The related properties of mass and weight are
frequently confused. Mass is the measure of the inertia of an object, which
is essentially a measurement of the density of matter within it. The mass of
an object is constant, regardless of its location. Weight is a measurement
of the force an object feels due to gravitational attraction, for example
the pull a planet exerts on a person standing on its surface. The weight of
an object can change, as the masses of the objects involved change. For
example, a boy who weighed 30 kilograms on Earth would weigh only 5
kilograms on the Moon, because the mass of the Moon is only about 1/6th that
of the Earth.
The units of mass and weight are different. In the
metric system, grams and kilograms measure mass. The metric units of weight
are called dynes (gram equivalents) and newtons (kilograms equivalent).
These metric units are not used consistently, because on the surface of the
Earth, 1 dyne of weight is almost equivalent to 1 gram of mass. People
(scientists included!) often use grams and kilograms as units of weight. The
source of this confusion is historical, and is related to efforts to
equilbrate metric and English measurement systems. In the English system,
2.2 kilograms of weight are equivalent to 1 kilogram of mass. The students
will make use of this relationship in the lab exercise.
- In this exercise, the students measure items using
a spring balance, and then calculate what the itemís weight would be
on the different planets. The weight changes, as the masses of the
planets are different. The spring balances measure the weight of objects
in grams. This is an example of the confusion cited in the Background.
However, in the Lab, the students are calculating weight, not mass. Note
that the weight conversion for Pluto is poorly known, because no
spacecraft has visited the planet (yet).
- Explain weight and mass to the class. Make sure
that they understand the distinction between these properties. Tell them
that they will be calculating the weights of different objects on various
Solar System bodies.
- Since many students may never have seen a spring
balance, we recommend that you explain how to use one. Be sure to tell
them that it measures weight, not mass. Remind students that the springs
in the balances can be pulled out of shape easily, so they should not
weight objects that are heavier than 500 grams.
- For Exercise 1, give the students different objects
to weigh. Have them then convert the metric weights to ounces by
multiplying. This will give the students a way to "feel" the
relationship of grams to pounds.
- For Exercise 2, give the students the two items to
weigh. Have them measure their own weight as well (this can be done in
advance for homework) and then determine their own and the itemís
weights on other planets. Conversions are given in the
"calculation" column for each planet.