Procedure
The requirements for the apparatus, and how they were implemented, are as follows.
1. A secure and well-centered
cylinder, to allow the cylinder and water to be spun at a fairly high rate of
rotation without losing stability.
First, a wooden disk of radius 7 inches and approximately an inch thickness was affixed to a metal turntable using screws. This allowed the wooden disk to sit securely on a flat surface but still rotate freely. To center the wooden disk on the turntable, a mark was drawn on the disk’s center, and its final position was adjusted so that when the turntable and disk were spun, the center mark did not rotate.
The cylinder that was used to hold the water—a plastic jar with radius 1.91 inches and 5.75 inches tall—was mounted to the center of the wooden disk. Double-sided tape was used to attach the bottom of the cylinder firmly to the wood. Final adjustments to the position of the cylinder were made by spinning the disk, then gradually adjusting the cylinder position if motion other than rotational motion was observed.
The cylinder was filled with 3.375 inches of water, which was dyed blue with a few drops of food coloring to increase the contrast between the water and its surroundings, thereby easing observation.
2. A method of driving the
rotation of the cylinder at a constant, fairly high rate of rotation.
The cylinder was rotated using a Dremel tool and a large motor hooked up to a DC power supply that was provided in the ACE lab. The tips of both the dremel tool and the motor were covered with a layer of double-sided tape in order to soften the grinding of the metal tip against the wood of the cylinder. To drive the rotation, the motor tip was placed flush against the surface of the wooden disk so that the rotation of the tip drove the rotation of the wooden disk and cylinder. This worked fairly well, as the dremel tool and the motor were both powerful enough to drive the rotation at a high speed.
3. A method of measuring the
angular velocity of the cylinder’s rotation.
To measure the angular velocity of the cylinder’s rotation, the motion sensor provided with the edition of Science Workshop installed in the ACE Lab was used. The motion sensor was a gear, and to improve traction between the gear and the wooden disk surface, a rubber wheel one inch in radius was attached to the gear.
No attempt was made to control the speed of either the dremel tool tip or the motor; instead, a moderate power level was used and then the angular velocity was measured separately using the motion sensor. Like the motors, the rubber wheel was held firmly against the surface of the wooden disk so that its rotation would be driven by the rotation of wooden disk.
4. A method of measuring the
curve that the water surface forms upon rotation.
In a previous semester, a student had done this experiment using photographs to capture the water surface curve. However, the problem that arose with that method was the distortion that was caused by the refraction of light through water. Therefore, for this experiment, photography methods were avoided. Instead, the surface of the water was measured manually using wooden pickup sticks.
A ring support was clamped to two stands so that it was suspended 1.5 inches above the lip of the cylinder, and a block of foam was fitted firmly inside the ring. 11 pickup sticks were inserted into the foam at approximately equal distances apart so that they formed a line that spanned the diameter of the lip of the cylinder. The sticks were approximately 1/8 inch in diameter and 6 inches long. To determine the level of the water at the location of a stick, the stick was pushed down until it caused visible interference in the water. The stick was then pulled back up until the interference disappeared. This process was repeated for each stick, and measurements from each stick were taken as discussed below.
Running the Experiment
The apparatus was initialized before each trial by bringing all of the measuring sticks above the equilibrium level of the water.
One group member would begin driving the rotation of the cylinder. Because access to equipment was limited, an adequate way to securely align the different parts of this experiment (cylinder, motion sensor, and driving motor) to avoid human interaction with the apparatus could not be designed. Therefore, both the motion sensor and the driving motor were handheld. However, in both cases, the group member holding the motion sensor and motor made a conscious attempt to keep the driving force and the motion sensor as steady as possible, and for each trial, the angular velocity was monitored using Science Workshop. Trials were only considered valid if the measured angular velocity stayed relatively constant (plus or minus 50 degrees/sec) during the entire run.
When the water surface curve had stabilized (which would take approximately 10 seconds or so), another group member would push each of the sticks down, one by one, until the point of the stick caused visible interference with the water. The stick was then pulled back up slightly until the interference disappeared.
When this was complete, the ends of the 11 sticks would effectively take a sampling of points that represented the surface of the water. Rotation was stopped, and the foam supporting the sticks was removed from the ring so that the length of the sticks could be measured for data collection.