By default, force sensors are set up in Coach software to measure both positive and negative forces. A force pushing on the sensor (as in an impact or collision) displays a positive force. A force pulling on the sensor (as in using the sensor as a spring-scale or strain gauge) displays a negative force. The sensor at rest should display zero force. The positive and negative numbers simply refer to the direction of the force.
If you teach concepts such as simple machines to elementary or middle school students the concept of a negative force will probably be quite confusing. In most cases you will be using the force sensor to pull carts or weights in experiments showing how much force is used to move an object (such as pulling a cart up an inclined plane). Direction of the force is not critical in this case and so should be displayed as a positive number to avoid confusion.
Coach software can do this for you in about three clicks of the mouse!
This example uses the CoachLab II interface panel but will work the same way for ULAB, TI-CBL, TI-CBL2, and Vernier LabPro interfaces.
Screen shot of the CoachLab II Interface with a Force Sensor attached to input #1. The value on the sensor icon reads 0.00N indicating that there is zero force being applied to the sensor.
Screen shot of the CoachLab II Interface while a "pulling" force is applied to the force sensor. The value on the sensor icon reads -1.04N indicating a force of 1.04 N pulling on the sensor.
In order to reverse the data display from a force sensor so that a positive number is displayed and recorded when pulling objects instead of a negative number simply:
Right click on the Sensor Icon and choose "Edit Properties"
Look at the "Calibration" section at the bottom of the dialog that appears
X0 and X1 are two data points for input voltages from the sensor, Y0 and Y1 are two data points for the corresponding force that Coach should display along this linear calibration.
Simply reverse Y0 and Y1 so that Y0 now equals +10 and Y1 now equals -10
Then click "OK" and save the activity (using the "File Menu")
If you pull on the force sensor in your experiment it should now display a positive value.
Some force sensors are very sensitive devices and produce a little noise due to vibrations in the hands holding them, or even electrical noise from florescent lights and motors. This can be seen as a sensor at rest oscillating between -0.00N and +0.00N or a sensor in use that has a steady force applied but displays an error of +/- 0.01 N. This can be confusing to younger students and so to fix this problem simply use the Sensor Properties dialog to change the number of decimal places to 1 instead of 2.
To learn more about sensor calibration, and how to create your own sensor please click here.
