of the Universe. The book was called De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres), and the planets went around it in circlular orbits. That was in 1543.


Johannes Kepler (1571-1630)

Astronomers of the time Copernicus had collected a lot of data about the movement of the heavens and they found that his description matched their observations better than the old Greek theory. But the match wasn't perfect, and in 1609 Johannes Kepler produced a new model, which turned Copernicus' circles, into elipses - stretched circles. The match between observation and prediction was now almost perfect. Kepler's idea became accepted over the next half century, but he could not explain why the heavens were seen to move as they did, but not how.

Kepler also formulated mathematical laws for predicting the motions of orbiting objects. These can be found here.

Galileo Galilei ( 1564-1642)

Galileo was the first astronomer to use a telescope and he confirmed that Kepler was correct when he discovered four moons going round, or orbiting, the planet Jupiter. But Galileo wasn't just interested in astronomy, he wanted to explain the rest of physical world too. He made two important observations.

First, he noted that things, no matter what they were made of, always fall at the same rate. He demonstated this by simultaneously dropping different weight objects from the Leaning Tower of Pisa, so the story says.

He is also credited with the idea of "Galilean relativity", which is nothing but his perception that by watching the motion of objects in an enclosed room, there is no way to tell if the room is at rest, or whether it is, in fact, in a boat on, say, a canal, moving along at a steady pace in a fixed direction. Everything looks and behaves the same in a room under steady motion as it does in a room at rest.

((Right) A replica of Galileo's telescope is in the London Science Museum. © J W Hodges

Isaac Newton (1642-1727)

Isaac Newton knew of the work of all his predecessors, and this helped him formulate two great theories that changed the way we think.

These were the Universal Laws of Gravitation and the Laws of Motion.

Today, these ideas are considered to be the backbone of physical science. They were major break-throughs that set history on a new course, and has let us start mankind's greatest adventure - the exploration of space.

Now lets try and understand both these laws.


Universal Laws of Gravitation

Many people have heard the story of Newton and the apple. Whether it is true is matter of speculation, but one thing is certain, Newton's laws of gravity explained why the planets and heavens are arranged and move in the way they do.

Newton said that anything in the Universe that has mass, has a property of attraction for any other mass in the Universe. This attraction is a force that wants to pull masses together. The size of the pull depends on how big the masses are, and how far apart they are. A full mathematical explanation is given here.

Laws of Motion

To understand motion, lets take a step back to Galileo. Consider his moving and stationary rooms. Why can you not tell the difference?

You may have witnessed the effect Galileo describes on a bus or train when another bus is next to you and your bus moves away...

Or is it the other bus that is moving?

It is difficult to tell sometimes...

You are easily fooled, because your senses contradict one another. You usually believe your eyes and they can mislead you.

Note, however that if you close your eyes, you can usually tell if your "room" is accelerating - changing speed or turning around or heaving up and down. You will certainly know this if you suffer from motion-sickness!

So why can you tell if you are changing speed or direction and not whether you are moving (at a speed in a straight line)?

The great leap of knowledge that Newton made was in the understanding of the role of Force in making objects change their rate or direction of movement. We call a change in movement rate, Acceleration. When something speeds up, slows down or changes the direction it is moving, it is said to be Accelerating. Slowing down, is of course just negative acceleration. We call this Deceleration, but it is essentially the same thing. A force is being applied whether we accelerate or decelerate.

.... And we can feel or sense force! Even quite tiny forces.

Now, every object in the Universe - such as you - (or any substance, such as a cloud of gas or a raindrop) has the property of mass. Mass is a measure of the quantity of matter in something. We have a good idea of what mass is even if we can't define it precisely in words!

Think of a mass - the bus will do. Now let us imagine that it is moving. If we multiply the mass by its velocity - speed in a straight line - we can say the bus has a certain Momentum. In other words, momentum is mass that is moving. Now you will understand that there is a major difference between a moving bus and a stationary bus, at least in terms of trying to get it moving in the first place, or in trying to stop it if it is moving. You need to apply Force.

Newton understood this and was able to write down three simple laws of physics that relate Momentum to Force.

Basically, Newtons Laws of Motion say three things.

    To change momentum a force must be applied.

    The applied force is proportional to the change in momentum.

    Every force that is applied produces a resisting force, equal and in opposite direction to the applied force.

It is possible to create a mathematical formula from the Second Law:


where F is the applied Force, m is the mass being accelerated and a is the acceleration. This basic relationship is the foundation for all calculations associated with things, such as space vehicles, in motion. Without this knowledge the engineers and scientists of NASA, ESA and the other space agencies would not be able to send spacecraft to the distant planets.

Albert Einstein (1879-1955)

Einstein formulated his two great theories in the early part of the twentieth century. Like Newton's theories they were about motion and gravity. They allowed context and limits to be put on the entire Universe - the vast, active Universe that telescopes were beginning to let astronomers see at that time. The consequences of Einstein theories are profound on our understanding of the macro - large scale - Universe, but for all practical everyday purposes they need not be considered by most of us for most of the time.

But since Einstein's Laws of Relativity, isn't Newton wrong? The answer is no! Newton's laws can be considered as a special case of Relativity Theory, where speeds are low and the strange effects produced by high speeds are not apparent or important enough to be relevant in any calculation. There is no point in using the complexity of Einstein's ideas to predict the motion of objects in our neighbourhood of space. On a grander scale - looking out into the depths of the cosmos, things are a little different, though. But while we can see, we can't reach that far yet!

It should be remembered that Newton's laws were used to calculate the trajectories - the flight-paths - of all the spacecraft that have visited the outer reaches of the Solar System - the Pioneers, Voyagers, Galileo, Cassini and so on. The calculated trajectories, using Newtons Laws, proved to be highly accurate in every case. One Voyager even passed through a small gap in Saturn's rings, without incident. And that was using Newton's laws of motion over a distance of more than 1,280,000,000 km (800,000,000 miles).

Motion in Space

After Newton formulated his Laws of Motion, describing how objects move in response to forces; Galileo's observation were reformulated in a slightly more technical, but equivalent way, by renaming the room as a calibrated "frame of reference". It was now said that the Laws of Physics are the same in a uniformly moving frame of reference, as they are in a frame at rest. The same force produces the same acceleration, and an object experiencing no force moves at a steady speed in a straight line in either case. In short, as Einstein's Theory of Special Relativity puts it,

The Laws of Physics are the same in all Inertial Frames.

Let's try and understand what we mean by an Inerial Frame.

What exactly do we mean by a frame "at rest" anyway? This seems obvious from our perspective as creatures who live on the surface of the Earth - we mean, of course, at rest relative to fixed objects on the Earth's surface. Actually, the Earth's rotation means this isn't quite a fixed frame, and also the Earth is moving in orbit at nearly 30 km per second. From an astronaut's point of view, then, a frame fixed relative to the Sun might seem more reasonable. However, the Sun is located in an outer spiral arm of the Milky Way Galaxy, and these arms are rotating. Similarly, our Galaxy is one in the Local Cluster, and each of these is moving in relationship to one another. Likewise, the local cluster is in motion too, as are all the other clusters of galaxies. In other words, nowhere is at rest and as we know from the astronomer Edwin Hubble, the Universe is expanding. Overall, everything is moving away from everything else. So, as we believe the laws of physics are good throughout the Universe, we can take any point we like, in space, as the origin or start position, for our frame of reference.

Suppose we now use this frame of reference and check that Newton's laws still work. In particular, we check that the First Law holds - that a body with no force acting on it moves at a steady speed in a straight line. This First law is often refered to as The Principle of Inertia, and a frame in which it holds is called an Inertial Frame. Then we set up another frame of reference, moving at a steady velocity relative to the first one, and find that Newton's laws hold true in this frame as well. The point to note here is that it is not at all obvious which - if either - of these frames is "at rest". We can, however, assert that they are both inertial frames, after we've checked that in both of them a body with no forces acting on it, moves at a steady speed in a straight line.

This business of frames of reference may seem a little theoretical and unnecessary, but for any scientific theory to be valid it must go hand in hand with observational measurement, and this is what we are trying to do here - we are trying to set some start point, some initial conditions, some position of reference for our observations.

Once we have this we can begin to understand the true nature and behaviour of the Universe.

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