The velocity of the boat relative to the water is 7 m/s
The velocity of the boat relative to the cyclist is 11 m/s
The velocity of the cyclist relative to the water is 4 m/s
All velocities seem to be relative to something else.
Introduction
From our work in AS we should be familiar with the idea of relative motion.
Consider the diagram below.
| The velocity of the boat relative to the bank is 5m/s The velocity of the boat relative to the water is 7 m/s The velocity of the boat relative to the cyclist is 11 m/s The velocity of the cyclist relative to the water is 4 m/s All velocities seem to be relative to something else. |
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Another important principle in physics, first suggested by Galileo, is that the laws of physics apply for any non-accelerating reference frame.
| If you drop a ball it falls to the floor directly below
you at 9.8m/s2 The path of the ball is linear to both the person who dropped it and another stationary observer. |
If you are a fast moving train and you drop a ball it
behaves in exactly the same way. The fact that you are moving does not
affect the apparent motion of the ball. To a stationary observer the path of the ball would actually be parabolic, just like a projectile.
|
If you were in an accelerating train then the ball would
behave differently. The path of the ball would be parabolic relative to the person who dropped it..
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The ideas above are very much what we call Classical Mechanics.
What led Einstein to suggest that light does not obey these principles?
It was the work of several others which mainly led him in this direction including Hendrik Lorentz and Henri Poincaré. They developed the mathematical techniques (beyond the scope of these web pages) which pointed the way. Maxwell's Equations are a set of mathematical equations which describe the nature of electromagnetic waves. Applying the mathematical techniques of Lorentz and Poincaré to these equations suggests that the speed of electromagnetic waves in a vacuum may be a universal constant.
This was the mathematical backbone of Einstein's special theory of relativity.
You may have heard of Einstein's General Theory of Relativity. The difference between this and the special theory is that it takes into account gravity. Because of this it is much more mathematically demanding. Beyond me, sorry!