Lunar and Planetary Institute

# Weighing In the Planets

Students gain a better understanding of the concept of gravity.

The Activity

Step 1: Examining the Data
Show.students the weight chart and the planetary data chart and ask them to examine the information.

• Why does our sample student weigh different amounts on the different planets?  (answer: different gravity)
• Why might someone weigh so much on the Sun and Jupiter, or the same amount for Mercury and Mars?  Which planet characteristics might be important?

Answer:  The objects include the Sun and Moon and planets; we call them “planetary bodies.” The weight depends on the planetary body’s gravity, which is proportional to the mass of the planetary body and inversely proportional to the squared radius of that planetary body.

Step 2: Calculate the Weight Factor
The “Weight Factor” here is the ratio of what someone weighs on another planetary body to what they weigh on Earth.  Ask the students what they think they would need to do to find the “weight factor” for each planet and the Sun and Moon, using the data they have so far.

• What could they do with the weights on the planets and the 100 lb weight on Earth?  (add, subtract, multiply, divide?) Which answers make the most sense—should the “weight factor” for Jupiter be bigger or smaller than the “weight factor” for Earth?

Answer: To get the “Weight Factor” or the ratio for how much someone weighs on the other planet, compared to their weight on Earth, they should divide their weight for each planetary body by 100 pounds.

Step 3: Calculate Your Own Weight on Each Planet
Multiply another weight—possibly your own -- by each planet’s “Weight Factor” to get a new weight for each planet.

• If a student who weighs 100 pounds on the Earth weighs 40 pounds on Mercury, how much would a 90 pound student weigh on Mercury?

Extensions

Have each student calculate a range of weights (say 35-40 Earth pounds) for each of the planets, then combine their answers to create a chart of weights by planets.

Have the students compare their weights for each planet.  Why might they weigh the same on both Mars and Mercury?  Do any of the answers surprise them?