Physics land ^^

Thursday, July 1, 2010 @ 6:47 AM

* Enrichment Notes

Mass
In physics, mass refers to any of three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass and passive gravitational mass. In everyday usage, mass is often taken to mean weight, but in scientific use, they refer to different properties.
The inertial mass of an object determines its acceleration in the presence of an applied force. According to Isaac Newton's second law of motion, if a body of mass m is subjected to a force F, its acceleration a is given by F/m.
A body's mass also determines the degree to which it generates or is affected by a gravitational field. If a first body of mass m1 is placed at a distance r from a second body of mass m2, the first body experiences an attractive force F given by
enrichment
In the International System of Units (SI), mass is measured in kilograms (kg).
The gram (g) is 1⁄1000 of a kilogram.

Mass&Weight relationship
On the surface of the Earth, the weight W of an object is related to its mass m by

where g is the acceleration due to the Earth's gravity, equal to about 9.81 m s−2. An object's weight depends on its environment, while its mass does not: an object with a mass of 50 kilograms weighs 491 newtons on the surface of the Earth; on the surface of the Moon, the same object still has a mass of 50 kilograms but weighs only 81.5 newtons.

All Physicists Agree That:
Weight and mass are not synonymous.
Weight is a force.
The weight of an object depends on its location - things can weigh differently in different places.
Weight is NOT Mass
The first thing to realize about weight is that weight is not the same as mass, even though the terms are used synonymously in everyday life. Mass measures inertia - it is a property of an object. Weight is a force - something that happens to an object.
Weight is a Force - But WHICH Force?
Physicists and engineers all agree that weight is a force, but there is considerable disagreement and confusion about what force weight is - different textbooks say different things, and some textbooks say different things in different places. Mostly, opinion is divided into two camps. For some people, "weight is the force of gravity", and for others "weight is what a scale reads". You might think that the two statements are equivalent - but they aren't. There are advantages and (unfortunately) disadvantages with either point of view.
Weight as Gravitational Force
Our text, among others, says:
Weight: The force of gravity upon a body.1
Advantages:
Weight is always easy to calculate. The weight of an object is w = mg, where m is the mass of the object, and g is the acceleration of free fall.
Disadvantages:
This is not the intuitive, laymen's definition of weight. You can't feel the force of gravity. You feel the floor pushing up on you, not the downward pull of the Earth. You don't feel pulled toward the Earth, even when you jump into the air.
Weight as What a Scale Reads:
There is a standard definition of weight, given by the International Organization for Standardization (ISO) which says:
The weight of a body in a specified reference system is that force which, when applied to the body, would give it an acceleration equal to the local acceleration of free fall in that reference system. - ISO 31-3 "Quantities and Units. Part 3, Mechanics", 19922
Can this be simplified without losing precision? Yes! It turns out that this definition of weight is equivalent to saying:
Weight is what a scale reads.3
which is equivalent to saying:
An object's weight equals the force required to support it.4
Advantages:
It seems at first that this is a much more natural, practical and useful way to look at weight than associating weight directly with the force of gravity. ("What a scale reads" and "the force of gravity" are not always equivalent - see The Elevator Problem for details...) This definition - weight is what a scale reads - also fits quite naturally with the idea of "weightlessness".
Disadvantages:
In order to use this definition effectively in applications requires a pretty sophisticated understanding of Newton's Laws - particularly Newton's Third Law - which most beginning physics students generally don't have.