Force
Force is a fundamental concept in science, because force is what makes objects change their motion. This notion is summarized in Newton's second law, which states that "the net force on an object equals its change in momentum over time." The mass of an object is usually constant (but not always, such as the case of a rocket launched toward space, whose mass is decreasing as its fuel is burned), Newton's law can be stated in its more familiar form: F=ma. The net force on an object equals it mass multiplied by its acceleration.
Note that both force and acceleration are vectors, typically designated in bold. They have both direction and magnitude.
Many forces can act on an object— gravity, electromagnetic attraction between electrically-charged bodies, outside forces that push or pull, etc. Knowing them, we can determine the acceleration of the object. Using the calculus invented by Isaac Newton and Gottfried Wilhelm Leibniz in the 17th century, it is then possible to calculate the path the object will take as time transpires. This may be an easy calculation--as in an small object falling due to the force of the Earth's gravity--or it may be an extremely complicated calculation, as in the precise determination of the path a space probe should take through the solar system in order to reach a distant planet.
Your weight is the force you feel due to the earth's gravity. Since you are not accelerating there must be some balancing force(s), and in this case it is the ground or floor, which exert an "equal but opposite" force upward. Force is usually measured in "newtons," but also in pounds. One pound equals 4.45 newtons.
Statics is the study of systems which are at rest. Given a system at rest, a typical problem is the determination of all the internal forces and how they balance. An important example is the design of a bridge, where it is necessary to ensure that all supporting columns, arches, beams, etc. can withstand the force they must bear.
If an object rotates in a circle, such as a rock on a rope swung around overhead, it tends to want to travel in a straight line (that is, if the rope broke, the rock would immediately begin to travel straight ahead). The force needed to hold a particle in a circular path is called the centripetal force. In this example, it is exerted by the hand, through the rope.
In this example, the force the hand feels pulling it outward is called the centrifugal force. It is often referred to as a "fictitious force," since it is experienced by the hand only in certain reference frames--so called accelerating reference frames. Consider a rider in a windowless car, for example, and suppose there is a cup on the dash. The rider considers the cup to be at rest, since it is not moving with respect to him. But if the car accelerates forward, the cup slides and the rider must exert a force on it to keep it on the dash--a centrifugal force. From the rider's point of view, no motion has taken place (since the car is windowless), yet a force need be exerted on the cup. Such fictitious forces do not usually occur in the usual analysis of physical problems.
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