The Pendulum
How Does the Pendulum Work?
To understand the relationship between gravitational forces and the mass of objects, the changes in speed and direction of objects, and the distance between objects. This lesson helps students understand concepts related to how gravitational forces act on objects by exploring the motion of pendulums.
Everything in the universe exerts gravitational forces on everything else, although the effects are readily noticeable only when at least one very large mass is involved (such as a star or planet). Gravity is the force behind the fall of rain, the power of rivers, the pulse of tides; it pulls the matter of planets and stars toward their centers to form spheres, holds planets in orbit, and gathers cosmic dust together to form stars.
Gravitational forces are thought of as involving a gravitational field that affects space around any mass. The strength of the field around an object is proportional to its mass and diminishes with distance from its center. For example, the earth's pull on an individual will depend on whether the person is, say, on the beach or far out in space. The image of an astronaut floating in space illustrates this point.
Everything in the universe exerts gravitational forces on everything else, although the effects are readily noticeable only when at least one very large mass is involved (such as a star or planet). Gravity is the force behind the fall of rain, the power of rivers, the pulse of tides; it pulls the matter of planets and stars toward their centers to form spheres, holds planets in orbit, and gathers cosmic dust together to form stars.
Gravitational forces are thought of as involving a gravitational field that affects space around any mass. The strength of the field around an object is proportional to its mass and diminishes with distance from its center. For example, the earth's pull on an individual will depend on whether the person is, say, on the beach or far out in space. The image of an astronaut floating in space illustrates this point.
Galileo and the Pendulum
Galileo Galilei was one scientist who studied gravitational forces. In the late 1500s, Galileo began to study the behavior of falling bodies, using pendulums extensively in his experiments to research the characteristics of motion. At the time, virtually all scholars still followed the belief of Aristotle that the rate of fall was proportional to the weight of the body. Galileo showed that this conclusion was erroneous based on the fact that air resistance slowed the fall of light objects. Galileo was able to combine observation, experiment, and theory to prove his hypotheses.
VideoThe Pendulum
Pendulum Lab PhET Interactive Simulations
Play with one or two pendulums and discover how the period of a simple pendulum depends on the length of the string, the mass of the pendulum bob, and the amplitude of the swing. It's easy to measure the period using the photogate timer. You can vary friction and the strength of gravity. Use the pendulum to find the value of g on planet X. Notice the anharmonic behavior at large amplitude.
Foucault's Pendulum  by Sixty Symbols
Foucault's Pendulum is a clever way of demonstrating the Earth's rotation  but it won't work at the equator! More physics at
http://www.sixtysymbols.com/
http://www.sixtysymbols.com/


Questions?
 How would you define a pendulum?
(A pendulum is loosely defined as something hanging from a fixed point which, when pulled back and released, is free to swing down by gravity and then out and up because of its inertia, or tendency to stay in motion.)  How does a pendulum work? What are the parts of a pendulum?
(A simple pendulum consists of a mass (called the bob) attached to the end of a thin cord, which is attached to a fixed point. When the mass is drawn upwards and let go, the force of gravity accelerates it back to the original position. The momentum built up by the acceleration of gravity causes the mass to then swing in the opposite direction to a height equal to the original position. This force is known as inertia.)  What is the period of a pendulum?
(A period is one swing of the pendulum over and back.)  What is the frequency of a pendulum?
(The frequency is the number of back and forth swings in a certain length of time.)  What variables affect the rate of a pendulum's swing?
(Students may come up with a variety of answers, but the four that they will be testing in this lesson are:  Length of the pendulumChanging the length of a pendulum while keeping other factors constant changes the length of the period of the pendulum. Longer pendulums swing with a lower frequency than shorter pendulums, and thus have a longer period.
 Starting angle of the pendulumChanging the starting angle of the pendulum (how far you pull it back to get it started) has only a very slight effect on the frequency.
 Mass of the bob at the end of the pendulumChanging the mass of the pendulum bob does not affect the frequency of the pendulum.
 Force of gravityThis accelerates the pendulum down. The momentum built up by the acceleration of gravity causes the mass to swing in the opposite direction to a height equal to the original position.)
Extra Credit Video
Physics I Classical Mechanics  by Prof. Walter Lewin
Foucault's Pendulum is explained by Prof. Walter Lewin of MIT