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Sir Isaac Newton

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Sir Isaac Newton is an English mathematician and physicist, considered one of the greatest scientists in history, who made important contributions to many fields of science. His discoveries and theories laid the foundation for much of the progress in science since his time. Newton was one of the inventors of the branch of mathematics called calculus (the other was German mathematician Gottfried Wilhelm Leibniz). He also solved the mysteries of light and optics, formulated the three laws of motion, and derived from them the law of universal gravitation.

Newton ignored much of the established curriculum of the university to pursue his own interests: mathematics and natural philosophy. Proceeding entirely on his own, he investigated the latest developments in mathematics and the new natural philosophy that treated nature as a complicated machine. Almost immediately, he made fundamental discoveries that were influential in his career in science.

Newton's Three Laws of Motion

Newton's first law of motion states that if the vector sum of the forces acting on an object is zero, then the object will remain at rest or remain moving at constant velocity.

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Friction acts like a force applied in the direction opposite to an object's velocity. For dry sliding friction, where no lubrication is present, the friction force is almost independent of velocity. Also, the friction force does not depend on the apparent area of contact between an object and the surface upon which it slides. The actual contact area—that is, the area where the microscopic bumps on the object and sliding surface are actually touching each other—is relatively small. As the object moves across the sliding surface, the tiny bumps on the object and sliding surface collide, and force is required to move the bumps past each other. The actual contact area depends on the perpendicular force between the object and sliding surface. Frequently this force is just the weight of the sliding object. If the object is pushed at an angle to the horizontal, however, the downward vertical component of the force will, in effect, add to the weight of the object. The friction force is proportional to the total perpendicular force.

Where friction is present, Newton's second law is expanded to


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Another conserved quantity of great importance is angular (rotational) momentum. The angular momentum of a rotating object depends on its speed of rotation, its mass, and the distance of the mass from the axis. When a skater standing on a friction-free point spins faster and faster, angular momentum is conserved despite the increasing speed. At the start of the spin, the skater's arms are outstretched. Part of the mass is therefore at a large radius. As the skater's arms are lowered, thus decreasing their distance from the axis of rotation, the rotational speed must increase in order to maintain constant angular momentum.

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