Sir Isaac Newton's Laws of Motion
Sir Isaac Newton's laws of motion began a revolution in the science of physics that lasted from the 17th century until the present day.
Newton's laws of motion, originally given in the Principia are:
- Newton's first law of motion. The first of Newton's laws of motion states: An object moves in a straight line at the same speed unless acted upon by an external force. This is also known as Galileo's law of inertia.
- Newton's second law of motion. The second of Newton's laws of motion states: The force applied to an object is equal to the mass of the object multiplied by the acceleration induced by the force.
- Newton's third law of motion. The third of Newton's laws of motion states: For every action there is an equal and opposite reaction. That is, if a force is exerted on object B by object A, an equal and opposite force is exerted on object A by object B.
We can combine Newton's laws of motion to describe the progress of a sprinter. (Assume there's no air or ground resistance). The sprinter needs to fix her blocks firmly to the track because when she hurls herself forwards there is an equal force applied backwards onto the blocks (3rd law). The force the sprinter generates divided by her mass gives us her acceleration from the blocks (2nd law). Between the blocks and her first step she is coasting at a constant velocity (1st law).
An object moves relative to some observer, but you could equally say the observer is moving relative to the object. People on a plane moving at a constant velocity do not feel that it is in motion, just like we do not feel the earth is in motion. If they look out of the window it appears to them that the earth is moving - and it is, relative to them.
If a child jumps up and down on his seat in the plane, this event seems to be happening in the same position in space. But to someone on the ground, the child looks to be jumping up and down over a town one minute, and over the sea the next minute. His position in space relative to the earth varies by several miles every few minutes.
So, well before Einstein, Newton's laws of motion implied a relativity of space. But he did not accept this himself. He believed in absolute space, like Aristotle. For Aristotle the absolute frame of reference was the Earth, for Newton it was some absolute, fixed frame in which the Earth and Sun moved.
Also like Aristotle, Newton believed in absolute time. They assumed that anyone would measure the same time between two events, even if one were on earth and the other travelling to the stars at close to the speed of light. This was shown to be fallacious by *Albert Einstein.
Almost as important as Newton's laws of motion was Newton's universal law of gravitation.
