Student Force Sensor (Order Code SFS-BTA or SFS-DIN) The Student Force Sensor is a general-purpose device for measuring forces. It can be used as a replacement for a hand-held spring scale, or it can be mounted on a ring stand. It can measure both pulls and pushes. Forces as small as 0.01 newton and as large as 20 newtons can be measured. The Student Force Sensor can be used for a variety of experiments including Studying force and impulse during collisions Studying simple harmonic motion Monitoring frictional force Studying Hooke s law Monitoring the thrust of rocket engines Measuring the force on a dynamics cart at the same time you monitor acceleration Measuring the force required to lift a known weight using simple machines Measuring the heat of vaporization of liquid nitrogen This Student Force Sensor is designed for use with the following interfaces available from Vernier: Vernier LabPro (for use with computers or TI Graphing Calculators) Texas Instruments CBL 2 or CBL System Universal Lab Interface (ULI) Serial Box Interface MultiPurpose Lab Interface (MPLI) The Student Force Sensor was designed by Bruce Lee of Andrews University and is manufactured by A.U. Physics Enterprises. How the Student Force Sensor Works The Student Force Sensor uses strain gage technology to measure force, based on the bending of a beam. Strain gages attached to both sides of the beam change resistance slightly as the beam bends. These resistors are built into a bridge circuit such that the changes in resistance result in a small change in voltage. An amplifier circuit inside the sensor steps up this voltage so that it can be measured by the lab interface. Potentiometers are included in the amplifier circuit to allow adjustment of the sensitivity and the voltage produced when no force is applied. The Student Force Sensor was designed to produce a voltage that varies in a linear way with force. NOTE: This product is to be used for educational purposes only. It is not appropriate for industrial, medical, research, or commercial applications. If you purchased the SFS-BTA, you will find a DIN to BTA adapter in the box with your sensor. Use the adapter to connect the sensor to a LabPro, CBL 2 or CBL.
Calibration In most cases, you can simply load an experiment file that is designed for use with the Student Force Sensor and calibration is taken care of. If you do not have these calibration files, contact Vernier and we will send them to you. These calibration files are only correct if you do not adjust the potentiometers on the Student Force Sensor. If you adjust the potentiometers on the Student Force Sensor, or if you just want to improve the calibration, you can easily recalibrate. Simply follow the same procedure used in calibrating most Vernier probes a two-point calibration. One of the points is usually with no force applied. Select the calibration option of the program you are using and remove all force from the Student Force Sensor. Enter 0 as the first known force. Now apply a known force to the Student Force Sensor. The easiest way to do this is to hang a labeled mass from the hook on the end of the beam. Enter the weight of the mass (note: 1 kg weighs 9.8 newtons). For most calibrations, we recommend using a 500-gram mass (4.9 newtons) for this second calibration point. Be careful not to use too much force during calibration. Adjustments In general, no adjustments are necessary when using this sensor. However, for special situations, they can be made. There are two adjustment screws on the Student Force Sensor, which are labeled on the side of the large part of the sensor. One is for sensitivity and one is for offset. These adjustments can be turned with a small jeweler's screwdriver. When the sensor is shipped, it is set for 1-volt output with zero force and 2-volt output with 9.8 newtons of push. This gives the sensor a range of 10 newtons of pull to about 24 newtons of push. The two adjustments on the Student Force Sensor are: Sensitivity: Turn the adjustment clockwise to increase the sensitivity. Note that increasing the sensitivity decreases the range of forces that can be measured. When the gain control is in the full clockwise (maximum gain) position, the Student Force Sensor produces about 0.065 volts/n and the range is about 17 N. When the gain control is in the full counterclockwise (minimum gain) position, the Student Force Sensor produces about 0.2 volts/n. Increasing or decreasing the sensitivity will change the offset. Zero Adjust (Offset): This adjustment does not change the sensitivity significantly, but it increases or decreases the voltage produced by the Student Force Sensor for a given applied force. When the Student Force Sensor is shipped, the offset adjustment is set so that the output is one volt with no force applied. Turning the zero adjust offset screw clockwise increases the voltage out. Mounting on a Ring Stand The Student Force Sensor is designed to be mounted on a ring stand in several different ways: 2
For measuring horizontal forces, you can use a standard 13-mm rod extended through the hole in the Student Force Sensor. Tighten down on the thumb screw. Note that the hook for connecting strings to the Student Force Sensor extends above the top of the body of the sensor, so you can pull in either direction. 13-mm rod 13-mm rod For measuring vertical forces, there are two ways to mount the Student Force Sensor. You can use a standard 13-mm rod extended through the hole in the Student Force Sensor. Tighten down on the thumb screw. You can connect a string to the hook on the end of the beam and pull either down or up. You can also use a 1/4-20 threaded rod in the threaded hole in the Student Force Sensor. 1/4-20 threaded rod Mounting on a Dynamics Cart The Student Force Sensor can be mounted on a dynamic cart, either with the strain gage bar vertical or horizontal. A special bracket (CI-6544) is sold by PASCO scientific for mounting the Student Force Sensor to a PASCO Dynamics cart. 3
Some Suggested Experiments Collision Experiments Mount the Student Force Sensor for the measurement of horizontal forces with the hook at the end of the beam in position to act as a bumper for an air track glider or dynamics cart. This experiment requires a lab interface that can sample fast enough to get many readings during the collision of the cart/glider and the Student Force Sensor. If you want to compare the integral of the force vs. time graph with the change in momentum of the cart/glider, there are two ways to do so: 1. Use a photogate and timing software to measure the speed of the cart before and after the collision. 2. Use a Motion Detector to make graphs of the motion and the force at the same time. One limitation of using a Motion Detector for these collision experiments is that the data sampling rate must be slowed down considerably. This means that the number of force measurements during the collision will also be reduced. Studying Friction Use the Student Force Sensor as a replacement for a spring scale. Run a string from the Force Sensor to a block of wood. Measure the force as you pull the block along a horizontal surface. You can investigate how frictional force is affected by surface area and type of surface. You can also compare static and dynamic friction. Simple Harmonic Motion Mount the Student Force Sensor and hang a weight from a spring connected to the beam as shown here. Start the weight oscillating up and down and plot the force vs. time. A sinusoidal graph will be produced. Using most programs, you can plot graphs of the motion of the weight and monitor force at the same time. This allows you to compare the phase of the force and motion graphs. 13-mm rod Comparing Force and Acceleration Mount the Student Force Sensor on a dynamics cart. Place the cart on a level surface. Place a Motion Detector in position to monitor the distance to the cart. Hold the Force Sensor in your hand with the hook connected to the cart. Start data collection. Pull and push horizontally on the cart using the Force Sensor as the program measures force, distance, velocity, and acceleration. Plot graphs of force vs. time and acceleration vs. time. Try plotting force vs acceleration. The slope of this graph should be the mass of the cart. 4
Motion Detector Using the Student Force Sensor with a Computer This sensor can be used with a Macintosh or PC computer and any of the following lab interfaces: LabPro, Universal Lab Interface, or Serial Box Interface. Follow these general procedures to use the Student Force Sensor with a computer: 1. Connect the Student Force Sensor to the appropriate port on the interface. 2. Start the data collection software on the computer. If you are using a Power Macintosh or Windows computer, run the Logger Pro software. If you are using an older Macintosh, DOS, or Windows 3.1 computer, run the Data Logger program. 3. Open an experiment file in the Logger Pro or Data Logger folder, and you are ready to collect data. Using the Student Force Sensor with TI Graphing Calculators This sensor can be used with a TI Graphing Calculator and any of the following lab interfaces: LabPro, CBL 2, or CBL. Follow these general procedures to use the Student Force Sensor with a graphing calculator: 1. Load a data-collection program onto your calculator: LabPro or CBL 2: Use the DataMate program. This program can be transferred directly from LabPro or CBL 2 to the TI Graphing Calculator. Use the calculator-to-calculator link cable to connect the two devices. Put the calculator into the Receive mode, and then press the Transfer button on the interface. Original CBL: Vernier has several programs that support this sensor and other sensors. If you are using the Student Force Sensor with the original CBL, we recommend the PHYSICS program. This program is available free on our web site at www.vernier.com. Our programs can also be obtained on disk. (Contact us for more information.) Load the program into a calculator using TI-GRAPH LINK. 2. Use the calculator-to-calculator link cable to connect the interface to the TI Graphing Calculator using the I/O ports located on each unit. Be sure to push both plugs in firmly. 3. Connect the Student Force Sensor to any of the analog ports on the interface. In most cases, channel CH 1 is used. 4. Start the data-collection program and you are ready to collect data. LabPro and Logger Pro are trademarks of Vernier Software & Technology. CBL and TI-GRAPH LINK are trademarks of Texas Instruments. 5
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