![]() For example, A man leaves his car parked outside his house. Therefore, if no force is applied and a body is in a state of rest, it will remain so if a body was in motion, it will continue to be in uniform motion at a constant speed. According to this first law, a body cannot change its state by itself for it to come to rest (initial velocity: 0) or uniform rectilinear motion, it is necessary for some force to act on it. Newton’s first law states that a body only varies its speed if an external force acts on it. Inertia is the tendency of a body to continue as it is. Newton’s First Law – The Inertia Principle This principle was adopted by Newton as the first of his three laws of motion: Newton’s laws marked a revolution within the field of physics. They formed the foundations of dynamics (part of the mechanics that studies movement according to the forces that originate it). Furthermore, by combining these principles with the law of universal gravitation, the laws of the German astronomer and mathematician, Johannes Kepler, on the motion of planets and satellites could be explained. The net force on the block can then be zero, and the block can move with constant velocity. For example, the push of our hand on the sliding block can exert a force that contracts the force of friction on the block, and an upward force of the horizontal plane contracts the force of gravity. We usually refer to the resultant of all the forces acting on the body as the “net” force. Fortunately, we need not go to the vacuum of distant space to study motion free of external force, because, as for as the overall transnational motion of the body is concerned, there is no distinction between a body on which no external force acts and a body on which the sum or resultant of all the external force is zero. ![]() The force of gravity will act on an object on or near the earth, and resistive forces such as friction on air resistance oppose the motion on the ground or in the air. It is difficult to find a situation in which no external force acts on a body. An external force is needed to set the body in motion, but no external force is needed to keep a body moving with constant velocity. We can now extrapolate and say that if all friction could be eliminated the body would continue indefinitely in a straight line with constant speed. You may have experimented with an air track, on which objects can be made to float on a film of the air such a device comes close to the limit of no friction, as even a slight tap on one of the gliders can send it moving along the track at a slow and almost constant speed. We find that the block decreases in velocity at a slower and slower rate and travels farther each time before coming to rest. Let us used still smoother blocks and surfaces and better lubricants. We note that the velocity decreases more slowly than before. Let us repeat our experiment, now using a smoother block and a smoother plane and providing a lubricant. We can argue against this idea, however, by reasoning as follows. This observation was used, in fact, to support the idea that motion stopped when the external force, in this case, the hand initially pushing the block, was removed. If we let the block slide along this plane, we note that it gradually slows down and stops. Let us place our test body, say a block, on a rigid horizontal plane. In his Principia Newton stated the three fundamental laws of motion, which are the basis of Newtonian mechanics. ![]() His three laws first presented (in 1686) in his Philosophiae Naturalis Principia Mathematica, usually called the Principia. ![]() He carried to full fruition the ideas of Galileo and others who preceded him. Isaac Newton, born in England in the year of Galileo’s death, is the principal architect of classical mechanics. It was not until the time of Galileo and Newton, however, that dramatic progress was made. History For centuries the problem of motion and its causes was a central theme of natural philosophy, an early name for what we call physics.
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