DO WE NEED TO DIET ON MARS?
What would you weigh if you were on Mars? Is there a difference between mass and weight? These questions and others will be answered as students are introduced to the concept of gravity as it applies to astronomy, and determine what their weight and mass would be on other planets.
Worksheet / Math
To explain the difference between mass and weight and how weight varies in different gravitational fields.
Students will be able to:
- Define mass vs. weight including their relationship to gravity
- Describe Earth's size in relation to the sun, moon, stars, and other planets
- Calculate their weight on other planets, using the student sheet provided
- Explain in their own words why they would weigh more on Jupiter and less on the moon, than they do here on Earth
Preparation: 20 min.
Activity: 45 min.
Discussion 10 min.
All objects, living or non-living, have both mass and weight. Mass is always constant and does not change, unless we physically remove the "stuff" by cutting off an arm or by dieting. Mass is a measure of the amount of "stuff" that makes up objects. Weight is a measure of how much that "stuff" is pulled by gravity. An object's mass is the same on Earth, on other planets and in space. However an object's weight does change from planet to planet and in space. If a person who weighs 150 pounds has 68.25 kilograms of "stuff" and travels to other planets, the gravity on those planets changes and that person's weight on that planet changes too. Our mass always stays the same from planet to planet.
All mass has an amount of pull. Larger objects have more pull than smaller objects do. When an object is located close to another object, both are slightly attracted to each other, even if we do not notice it. For more information on this subject refer to the activity "Great Galactic Glue".
Larger bodies (objects with more mass) have more "pull" towards Earth than smaller bodies do, so not everything is pulled at the same rate. This "pulling" is actually gravity. The larger the object the more gravity it has affecting it and the more it pulls towards other objects.
We don't notice our own individual gravity even though we have it because Earth's pull overwhelms an individual's pull. Similarly, Earth's gravity is minimal compared to the sun. Even though gravity keeps planets in orbit, it is thought to be one of the weakest forces known. It takes an extremely large object with lots of mass, like a moon, star, or planet, to exert a noticeable pull on another object.
Large bodies pull other objects towards the center of that mass at an even force in all directions. Planets are spherical because of this. There is an equal pull on all sides of a sphere; no one side is being pulled more than another side. However, the farther you move from the center of the body, the weaker the gravitational force. What you and I call "up" is actually just away from the center of the sphere we call Earth. Obviously, "up" to someone on the other side of the planet is a much different direction than it is to us.
Gravity is what keeps Earth in orbit around the sun, and the moon in orbit around Earth. The sun has gravity much greater than Earth's because of its size. Likewise, the moon has much less gravity due to its smaller size. If we were to walk on the moon, it would comparatively feel like we were floating in the air, only partly weighted down by our own mass. Even though the moon's gravitational pull is weak, it does affect fluids on Earth's surface. Waves and tides are affected by the moon's gravitational pull.
A planet's composition also has an effect on the amount of pull a planetary body has on another object. Black holes are bodies whose mass is incredibly great, many times more than that of our sun, yet has a incredibly small volume. (Volume is the space an object takes up). The pull of gravity on the existing mass caused the matter to collapse on itself into something more dense than anything to which we can relate. The pull of gravity around a black hole is so intense that it will pull anything within its vicinity towards it, including photons, or light. Because it also pulls in light, we can't see it. That is why we call it a black hole. We can only hypothesize as to what a black hole in space actually looks like.
The mass of an object determines how much pull each object has. The more massive an object, the more its gravity pulls on other objects around it. Extremely massive objects like stars tend to produce the most gravity. Neutron stars with the same mass as our sun are about the size of the city of San Francisco.
The following activity provides students with information about different planet's pull on objects and allows them to determine the student's weight on those planets. A worksheet is provided in Adobe PDF form along with the needed calculations.
- Have students define mass and weight, as they understand it. Explain that mass does not change, but weight will change based on the amount of relative gravity.
- Show students a diagram of our solar system and have the students identify Earth. Discuss how each of the planets are different sizes and have different masses.
- Ask students to define gravity, as they understand it. Explain that all things have gravity, not just our planet.
- On the student sheet, have students determine their mass on Earth and then on each planet in kilograms.
- Have students then predict what they would weigh on each planet or star on the student sheet: more, less, the same.
- Using a calculator and the chart provided with this lesson plan, have students calculate their weight on each planets or stellar object.
Why do you think we would weigh less on the moon? How about on Jupiter? Would we need to go on a diet if we were on Mars? Jupiter? Why not? Why doesn't our mass change? Do you have your own gravity? If we were weightless floating around in space about three feet apart from each other with no gravity acting on us, what would happen? Who would have more gravity? Would we eventually bump into each other? What object that you can see from Earth everyday has the largest amount of gravity that affects our lives? Why don't we just get pulled into the sun?
Included National Parks and other sites:
Utah Science Core:
3rd Grade Standard 1 Objective 1, 2
3rd Grade Standard 3 Objective 1, 2
3rd Grade Standard 4 Objective 1, 2
6th Grade Standard 3 Objective 3