Eric Murray, Spring 2006
Question these experiments will enable you to answer: If an object has an acceleration of zero, is the net force on it also zero?
Features: The object, a knot or small plastic ring, is on a force table.
This device allows forces of various magnitudes and directions to be applied to the object. Two of the forces will supplied by masses hanging from strings over pulleys. The third force will be supplied and measured by a force sensor.
Approach: The two forces supplied by the hanging masses are applied at particular angles. The table is positioned so that the object is centered, and the string from the force sensor is parallel to the sensor axis. While there are many positions in which the acceleration of the object is clearly zero, centering it on the force table is necessary for accurate direction measurements to be made. The most accurate measurements can be made if the strings just skim the top of the table without touching it. The string from the force sensor must be parallel to the sensor axis for an accurate force measurement to be made. Because the force sensor is quite sensitive, an average force magnitude will be measured over ten seconds.
Preliminaries: Make sure the motion sensor is plugged in to channel A of the interface unit. Open the data template. You'll find both a numeric and a graphical display of force. The force sensor has been set to record data at 10 Hz, for 10 s after clicking on Record
. Clicking the &Sigms
icon will display the mean value on the graph. Calibrate the force sensor by clicking on Calibrate
, then choosing Force
, Force Measurement
, Two Standards
, and Two Points
. With the force sensor mounted horizontally and no force applied, press the tare
button on the body of the sensor. Type 0
in the filed for Calibration Point 1, and click Next
. This tells the Capstone software what sensor output corresponds to zero force. Next, put 200 g on a mass hanger (for a total of 205 g and a weight of 2.009 N). Hang this mass from a string that goes over a pulley and is attached to the force sensor, parallel to the sensor axis. Type 2.009
in the field of Calibration Point 2, and click Next
. This tells the Capstone software what sensor output corresponds to a force of 2.009 N. The force sensor is now calibrated. Click Calibrate
again to and close the calibration window.
If there is a second mass hanger attached to your string, you may either remove it, or set it atop the force table so it applies no force to the force sensor. Also, you may wish to slacken the string by hand and tare the force sensor just before clicking Read from Sensor
for Calibration Point 2. While the calibration of the force sensor is quite stable over time, the tare is not. You should slacken the string by hand and tare the force sensor just before every measurement.
Experiment 1: Put 100 g on a hanger (for a total of 105 g), and place it at zero degrees. Let that be mass A. Put 100 g on the other hanger and place it at 20°. Let that be mass B. Position the force table so that the object is centered, and the string from the force sensor is parallel to the sensor axis. Note that when the object is truly centered, the strings leading to the pulleys will pass directly over and along the angle marks on the table. Slacken the string and tare the force sensor, minimize any swinging of the hanging masses, and click Start
to measure the force magnitude applied by the force sensor. Read the force direction applied by the force sensor from the force table.
Repeat the force measurement with mass B postioned at 40°, 60°, 80°, 100°, 120°, 140°, and 160°.
Calculate the x and y components of the horizontal force on the object for each of these eight trials, using vector algebra. (The x component is parallel to 0° and the y component is parallel to 90°.) Find the average and the standard error of each component. You may find an Excel spreadsheet to be helpful.
Experiment 2: Repeat Experiment 1 with 150 g on hanger A (for a total of 155 g) at zero degrees, and 50 g on hanger B.
Summary: Review your worksheet. Think about the goals of these experiments, your results, and the expectations from theory while writing your discussion.