Universe Cycle - Solar System (6)

  • Exploring the difference between mass and weight.
  • Calculating the weight of objects on different planets.
  • mass
  • spring balance
  • weight

Students calculate the weight of objects on different planets


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.

  1. 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).
  2. 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.
  3. 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.
  4. 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.
  5. 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.

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