Resource: Virtual Pendulum

While feathers float to the ground when they fall, rocks drop like, well, rocks. Such observations of lightweight objects drifting slowly to the earth and heavy objects crash-landing have led many of us, including the earliest scientists, to infer that heavy, or more massive, objects fall faster than lighter objects. Sometimes, of course, they do, but this has nothing to do with how heavy or massive they are.

Wind resistance is the force that causes feathers and leaves to fall slowly to the earth. In the absence of this force, as in a vacuum, these objects would accelerate toward the ground at the same rate as a bowling ball or a car -- 9.8 m/s² -- though their impact would certainly be less dramatic.

For the same reason, the mass of the bob, the heavy part at the end of a pendulum, has no effect on the period, the amount of time it takes the pendulum to complete one back-and-forth swing.

Perhaps even more surprisingly, the distance you pull a pendulum aside before you release it, called the amplitude or starting angle, has little effect on the period, as long as you don't pull too far. In some ways this is counterintuitive, since the bob travels farther in each period when released from a greater distance. However, at that greater distance, it starts out at a steeper angle and accelerates at a higher rate, causing it to reach a faster speed. The increase in speed allows the bob to travel the longer distance in nearly the same amount of time.

This helps to explain why a pendulum's length is the most important factor in determining the period of that pendulum. Increasing the length causes the bob to travel a greater distance without increasing the angle or the pendulum's acceleration.

Which feature, or variable, of the pendulum surprised you the most? Why?

Which variables don't make a difference? Why not?

Using sketches, explain why the rotation of Earth is the explanation for the pendulum not leaving a straight line.