Isaac Newtons three laws of motion, first published in 1687, describe the relationship between a force acting on a body and the bodys motion due to that force.
The first law says that a body moving with a given speed will maintain that speed in a straight line unless a force acts upon it no force means no acceleration. The second law states that a force (F) will make a body accelerate by an amount (a) that's inversely proportional to its mass (m): F=ma. The third law says that whenever one body applies a force (the action force) to a second one, the second one simultaneously applies an equal and opposite reaction force on the first. Stepping off a boat onto a pier will make the boat move away, for instance.
Newton showed that these laws neatly explain the orbits of the planets around the Sun when combined with Newtonian gravity (see page 14). But they are not valid for objects moving at very high speeds or in very intense gravitational fields, when relativity theory is required (see pages 16 and 18).
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