To explore the role of gravity in falling.
During the elementary years, force may be treated as the originator of motion. An explanation of force itself can be postponed until middle school, and even high school. It is important, however, to help students broaden their understanding of the fundamental forces of nature, with a particular emphasis on gravitational and electromagnetic forces. This lesson introduces students to gravity as a force, focusing on the concept of falling.
Elementary-school students typically do not understand gravity as a force. They see the phenomenon of a falling body as "natural" with no need for further explanation or they ascribe to it an internal effort of the object that is falling (Ogborn, 1985). If students do view weight as a force, they usually think it is the air that exerts this force (Ruggiero et al., 1985). Misconceptions about the causes of gravity persist after traditional high-school physics instruction (Brown & Clement, 1992) but can be overcome by specially designed instruction (Brown & Clement, 1992; Minstrell et al., 1992). (Benchmarks for Science Literacy, p.340)
Students of all ages may hold misconceptions about the magnitude of the earth's gravitational force. Even after a physics course, many high-school students believe that gravity increases with height above the earth's surface (Gunstone & White, 1981) or are not sure whether the force of gravity would be greater on a lead ball than on a wooden ball of the same size (Brown & Clement, 1992). (Benchmarks for Science Literacy, p.340)
The class should begin by discussing the motions of objects that they know about. Ask students to describe how these common objects move:
- Balls that have been thrown
- Sliding or rolling down ramps
- Pedaled bicycles
- Moving cars, boats, and planes
- Orbiting satellites
Students should describe the path that the motion would take as well as what might begin or stop the motion. The common characteristic of all the motions that students have experienced directly is that one or more forces are acting on the moving body.
All are being acted upon by the pull of the Earth's gravity. Most also are being acted upon by other forces (pushes and pulls) as well, for example, friction, motors, and human pushes or pulls. All of these pushes and pulls combined give each object its characteristic motion: the arc of the flight of a basketball or the path of a toy car down a ramp and across a level surface.
Students hold various meanings for the word "force." Typically, students think force is something that makes things happen or creates change. Their descriptions of force often include related words such as energy, momentum, pressure, power, and strength. Younger students associate the word "force" with living things (Watts, 1983b). In addition, students tend to think of force as a property of an object ("an object has force," or "force is within an object") rather than as a relation between objects (Dykstra, Boyle, & Monarch, 1992; Jung et al., 1981; Osborne, 1985).
Students also tend to distinguish between active objects and objects that support or block or otherwise act passively. Students tend to call the active actions "force" but do not consider passive actions as "forces" (Gunstone & Watts, 1985). (Benchmarks for Science Literacy, p.340)
The laws governing forces and motion are among the most fundamental concepts in natural science. Students can understand these concepts better when they explore them in the context of familiar, everyday life.
If an object is pushed off a table or held in one's hand and then released, it moves toward the center of the Earth. It is being pulled (as all other material objects are being pulled) by the force of gravity, the attraction between the masses of the two bodies, the object and the Earth. The movement of the object toward the Earth is called "falling."
To demonstrate to students that falling and gravity can be very useful, explore with students how "falling" is involved when they use:
- Playground slides
In their science journals, students should write an explanation for how falling is involved in each of the above.
Say to students, "Common sense suggests that heavy objects are pulled toward the Earth with greater force than lighter objects. That is why they feel "heavier"; they are pulling harder against our hands as they try to fall to Earth. Does this mean that a heavy object will fall faster than a light object?"
Students should discuss this and make a prediction based on their personal experiences.
Note: The term gravity may interfere with students' understanding because it often is used as an empty label for the common (and ancient) notion of "natural motion" toward the earth. The important point is that the earth pulls on objects. (Benchmarks for Science Literacy, p. 94)
Provide your students with the activity sheet called Falling for Gravity and ask them to try the suggested activities. They should use these experiences to revise their predictions about whether or not a heavy object will fall faster than a light one. Discuss with students how their predictions changed based on these experiences.
Now students should use their Falling student esheet to go to WaterWorks from the Oregon Museum of Science. This page describes a water fountain students can make. Students should read the page, decide if it would work, and describe the role that gravity plays in making the fountain work. You can follow up the activity by having students actually make the fountain. However, the point is to see if students can explain why it works based on their understanding of gravity.
Students should use their esheet to go to and read Gravity in Orbit from the How Things Fly exhibit on the National Air and Space Museum website.
Then they should write a paragraph explaining the animation that is shown on the page. It depicts what is described in the text—a person throwing baseballs at different speeds.
Students should also explain how the analogy of the baseballs relates to a spacecraft orbiting around the earth.