KEY QUESTION
How much would an object weigh on different planets?

POSSIBLE PRECONCEPTIONS
Some people think that weight is related to air pressure, an object’s distance from the Sun or that Earth is the only planet with gravity.

KEY CONCEPTS
An object’s weight depends on the object’s mass and the mass and radius of the large body it is near.  Thus, in general, objects will have different weights on planets with different masses and radii.

GENERAL INFORMATION
Grade Level: 6-12 (Middle-High School)
Group Size: Individual & whole class
Length of Activity: One class period
Setting: Indoors
Method: Teacher-guided discovery or demonstration
Focus: Gravity, planets
Skills: Observing, questioning, classifying, using numbers, controlling variables, interpreting data, using logic, inferring, predicting

MATERIALS LIST
       Two six packs of any soft drink (diet drinks are best)
       A metal punch capable of opening aluminum cans
       1,080 pennies ($10.80) (1,030 [$10.30] for the planets only)
       Marking pen
       Masking tape
       Optional-   Gram balance

PROCEDURE
Preparation
Leaving one can untouched, use the metal punch to open a coin-sized slit in the bottoms of 11 cans.  The object is to make the cans appear unopened. Do not remove any metal when opening the cans.  Drain the cans and rinse if necessary. (Rinsing is necessary if you used a sugared soft drink.  If you use diet drinks, the inside will not be sticky.)  Let dry for several days.

Fill the cans with pennies according to the following chart.  As you fill each can, place a piece of masking tape over the slit and write a code letter or number on the tape so you don’t lose track of which can represents which planet or moon.  This tape also covers the hole in the can's bottom.
 

Doing the Activity
Arrange the cans in random order on a demonstration table.  Explain to your students that each can represents a full can of soft drink – though they all look the same, they weigh different amounts because they represent what a full can would weigh on another planet or moon.  Pass the cans among the students.  Each student should have a chance to hold each can.  Challenge your students to match each can with the correct planet or moon.  If possible, allow the students to compare the cans over several days. 

After all the students have compared the cans, explain that gravity is a function of mass.   The more mass you have, the more gravity you have. However, the relative weight of an object depends not only the mass of the planet/moon (and therefore its gravity) but also on the planet/moon’s size. A large gaseous planet that isn’t very dense may have less surface gravity than a much smaller but denser planet.  The surfaces of the gaseous planets (Jupiter, Saturn, Uranus & Neptune) may be indistinct.  The radii used for the gas giants are the center of the planet to its cloud tops since gas giants have no solid surface, as we know it.

CLOSURE
Ask teams or individuals to match the cans to the appropriate planet or moon and ask them to explain how they arrived at that order.  Record the suggested matches on the board.  Post the correct order, and arrange the cans accordingly. Use your student’s explanations as a starting point to explain the reasons the cans are arranged as they are.  For example, a student might reason that since Uranus (pronounced Ur an us) is more massive than Earth, the can for Uranus would weigh more.  However, Uranus is much less dense than Earth and its radius is much larger so ‘surface’ gravity on Uranus is less than it is on Earth.

EXTENSIONS
This activity can be done with younger children using cans that represent only the nine planets and the Moon. A logical outgrowth of this activity is to have students calculate their own weights on the various solar system bodies.

BACKGROUND
The relative weight of an object on each planet or moon is calculated by dividing the planet or moon’s average mean density by Earth’s (Earth’s is 5.52 g/cm³). This percentage is then multiplied by the planet or moon’s diameter divided by that of Earth’s (12,756 km). Many basic astronomy texts report planetary diameters using the Earth’s diameter as 1.

You can substitute other types of cans or other objects instead of pennies with the following formulae. You can also use it to check your cans and their weights.