Gravity and Buoyancy

Grades:
Length: 60 Minutes

Overview

Students learn about environments with gravity and those with reduced gravity by observing the behavior of a water-filled plastic bag, both inside and outside of a container of water.

This activity is from The Science of Muscles and Bones Teacher's Guide, aand was designed for students in grades 6–8. Lessons from the guide may be used with other grade levels as deemed appropriate.

Teacher
Background
Objectives and Standards
Materials and
Setup
Procedure and
Extensions
Handouts and
Downloads

Teacher Background

All organisms on our planet are adapted to living with gravity, the force that pulls objects toward the center of the Earth. Gravity keeps objects from floating into space and it is the reason why “what goes up must come down.” It is not exclusive to the Earth. Amazingly, all objects in the universe attract each other. The force of the attraction depends on the distance between the two objects and their masses. Gravitational forces are normally too tiny to notice, unless one of the objects has a lot of mass (such as a planet or moon).

Many students have difficulty with the concepts of mass and weight. All objects in the universe have mass, which can be understood as a measurement of how difficult it is to set an object in motion or to stop it once it is moving. The mass of an object, measured in kilograms, is constant no matter where the object is.

Weight, on the other hand, varies with the amount of gravity and can be measured in units called “newtons” (named after the famous physicist). On Earth, something with a mass of 1 kg weighs about 10 newtons. On the Moon, where gravity is less, the same object still has a mass of 1 kg but weighs less than two newtons. It is important to note, however, that in everyday language people are much more likely to say that “something weighs two kilograms.” For ease of understanding, in this guide we use the words “weigh” and “weight” in their everyday sense instead of their strictest scientific interpretation.

Understanding the difference between mass and weight is important if you go into space. Deep in space, something can be virtually weightless because it is too far away from other objects to be affected by their gravity. An object in orbit around Earth (or other celestial body) also is weightless, but for a different reason. Though this object is close to the Earth, it circles the planet at a velocity that overcomes the downward pull of Earth’s gravity. In other words, orbiting bodies fall freely toward the Earth, but because they have so much forward speed, their trajectories follow the curvature of the Earth’s surface.

This activity allows students to observe and compare the pull of gravity on water contained within a plastic bag when the bag is standing alone and when it is submerged in water, at which time, the force of gravity is counteracted by buoyancy.

Objectives and Standards

Concepts

Gravity holds us to the Earth’s surface.
The force of gravity can be counteracted by other forces.

Science, Health and Math Skills

Measuring
Predicting
Observing
Comparing
Inferring

Materials and Setup

Materials per Group of Students (see Setup below)

Clear container with straight sides that holds at least 1 liter of water (or a glass aquarium in a central location)
Food coloring
Paper towels
Plastic zip-top bag, snack-size
Water
Copy of the student sheet

Setup

Depending on time and your students’ ages, you may want to fill the bags for students. Fill each bag with as much water as it will hold and add a drop of food coloring. Zip the top tightly closed, while removing as much air as possible.
Place the bags and other materials in a central location.
Have students work in groups of four.

Safety

Please follow all school district and school laboratory safety procedures. It always is a good idea to have students wash hands before and after any lab activity.

Procedure and Extensions

Begin a class discussion of gravity by asking questions such as, What keeps us and other objects from floating off the Earth and into space? What happens when you throw a ball into the air? Does it fly into outer space? How could we explore the pull of the Earth on objects near its surface? Tell students that they will be investigating gravity in action.
Have the Materials Manager from each group collect a container of water and a water-filled plastic snack bag, or have students fill the bags following the directions given under Set-up and Management.
Tell students that they will be investigating the behavior of the water bag in two different environments: resting on a flat surface and floating in water. They should record their predictions and observations on a copy of the student sheet.
Have each group predict what will happen to the shape of the bag when it is placed on a hard, flat surface. Let each group set its bag on the table and record the bag’s appearance. Groups may choose any orientation for their bags (on the side or with zip top “up” works best). Students will note that the bottom of the bag is flattened. Ask, Why do you think the bottom of the bag is flat? What would happen to the water if it wasn’t in the bag? What would happen to the bag if it wasn’t filled with water?
Next, have the students predict what might happen when the bag is placed in the water. They should consider where they think the bag will sit in the container (floating on the surface, at the bottom, etc.), and what shape they think the bag might have
After they have made their predictions, direct students to place the bags gently in the containers of water. They should orient their bags in the same position that was selected for the observations on the table.
After each group records its observations, ask, What happens to the shape of the bag in the water? Students will observe that the lower surface of the bag is not flattened in the water. Also ask, Where does the bag rest in the water? Unless the bags contain large air bubbles, they will float completely or almost completely submerged in an upright or sideways position. Help students understand that the bags float freely under water because buoyancy counteracts the downward pull of gravity. On the table, however, gravity is able to pull the water within the bag toward the Earth’s surface without the counteraction of buoyancy.
Conclude by leading students in a discussion of what the water in the bags might look like in a microgravity environment, such as in space. Help them understand that water bags in space probably would look similar to the bags as they floated under water OR discuss what might happen if they tried to weigh the bags under water, using a small scale. Students should be able to predict that they would be unable to weigh the floating “underwater” bag.

Extensions

Challenge students to come up with other examples in which gravity’s pull is counteracted. Examples include: flight of birds and insects, hot air balloons, kites and airplanes, jumping into the air (temporarily overcomes gravity), fish swimming upward, etc.
Have students visit NASA’s web site (www.nasa.gov) to investigate how astronauts practice tasks underwater to prepare for future work in space.
If students have not investigated buoyancy prior to this activity, help them understand concepts related to floating and sinking by using snack bags filled with sand, water, air and any other substances. Students should weigh each bag, including the one with water, and predict which bags will float and which will sink. Any bags that weigh more than the bag of water will sink. Bags that weigh less than the bag of water will float on the surface.