This video clip shows a pendulum bob consisting of a sphere filled with concrete, suspended from a string attached to the ceiling. A thread is attached to a bead glued to the side of the sphere and runs across the lab to a light, very low-friction pulley. The thread goes over the pulley, and from the thread is suspended a number of paper clips. These paper clips create a tension in the thread, so that the thread stretches slightly. At its ends it thus pulls back with very nearly equal forces. At the end supporting the paper clips, it is this force that counters the force of gravity on the clips. At the end attached to the sphere, this force pulls the sphere away from its natural equilibrium position.
We will observe the displacement from equilibrium as a function of the number of suspended clips.
In order to observe the positions of the sphere quickly and easily, the sphere is partially submerged in water. This damps the motion of the sphere (it also dampens the sphere a bit), so that it doesn't oscillate as freely in response to the air currents in the room. We do pose the question of whether the buoyant force exerted by the water on the sphere makes any difference in our observations.
Around clock times 3:02, 3:40, 4:30, 5:00, 5:45 and 6:30, as indicated by your MPEG player, five additional paper clips are added to those already suspended and the pendulum oscillates a bit then comes to restat a new position. It is possible that 5-clip weights are added at other clock times--observe the system carefully.
The data required includes the original, equilibrium position of the pendulum and the position after the addition of each 5-clip weight.