# PBS Teachers™

Activity 2: How Much Do You Weigh in Outer Space? (Grades 5-8)

Concepts Home | How Long is a Light Year? | How Much Do You Weigh In Space? | The Capacity of Planets | More Math Concepts

Objective:
The students will demonstrate the ability to apply the use of decimals, fractions, ratios, and proportions to situations based upon given information about planets in the solar system.

NCTM Standards:

• Understand numbers, ways of representing numbers, relationships among numbers, and number systems;
• Understand the meaning of operations and how they relate to each other;
• Use computational tools and strategies fluently and estimate appropriately

Note to teachers:
For this activity, students will be using a chart to compute their weight on different planets and the sun. Students also need a scale to measure their current weight. If you teach students for whom weight is a sensitive subject, modify the activity so that it focuses on the weight of a car, for example, or some other object. Alternatively, students might complete the assignment at home for extra credit.

How Much Do You Weigh In Outer Space?

Gravity is a universal, natural force that attracts objects to each other. Originally defined by Isaac Newton and later redefined by Albert Einstein, gravity is basically the natural force of attraction between two objects. Two factors determine the magnitude of the gravitational force between two objects: 1) their masses and 2) the separation distance between them. Gravity is the pull toward the center of an object, for example a planet or a moon. When you weigh yourself, you are measuring the amount of gravitational attraction exerted on you by Earth. The moon has a weaker gravitational attraction than the Earth, so you should weigh less on the moon.

Isaac Newton showed that the planets do not fly off into space because the gravitational attraction between the sun and each planet hold them close to each other. This attracting force exists between objects because of their mass. The greater the mass, the greater the attraction of gravity. Since every planet has mass, every planet exerts a gravitational force on nearby objects.

For this activity, we say that planets have gravity, but actually what we really mean is that there is a gravitational force of attraction between the planet and a person standing on that planet's surface. Therefore, people have different weights on different planets.

Complete the chart below (show all work) and then answer the questions that follow.

 PLANET Multiply your Earth weight by: Your "new" weight Mercury 0.4 Venus 0.9 Earth 1 Moon 0.17 Mars 0.4 Jupiter 2.5 Saturn 1.1 Uranus 0.8 Neptune 1.2 Pluto 0.01

1. Identify a planet that has a similar gravitational attraction to that of Earth.

2. List the planets' gravitational attraction from least to greatest.

3. Compare the multiplication factors in the chart. Judging from these factors, which planet probably has the greatest mass?

4. Another student claims that the Moon's gravity is 1/6 of the Earth's gravity. Is this a true statement? Look at the chart and use mathematics to support your answer.

5. If your doctor told your aunt that weighing 165 pounds at 5'4" makes her 20 pounds overweight, on which planet would this be an acceptable weight? Justify your answer.