To weight or not to weight, that is the question!

Although the confusion over weight and mass is an old issue, it still has the potential to confuse students that are new to science.

In the simplest terms - the stuff or matter that makes up an object is its mass. Unless an object is scuffed, worn down, or is radioactive, its mass does not change.
On account of having mass, (stuff, matter) objects are tugged at by gravity. The resultant force felt by a body is called its weight. The Earth has mass, so everything else with mass is attracted by the Earth.

The weight of a body therefore depends on the gravitation of its environment - a body that is in the weaker gravitational field of the Moon weighs less. Similarly, it weighs more on Jupiter.

What actually causes confusion in science is this; the units of mass, the Kilogram (kg) and the pound (lb.) are used
conventionally as units of weight. A statement such as  "I bought 10 kg of groceries" is fine for everyday conversation. What it means in scientific terms is this - "I bought groceries with a mass that exerted a force of 98.1 Newtons (or 10 Kg x 9.81 m/s^2) force on the weighing scale". You can see why the short form was adopted for conventional use.

Our assumption that the force of gravity (9.81 m/s^2) is the same everywhere on Earth, lets us measure weight but pretend that it is mass - this being a "no harm, no foul" situation because the error is equal for everyone on Earth.
In other words, units of mass are used for a property of mass that translates to its weight.

NOTE
: A two-armed pan balance neutralizes the force of gravity by letting it act on both arms equally, effectively measuring the mass of an object. On the other hand a single-pan balance, or a spring balance, effectively measures the weight of the object. A two-pan balance used on an alien planet would show the same reading as it does on Earth, since it is effectively measuring mass. A spring balance would show a variance depending on the acceleration due to gravity.

Quoting a mass-weight reference from the State of Florida's
standards:

Mass is the amount of matter (or "stuff") in an object. Weight, on the other hand, is the measure of force of attraction (gravitational force) between an object and Earth.

The concepts of mass and weight are complicated and potentially confusing to elementary students. Hence, the more familiar term of "weight" is recommended for use to stand for both mass and weight in grades K-5. By grades 6-8, students are expected to understand the distinction between mass and weight, and use them appropriately.

Our convention of using mass units for weight works as long as everything is measured on Earth. In a future where interplanetary travel is common, the two pan balance may become the only acceptable way of measuring consumables for commerce. A can of paint bought from the Walmart in Iowa can only be sold to the base near Cydonia, Mars if mass is being measured, and not weight.

A kg on Mars is the same as a kg on Earth.

To experience these insights, try our free Mass & Weight app for iPad:

http://itunes.apple.com/us/app/exploriments-weight-mass-force/id483875230?mt=8