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Laws of Motion
Roemer discovers that the speed of light is a constant
Albert Einstein's Fundamental idea of relativety
Demonstration the effect of "Free Fall"
See how light rays follow curves in spacetime
Einstein imagines riding on a beam of light
For many years Aristotels was seen as the great philosopher , with a great understanding of the universe , however , as science became more popular and people got a better understanding of space and time mathematicians started to explore many of known science by applying mathematical formulas. In this respect Isaac Newton played an important role.
Laws of Motion - featuring Isaac Newton and Galileo Galilei
The Aristotelian tradition held that one could work out all the laws that govern the universe by pure thought : It was not necessary to check by observation so no one until Galileo Galilei bothered to see weather bodies of different weight did in fact fall at different speeds. Galileo Galilei rolled balls of different weights down a smooth slope. His measurements indicated that each body increased its speed at the same rate no matter what its weight. Galileo Galilei measurements were used by Newton as the basis of his laws of motion. In Galileo Galilei experiments proved that as a body rolled down a slope it was always acted on by the same force (its weight). This showed that the real effect or weight is a force always to change the speed of the body , rather than just set it moving, as was previously thought. It also meant that whenever a body is not acted on by any force it will keep on moving in a straight line at the same speed. This observation is known as Newton's first law.
What happens to a body when a force does act on it is given by Newton's second law. This states that the body will accelerate , or change its speed , at a rate that is proportional to the force. You can see this with cars. The more powerful the engine ,the greater the acceleration , but the heavier the car , the smaller the acceleration .Newton's law of gravity tells us that the further apart the bodies, the smaller the gravitational force. If the law were that gravitational attraction of a star went down faster with distance, the orbits of the planets , would not be elliptical , they would spiral into the sun ...
Roemer discovers that the speed of light is a constant
The fact that light travels at a finite , but very high speed was first discovered in 1676 by the Danish astronomer Ole Christensen Roemer . He observed that the times at which the moons of jupiter appeared to pass behind Jupiter were not evenly spaced, as one would expect if the moons went around Jupiter at a constant rate. Roemer noticed that eclipses of Jupiter's moons appeared later the further they were from Jupiter. He argued that this was because the light from the moons took longer to reach us when we were further away from the moons.
Roemer's achievement is not only proving that light travels at a finite speed , but also in measuring that speed , was remarkable coming as it did 11 years before Newton's publication of "Princepia Mathematica"
Albert Einstein's Fundamental idea of relativety
In 1905 a clerk in Swiss patent office , called Albert Einstein , put forward a much more elegant idea which he called the "Theory of Relativity". The fundamental postulate of the theory was that :
The law of science should be the same for all freely moving observers , no matter what their speed.
This was true for Newton's laws of motion , but now the idea extended to include Maxwell's theory , and the speed of light : all observers should measure the same speed of light , no matter how fast they are moving , this simple idea , has some remarkable consequences. Perhaps the best known are the equivalence of mass and energy , summed up , in Einstein's famous equation E = mc2 ( where "E" is energy , "m" is mass , and "c" is the speed of light) , and the law that nothing may travel faster than the speed of light.
This special theory of relativity , was inconsistent with the Newtonian theory of gravity , which said that objects attracted each other with a force that depended on the distance between them.
In 1915 Einstein made the revolutionary suggestion that gravity is not a force like other forces , but is a consequence , of the fact that space time is not flat , as had been previously assumed. It is curved or "warped" by the distribution of mass and energy in it.
Demonstration the effect of "Free Fall"
The effect of free fall can be demonstrated by imagining Einstein playing Ping - Pong in a tram suspended on a railway track on the edge of a cliff. A ping - pong ball appears to the observer to move differently , when it is not in free fall - When the tram is still on the rails. The Ping – Pong ball appears to move in an arc. When the tram go off the rails in its free fall , the ball appears to move in a straight line. Yet in the two situations , the ball arrives at the same time , in the same location in space time. It is just that the motion of the ball appears different to an observer in free fall.
