Determining weight
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From: Don1 on 28 Jul 2005 17:56 An object's weight (w) is the net force (f) that it exerts on a weight-scale or other support, divided by the acceleration of free fall (g) at that location: Mathematically it can be shown as: Weight=f/a/g; or more concisely as w=(fg/a). NOT as w=mg, because m=w/g=f/a. A quick, easy way to determine an object's weight is with a spring scale like those in the produce section of a grocery market, or a steelyard type of scale like doctors use; if they are made, and calibrated to show weight; which is the force due to gravity at Earth's surface. Put your old balance scales away with all of your other antiques. While you are at it, what _is_ the volume and weight of a slug of water at 39.2 degrees F, and atmospheric (sea level) pressure? Don
From: Sam Wormley on 28 Jul 2005 18:05 Don1 wrote: > An object's weight (w) is the net force (f) that it exerts on a > weight-scale or other support, divided by the acceleration of free fall > (g) at that location... Not in the lingo of physics and mathematics, Shead! Weight http://scienceworld.wolfram.com/physics/Weight.html Inertia http://scienceworld.wolfram.com/physics/Inertia.html The resistance to change in state of motion which all matter exhibits. It's a concept, Shead, not a number with units, not a ratio. Newton's First Law http://scienceworld.wolfram.com/physics/NewtonsFirstLaw.html Also called the "law of inertia," Newton's first law states that a body at rest remains at rest and a body in motion continues to move at a constant velocity unless acted upon by an external force. Newton's Second Law is about "inertial mass" http://scienceworld.wolfram.com/physics/NewtonsSecondLaw.html A force F acting on a body gives it an acceleration a which is in the direction of the force and has magnitude inversely proportional to the mass m of the body: F = ma Inertia is an intrinsic property of mass. Most of what follows is quoted from http://www.physlink.com/ae305.cfm Gravitational Mass F = GmM/r^2 Inertial Mass F = ma Acceleration a = dv/dt 1) Inertial mass. This is mainly defined by Newton's law, the all-too-famous F = ma, which states that when a force F is applied to an object, it will accelerate proportionally, and that constant of proportion is the mass of that object. In very concrete terms, to determine the inertial mass, you apply a force of F Newtons to an object, measure the acceleration in m/s^2, and F/a will give you the inertial mass m in kilograms. 2) Gravitational mass. This is defined by the force of gravitation, which states that there is a gravitational force between any pair of objects, which is given by F = G m1 m2/r^2 where G is the universal gravitational constant, m1 and m2 are the masses of the two objects, and r is the distance between them. This, in effect defines the gravitational mass of an object. As it turns out, these two masses are equal to each other as far as we can measure. Also, the equivalence of these two masses is why all objects fall at the same rate on earth. The only difference that we can find between inertial and gravitational mass that we can find is the method. Gravitational mass is measured by comparing the force of gravity of an unknown mass to the force of gravity of a known mass. This is typically done with some sort of balance scale. The beauty of this method is that no matter where, or what planet, you are, the masses will always balance out because the gravitational acceleration on each object will be the same. This does break down near supermassive objects such as black holes and neutron stars due to the high gradient of the gravitational field around such objects. Inertial mass is found by applying a known force to an unknown mass, measuring the acceleration, and applying Newton's Second Law, m = F/a. This gives as accurate a value for mass as the accuracy of your measurements. When the astronauts need to be weighed in outer space, they actually find their inertial mass in a special chair. The interesting thing is that, physically, no difference has been found between gravitational and inertial mass. Many experiments have been performed to check the values and the experiments always agree to within the margin of error for the experiment. Einstein used the fact that gravitational and inertial mass were equal to begin his Theory of General Relativity in which he postulated that gravitational mass was the same as inertial mass and that the acceleration of gravity is a result of a "valley" or slope in the space-time continuum that masses "fell down" much as pennies spiral around a hole in the common donation toy at your favorite chain store. Useful references for Shead http://scienceworld.wolfram.com/physics/Inertia.html http://scienceworld.wolfram.com/physics/MomentofInertia.html http://scienceworld.wolfram.com/physics/Mass.html http://scienceworld.wolfram.com/physics/Momentum.html http://scienceworld.wolfram.com/physics/NewtonsLaws.html http://scienceworld.wolfram.com/physics/Weight.html
From: Don1 on 28 Jul 2005 19:33 Sam Wormley wrote: > Don1 wrote: An object's weight (w) is the net force (f) that it exerts on a weight-scale or other support, divided by the acceleration of free fall (g) at that location: Mathematically it can be shown as: Weight=f/a/g; or more concisely as w=(fg/a). NOT as w=mg, because m=w/g=f/a. A quick, easy way to determine an object's weight is with a spring scale like those in the produce section of a grocery market, or a steelyard type of scale like doctors use; if they are made, and calibrated to show weight; which is the force due to gravity at Earth's surface. Put your old balance scales away with all of your other antiques. Don SNIP< You'd better learn some modern techniques Sam: Spring scales and steelyard type scales are the way weight is determined these days; from the three fundamental variables: Force (f), displacement (s), and time (t). Don
From: odin on 28 Jul 2005 19:43 > You'd better learn some modern techniques Sam: Spring scales and > steelyard type scales are the way weight is determined these days; from > the three fundamental variables: Force (f), displacement (s), and time Spring scales? You must be joking. Even for every day objects, a balance beam scale is better than a spring scale. It is too hard to calibrate for the spring's dependence on temperature, variations in gravitational field, buoyancy effects, and so on. And then there are the not-so-every-day objects. Do you really think that the spring scale is the "modern technique" for determining the mass of a subatomic particle or the mass of planet?
From: Don1 on 28 Jul 2005 20:56
odin wrote: > > You'd better learn some modern techniques Sam: Spring scales and > > steelyard type scales are the way weight is determined these days; from > > the three fundamental variables: Force (f), displacement (s), and time > > Spring scales? You must be joking. Even for every day objects, a balance > beam scale is better than a spring scale. It is too hard to calibrate for > the spring's dependence on temperature, variations in gravitational field, > buoyancy effects, and so on. And then there are the not-so-every-day > objects. Do you really think that the spring scale is the "modern technique" > for determining the mass of a subatomic particle or the mass of planet? Heck no. bui its quicker, easier and accurate enough for most everyday measures in an environmentally controled (air conditioned) laboratory or supermarket. Think a doctors steelyard scale isn't accurate enough. Think again buster, they are made and calibrated to determine the weight-forces acting on them; wherever they are used. The steelyard isn't a new method either; but is a tried and true weight-scale. Don |