GRAVITY
Gravity is one of the oldest and most celebrated topics in physical science.
Early benchmarks about the earth's gravity, the relationship between forces
and motion, and observations of the sky support more sophisticated benchmarks
about orbits and universal gravitation in later grades. Often, the largely
descriptive benchmarks in the other three maps of this cluster depend
on the gravitational explanation presented here, and, reciprocally, benchmarks
about gravity build on observations of the sky and ideas about motion
and orbits.
Several historical episodes in Science for All Americans and Benchmarks
Chapter 10: HISTORICAL PERSPECTIVES could extend students' understanding
of gravity. They include the account of heavenly motion in Displacing
the Earth from the Center of the Universe, Newton's use of gravity
to explain heavenly motion in Uniting the Heavens
and Earth, and Einstein's theory of general relativity in Relating
Matter & Energy and Time & Space, which pictures gravity
as a distortion of space and time.
Notes
Universal gravitation, introduced in 6-8, is a very abstract idea and is especially
hard to grasp because gravitational forces between objects such as soda cans,
pencils, and people are not noticeable. Understanding changes in motion and
observations of the solar system, and the fact that at least one very large
mass must be involved for the effects to be easily detected, will make this
idea more credible.
The counterintuitive idea that the earth is a sphere—necessary for students
to understand that "down" is toward the earth's center—is supported
by a K-2 benchmark about shapes. (Students will, of course, need to have experience
with three-dimensional as well as two-dimensional shapes.)
In the early grades, students are probably not ready for a gravitational explanation
of the difficult idea that the earth orbits the sun, but they should get a
rough idea of the scientific description nonetheless. Even for this purpose,
students need an early sense of how apparent motion depends on frame of reference.
The relative motion strand includes a 3-5 benchmark
from the relevant essay in The Earth section of
Benchmarks Chapter 4 and a more sophisticated 6-8 benchmark from Benchmarks
Chapter 10: HISTORICAL PERSPECTIVES.
The true nature of the motions of the sun, stars, and planets is not easily
discovered just by observing them. This fact reveals the potential in instruction
for relating the benchmarks in this map to models. Instructional strategies
can also draw on the relationship between benchmarks in this map and benchmarks
about systems and scale in Benchmarks Chapter 11: COMMON THEMES.
| Research in Benchmarks
Student ideas about the shape of the earth are closely related
to their ideas about gravity and the direction of "down" (Nussbaum,
1985a; Vosniadou, 1991). Students cannot accept that gravity is
center-directed if they do not know the earth is spherical. Nor
can they believe in a spherical earth without some knowledge of
gravity to account for why people on the "bottom" do not fall
off. Students are likely to say many things that sound right even
though their ideas may be very far off base. For example, they
may say that the earth is spherical, but believe that people live
on a flat place on top or inside of it—or believe that the
round earth is "up there" like other planets, while people live
down here (Sneider & Pulos, 1983; Vosniadou, 1991). Research
suggests teaching the concepts of spherical earth, space, and
gravity in close connection to each other (Vosniadou, 1991). Some
research indicates that students can understand basic concepts
of the shape of the earth and gravity by 5th grade if the students'
ideas are directly discussed and corrected in the classroom (Nussbaum,
1985a).
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).
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). High-school students also have difficulty in conceptualizing
gravitational forces as interactions. In particular, they have
difficulty in understanding that the magnitudes of the gravitational
forces that two objects of different mass exert on each other
are equal. These difficulties persist even after specially designed
instruction (Brown & Clement, 1992). |
