TEACHER RESOURCES

State Standards | Classroom Demonstrations

State Standards

Grade 8 (eight)
Focus on Physical Science

Forces

2. Unbalanced forces cause changes in velocity. As a basis for understanding this concept:

a. Students know a force has both direction and magnitude.

b. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces.

c. Students know when the forces on an object are balanced, the motion of the object does not change.

g. Students know the role of gravity in forming and maintaining the shapes of planets, stars, and the solar system.

4. The structure and composition of the universe can be learned from studying stars and galaxies and their evolution. As a basis for understanding this concept:

a. Students know galaxies are clusters of billions of stars and may have different shapes.

b. Students know that the Sun is one of many stars in the Milky Way galaxy and that stars may differ in size, temperature, and color.

c. Students know how to use astronomical units and light years as measures of distances between the Sun, stars, and Earth.

d. Students know that stars are the source of light for all bright objects in outer space and that the Moon and planets shine by reflected sunlight, not by their own light.

e. Students know the appearance, general composition, relative position and size, and motion of objects in the solar system, including planets, planetary satellites, comets, and asteroids.

1. Newton's laws predict the motion of most objects. As a basis for understanding this concept:

e. Students know the relationship between the universal law of gravitation and the effect of gravity on an object at the surface of Earth.

f. Students know applying a force to an object perpendicular to the direction of its motion causes the object to change direction but not speed (e.g., Earth's gravitational force causes a satellite in a circular orbit to change direction but not speed).

h.* Students know Newton's laws are not exact but provide very good approximations unless an object is moving close to the speed of light or is small enough that quantum effects are important.

i.* Students know how to solve two-dimensional trajectory problems.

j.* Students know how to resolve two-dimensional vectors into their components and calculate the magnitude and direction of a vector from its components.

k.* Students know how to solve two-dimensional problems involving balanced forces (statics).

m.* Students know how to solve problems involving the forces between two electric charges at a distance (Coulomb's law) or the forces between two masses at a distance (universal gravitation).

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Classroom Demonstrations

1. Inverse Square Law Device
Using All Thread^TM (or equivalent) threaded rod, nuts, and small tiles available at area home improvement stores, the device shown in Figure 1 can be built. Teachers can use this device to demonstrate various inverse square relationships, including Newton's Law of Universal Gravitation. The first tile is located at a distance of "one unit" from the point of where the four rods are joined together. The second set of tiles, 4 total, is located at a distance of "2 units." And the third set, 9 total, is located at a distance of "3 units."

2. Cavendish's Experiment Simulator
A rough approximation of Sir Henry Cavendish's experiment in 1798 can be built using a meter stick, string, and two masses, as shown in Figure 2. Two students can hold the two additional masses while the teacher explains Cavendish's methodology: he knew the force required to twist the quartz fiber (represented by our supporting string) a given amount. Teachers may wish to discuss torsional pendulums at this point. Cavendish also used a very clever technique to "amplify" the minute amount of twist caused by the very small gravitational force caused by the mass's attraction. He reflected candlelight off a mirror attached to the quartz fiber and onto an enlarged scale in his laboratory.

3. Formation of Stars Activity
Students serve as matter distributed throughout the universe in this demonstration. In Figure 3, the students are uniformly distributed throughout the universe. However, the slightest imperfection in the uniform nature of this distribution causes unbalanced gravitational forces. The teacher can direct a few students to begin to "clump," as shown in Figure 4. More clumping, directed by the teacher, leads to further imbalances in the gravitational forces until eventually the matter in this part of the universe has been pulled tightly into stars and proto-stars. See Figure 5.

Figure 3
Figure 4
Figure 5

Lesson Developed by Dave Fleischman and William Harvie, physics teachers at Torrey Pines High School.

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