Newton’
s first law states that, if a body is at rest or moving at a constant
speed in a
straight line, it
will remain at rest or keep moving in a
straight line at constant
speed until it is acted upon by a force. This postulate is known as the law of inertia. The law of inertia was first formulated by Galileo Galilei for horizontal motion on
Earth and was later generalized by René Descartes. Before Galileo it had been thought that all horizontal motion required a direct cause, but Galileo deduced from his experiments that a body in motion would remain in motion unless a force (such as friction) caused it to come to rest.
Newton’s second law is a quantitative description of the changes that a force can produce on the motion of a body. It states that the time rate of change of the momentum of a body is equal in both magnitude and direction to the force imposed on it. The momentum of a body is equal to the product of its mass and its velocity. Momentum, is a vector quantity, having both magnitude and direction. A force applied to a body can change the magnitude of the momentum, or its direction, or both.
Newton’s second law is one of the most important in all of physics. For a body whose mass m is constant, it can be written in the form F = ma, where F (force) and a (acceleration) are both vector quantities. If a body has a net force acting on it, it is accelerated in accordance with the equation. Conversely, if a body is not accelerated, there is no net force acting on it.
spittintard:
Newton’s third law states that when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction. It applies to bodies in uniform or accelerated motion. The forces it describes are real, not mere bookkeeping devices.
Newton’s laws first appeared in his masterpiece, commonly known as the Principia. In 1543 Nicolaus Copernicus suggested that the
Sun, rather than
Earth, might be at the centre of the universe. In the intervening years Galileo, Johannes Kepler, and Descartes laid the foundations of a
new science that would both replace the Aristotelian worldview, inherited from the ancient Greeks, and explain the workings of a heliocentric universe. In the Principia
Newton created that new science. He developed his three laws in order to explain why the orbits of the planets are ellipses rather than circles, at which he succeeded, but it turned out that he explained much more. The series of events from Copernicus to
Newton is known as the Scientific Revolution. In the 20th century
Newton’s laws were replaced by quantum mechanics and relativity as the most fundamental laws of physics. Nevertheless,
Newton’s laws continue to give an accurate account of nature, except for very small bodies such as electrons or for bodies moving close to the speed of light. Quantum mechanics and relativity reduce to
Newton’s laws for larger bodies or for bodies moving more slowly.
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