Lab Partners LAB 3: PASSIVE FORCES Investigation 1: Tension Forces Objectives How to use a Force Probe to measure forces How to calibrate the Force Probe The characteristics and origin of tension Materials Logger Pro for Windows Universal Laboratory Interface (ULI) Vernier Dual-Range Force Sensor Set of assorted, slotted weights 2 short pieces of chain (same lengths but different weight/unit length) 2 rubber bands with different elasticities Two-inch diameter string loop Meter stick Introduction When you pull on one end of a rope attached to a crate a force is transmitted down the rope to the crate. If you pull hard enough, the crate may begin to slide along the floor. Tension is the name given to forces transmitted in this way along strings, ropes or wires. In this investigation you will first learn to calibrate and use the Force Probe. Then you will use the probe to examine tension forces. Activity 1 What Does a Force Probe Measure? In this activity you will discover how the force probe responds to pushes and pulls. 1. Start Logger Pro. Load file L03Al-2a(Measuring Force). 2. Set up the force probe. Plug the Force Probe into the Channel 1 port of the ULI. Place the Force Probe on the mounting post with the hook pointing vertically downward. Zero the probe by clicking on the Zero button located at the top of the screen. 3. Plot a force-time graph. With the hook of the force probe still pointing vertically downward, click on I Collect | to graph force (or just hit enter). Question Did the force probe read zero with nothing hanging from the hook? yes 4. Graph again. While graphing this time, gently push up on the hook and then gently pull down. Question Does the force probe record a push as a positive force or as a negative force? What about a pull? push-negative pull positive Version 09/27/07 6-1 Passive_Forces.docComment Zeroing the force probe assures that zero will be displayed on the screen when no force is applied to the probe. This is done electronically. Any small mechanical change in the force probe-including changing its orientation-will change the zero reading. Therefore, we must zero the probe every time before we take new measurements. When you zero the probe, you must do it with the probe in the same orientation in which it will be used to collect the next set of data. 5. Calibrate the force probe. Select Experiment > Show Sensors. In the window that pops up, click on the Dual Range Force Sensor icon in the channel 1 space (marked CH1). Select Calibrate in the drop down menu that appears. In the window that now shows up, make sure you are in the Calibration tab and that the drop down menu within the tab reads Live Calibration. For Reading 1, enter 0 N for Value 1 and make sure there is nothing attached to the probe. Click on Keep. For Reading 2, enter 4.9 N for Value 2 and hang 0.500 kg from the probe, making sure that the mass is steady. Then, click on Keep. When that is done, click Done. You may now close the Sensors window at this time. 6. Check the calibration. Remove all masses. Zero the probe. Double click on the graph, and set the Force axis from 0 to 10 N. Now set the device to record for 60 seconds. To do this, go to Experiment > Data Collection. On the Collection tab in the window that opens, make sure that the Mode is set to Time Based. Change Length to 60 seconds. Click Done. Begin graphing. Add 100 grams to the hook about every 10 seconds. Remember: the hanger is 50g. Therefore, the first 100g you add will be the hanger with a 50g mass. Select Analyze > Examine (Ctri-E). Read the Force values for each amount of mass that was added to the Force probe. Hanging Force = Weight Force Probe Reading Percent Mass (kg) = mg (N) (N Error 0 10 401400 0. 2 1 N 80g 200 0 '7 6 W 1209 300 160 2 .400 1. 7 qN 2009 500 2:03 Weight - Force Probe Reading Percent Error = . 100 Weight Question How accurately does your force probe measure force? Version 09/27/07 6-2 Passive_Forces.docActivity 2 Tension Forces In this activity, you will examine tension and its origin by hanging masses from the force probe at the ends of pieces of chain, string and rubber bands. 1. Measure the gravitational force exerted by the earth on a hanging mass. Double click on the graph and change the time axis to 0 to 20 sec. Also change the force axis to 0 to 5 N. 0150g Zero the force probe and then start graphing. After 5 seconds, hang 0.1 kg from it. After 12 seconds, add another 1kg. .509) Store your graph for comparison in the rest of this activity. Do this by selecting Experiment > Store Latest Run (Ctri-L). Then, rename the data by selecting Data > Dataset Options > Run 1. Make sure to give it a name that identifies how the data was collected. Sketch your graph on the axes below and label it. 5.00 3.75+ 2.504 1.25- 20 4 8_ Time (seconds) 12 16 Predictions If you hung the 100-gram masses from one of the chains (provided for this lab) attached to the hook, how would the forces measured by the probe compare to those measured with the masses hung directly from the force probe hook? Test your prediction using the two different pieces of chain. 2. Graph force. After zeroing the force probe with nothing hanging from it, hang the heavier piece of chain from it. Graph the force. After 5 seconds hang the first 0. 1 kg mass from the end of the chain, and add the second 0.1 kg mass after 12 seconds. Sketch the graph on the above axes, and label it. Question Compare the forces to the ones with the masses attached directly to the hook. Do the masses still cause the same increase in force? What is the effect of adding the chain? 3. Repeat (2) using the lighter piece of chain of the same length. Graph force. Sketch the graph on the same axes, and label it. Question How do the forces compare to the ones with the heavier chain? Version 09/27/07 6-3 Passive_Forces.docPrediction If instead of the chain, you used the two-inch diameter string loop, how would the forces measured by the probe compare to those measured with the masses attached directly to the hook? 4. Test your prediction. Remove the chain. Zero the probe with the hook pointed vertically downward, and with nothing hanging from it. Hang the string loop from the probe. soy Start graphing. Hang a O. Pkg mass from the string loop. After 10 seconds, add the second @. Pkg mass. Sketch this graph on the same axes, and label it. Questions How does this force graph compare to the graph measured when the masses were attached directly to the hook? When a force is exerted on one end of a string, how large a force is felt at the other end of the string? Prediction If you hang the 100-gram masses from a rubber band (of very small mass) instead of a string, how will the forces measured by the force probe compare to those measured with the masses attached directly to the hook? 5. Test your prediction using the more easily stretched rubber band. Start graphing. Hang a 0.1 kg mass from the string loop. Be sure that the mass does not oscillate at the end of the rubber band. After 10 seconds, add the second 100- gram mass. Store your run by selecting Experiment > Store Latest Run. Sketch the graph on the axes below and label it. 5.00 - 2 3.75- Force (N) 2.50- 1.25- 0 . 0 8 12 16 20 Time (seconds) Also measure the change in length of the band when one mass and then both masses are hanging from it, and record it below. Change in length O. Pkg: Change in length 0.2 kg: 8 cm 509 Version 09/27/07 6-4 Passive_Forces.docQuestion Did the measured force agree with your prediction? Explain. yes 6. Repeat (5) with the harder to stretch rubber band. Sketch your graph on the axes above, and label it. Change in length 0.1 kg: Change in length 0.2 kg: Questions How did the measured forces compare to those with the more easily stretched rubber band ? How did the changes in length compare between this one and the more easily stretched rubber band? How does the force get transmitted along the rubber band from the hanging mass to the hook of the force probe? Based on your observations, how much of the force at one end of a string or rubber band is transmitted to the other end? (What causes the tension to be transmitted along a string?) Hint: the string stretches like the rubber band, although it does not stretch nearly as much. Investigation 2: Normal Forces To find out The characteristics and origins of normal forces Materials Same for Investigation 1 plus: Two mounted metal sheets with different flexibility with screw eyes attached Introduction A book on a table, the top of a ladder leaning against a wall, your car driving down a highway--each of these objects feels a force exerted on it by the surface with which it is in contact. These forces exerted perpendicular to the surfaces of the tabletop; the wall or the highway are all classified as Normal forces. Here you will use the force probe to examine normal forces. Activity 1 Forces Exerted by a Flexible Metal Sheet In this activity you will examine the force exerted by a flexible sheet of metal when it is bent by pushing or pulling on it. Before you begin, clear the graphs from Investigation 1 using Data > Clear All Data. A time scale of 0 to 20 sec should work for the following activities. 1. Examine the force exerted when a flexible sheet of metal is pushed and pulled. Remove everything from the force probe hook. Version 09/27/07 6-5 Passive_Forces.docYou have two flexible metal sheets mounted in a holder made from a bent much thicker sheet of aluminum. First work with the more flexible sheet made of brass (yellowish metal). Start graphing force, hook the force probe into the screw eye on the metal sheet and push several times on the sheet with the force probe. Push just hard enough to display the largest force that will fit on your axes. After 10 seconds, pull several times on the force probe while still collecting data. Also use a transparent ruler to measure how far (in mm) the center of the sheet moves when you push or pull with the maximum force you used above. Displacement of the brass sheet with maximum force applied: _- 17mm Store your graph by Experiment > Store Latest Run. Rename it Brass. TP- IP Questions When you push on the sheet with the force probe, does the sheet push back? How do you know? Refer back to the Question 2 in Investigation 1. yes , the sheet pushed back As you increase the force that you exert on the sheet, what happens to the sheet? What happens to the force exerted by the sheet? The Sheet pushed back even more whe increasing the 2. Repeat the procedure described in 1, but use the stiffer silver colored metal sheet. force Apply the same maximum force. Displacement of the stiffer sheet with maximum force applied: - 6mm Questions For the maximum push (or pull) does the center of the stiffer sheet move more or less than the more flexible sheet? The center of the stiffer sheet moves less than the flexible What do you think causes the force exerted by the sheet on the probe? How does the sweet. sheet "know" how hard to push (or pull) to balance a force exerted on it? What happens to the sheet when you apply a force to it? Version 09/27/07 6-6 Passive_Forces.docActivity 2 Forces Exerted by a much less flexible aluminum sheet. Repeat the procedure described in 1, but this time pushing and pulling on the screw eye attached to one end of the aluminum holder. Apply the same maximum force. Be careful not to damage the force probe by pushing too hard. Questions Compare your graph to the graph for the more flexible sheet (Brass). Are the results similar or different? The graph looked different. What do you think causes the force exerted on the probe by the thick aluminum sheet? [How does the thick sheet "know" how hard to push (or pull) to balance a force exerted on it by the force probe?] What do you think happens to the thick aluminum sheet when you push or pull on it? Does it move as the thin metal sheets moved? Can you measure any movement? What should happen if you push on a hard wall? Does it move? Can you measure how far it moves