Expressing F in newtons we now get a-for any acceleration, not just for free fall-as "īy Newton's second law, the acceleration a of an object is proportional to the force F acting on it and inversely proportional to its mass m. "When you measure what you are speaking about and express it in numbers, you know something about it, but when you cannot express it in numbers your knowledge is of a meager and unsatisfactory kind. Lord Kelvin, leading British scientist in Queen Victoria's era, was quoted as once saying We now can express in numbers the dependence of acceleration on force and mass. Earlier this was called "a force of one kilogram of weight, " a convenient unit for rough applications (1 kg = 9.8 newton), but not for accurate ones, because of the variation of g around the globe. Equation (1) not only shows that weight is proportional to mass, but-assuming it is measured in kilograms-it introduces a unit of F, named (no surprise!) the " newton."īy that equation, a force of one newton acting on one kilogram of mass accelerates it by 1 m/sec 2, so the force of gravity on one kilogram of mass is about 9.8 newtons. In the MKS system the effective value of g varies from 9.78 m/s 2 on the equator to 9.83 m/s 2 at the poles, due to the Earth's rotation (see section #24a). if by mistake you mix MKS units with grams or centimeters (or pounds and inches), and you may end up with some mighty strange results! That convention is known as the MKS system: as long as one's formulas contain only quantities derived by that system, they will be consistent and correct. Let us therefore choose from now on to measure distance in meters, mass in kilograms and time in seconds. Yes, proportionality allows one to add on the right some constant multiplier, but we won't, because now we want to define some units of F.Īll quantitative formulas and units in physics depend on the units in which three basic quantities are measured- distance, mass. Where g is the acceleration of gravity, directed downwards. The force of gravity is proportional to mass m, so we can write In today's terms we say that both weight and inertia are proportional to the mass of the object, the amount of matter which is contains.Ĭonsider free fall due to gravity. Newton proposed that the reason was that although the force of gravity on the heavier object (its weight) was twice as large, so was its inertia. -If air resistance can be ignored, a light object falls just as fast as one twice as heavy.-The acceleration of such an object is limited by its own resistance to motion, which Newton named its inertia.-In the absence of opposing forces, if a force does act on an object at rest or moving at constant speed, it accelerates in the direction of the force.-In the absence of opposing forces, if no force acts on an object at rest or moving at constant speed, it continues to do so indefinitely ( Newton's first law).We can therefore measure force in grams or kilograms, units of weight, and loosely define force as "anything that can be matched by weight" (e.g. -The most familiar force is weight, the downward force on an object due to gravity.-A force is the name given to whatever causes motion.Isaac Newton on the (former) British pound noteįor more money bills with famous physicists, click here
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