1、Experiment18Physics with Computers 18 - 1Work and EnergyNo springsWork is a measure of energy transfer. In the absence of friction, when positive work is done on an object, there will be an increase in its kinetic or potential energy. In order to do work on an object, it is necessary to apply a forc
2、e along or against the direction of the objects motion. If the force is constant and parallel to the objects path, work can be calculated usingWFswhere F is the constant force and s the displacement of the object. If the force is not constant, we can still calculate the work using a graphical techni
3、que. If we divide the overall displacement into short segments, s, the force is nearly constant during each segment. The work done during that segment can be calculated using the previous expression. The total work for the overall displacement is the sum of the work done over each individual segment
4、:Fs()This sum can be determined graphically as the area under the plot of force vs. distance.1 These equations for work can be easily evaluated using a force sensor and a Motion Detector. In either case, the work-energy theorem relates the work done to the change in energy asW = PE + KEwhere W is th
5、e work done, DPE is the change in potential energy, and DKE the change in kinetic energy.In this experiment you will investigate the relationship between work, potential energy, and kinetic energy.OBJECTIVES Determine the work done on an object using a force vs. distance graph. Use the Motion Detect
6、or to measure velocity and calculate kinetic energy. Compare the work done on a cart to its change of mechanical energy.MATERIALSPower Macintosh or Windows PC masses (200 g and 500 g)LabPro or Universal Lab Interface spring with a low spring constant (10 N/m)Logger Pro masking tapeVernier Motion Det
7、ector wire basket (to protect Motion Detector)Vernier Force Sensor rubber banddynamics cart1 If you know calculus you may recognize this sum as leading to the integral finalitlsdFW)(.Experiment 1818 - 2 Physics with Computers PROCEDURECALIBRATE THE FORCE SENSOR1. Connect the Motion Detector to DIG/S
8、ONIC 2 of the LabPro or PORT 2 of the Universal Lab Interface. Connect the Vernier Force Sensor to Channel 1 of the interface, or PORT 1 if using a ULI Force Probe. If your sensor has a range switch set it to 5 or 10 N.2. Open the Experiment 18 folder from Physics with Computers. Then open the folde
9、r that matches the type of force sensor you are using. Then open the file Exp 18a for your force sensor. Three graphs will appear on the screen: distance vs. time, force vs. time, and force vs. distance. Data will be collected for 5 s.3. If you are using the ULI Force Probe, it is necessary to calib
10、rate the sensor. Other Force Sensors can optionally be calibrated using the same procedure, or you can skip this step. a. Choose Calibrate from the Experiment menu. Click on the PORT 1 icon (DIN 1 or CH 1 for Force Sensors other than the ULI Force Probe) so the port is highlighted. Click .b. Remove
11、all force from the Force Sensor. Enter a 0 (zero) in the Value 1 field. Hold the sensor vertically with the hook downward and wait for the reading shown for Input 1 to stabilize. Click Kep. This defines the zero force condition. c. Hang the 500-g mass from the Force Sensor. This applies a force of 4
12、.9 N. Enter 4.9 in the Value 2 field, and after the reading shown for Input 1 is stable, click Kep. Click OK to close the calibration dialog. 4. Hold the Force Sensor with the hook pointing downward, but with no mass hanging from it. Click and then to set the Force Sensor to zero.Part III Work Done
13、To Accelerate A Cart In Part III you will push on the cart with the Force Sensor, causing the cart to accelerate. The Motion Detector allows you to measure the initial and final velocities; along with the Force Sensor, you can measure the work you do on the cart to accelerate it.25. Open the experim
14、ent file Exp 18c for your force sensor. Three graphs will appear on the screen: distance vs. time, force vs. time, and force vs. distance. Data will be collected for 5 seconds. If you are using a ULI Force Probe, recalibrate it as you did in Step 3. During calibration enter the second force as a neg
15、ative number to make a push on the Force Probe read as a positive value.26. Remove the spring and support. Determine the mass of the cart. Record in the data table.27. Place the cart at rest about 1.5 m from the Motion Detector, ready to roll toward the detector.28. Click . On the dialog box that ap
16、pears, click . Logger Pro will now use a coordinate system which is positive towards the Motion Detector with the origin at the cart.29. Prepare to gently push the cart toward the Motion Detector using the Force Sensor. Hold the Force Sensor so the force it applies to the cart is parallel to the sen
17、sitive axis of the sensor.30. Click to begin data collection. When you hear the Motion Detector begin clicking, gently push the cart toward the detector using only the hook of the Force Sensor. The push Work and EnergyPhysics with Computers 18 - 3should last about half a second. Let the cart roll to
18、ward the Motion Detector, but catch it before it strikes the detector. 31. Examine the distance vs. time and force vs. time graphs by clicking the Examine button, . Identify when you started to push the cart. Record this time and distance in the data table.32. Examine the distance vs. time and force
19、 vs. time graphs and identify when you stopped pushing the cart. Record this time and distance in the data table.33. Determine the velocity of the cart after the push. Use the slope of the distance vs. time graph, which should be a straight line after the push is complete. Record the slope in the da
20、ta table.34. From the force vs. distance graph, determine the work you did to accelerate the cart. To do this, select the region corresponding to the push (but no more). Click the Integrate button, , to measure the area under the curve. Record the value in the data table.35. Print the graphs Part IV
21、 Work Done To Accelerate A Cart up an inclineThis time have the cart go up an incline. The difference between the work done by the force sensor and the change in Kinetic Energy of the cart should equal to the Work done by gravity (Gravitational Potential energy change). The procedure and analysis sh
22、ould be similar to Part III. Measure the weight of the cart in newtons and the height change in going up the ramp in metersPrint the graphsExperiment 1818 - 4 Physics with Computers DATA TABLE NAME_LAB PARTNERS _Part IIITime (s) Distance (m)Start PushingStop PushingMass (kg)Final velocity (m/s)Integ
23、ral during push (Nm)KE of cart (J)Part IVTime (s) Distance (m)Start PushingStop PushingMass (kg)Final velocity (m/s)Integral during push (Nm)KE of cart (J)Difference between work and KE of cart (J)Height change for the cart (m)Weight of cart (N)GPE of cart (J)Work and EnergyPhysics with Computers 18
24、 - 5ANALYSIS1 In Part III you did work to accelerate the cart. In this case the work went to changing the kinetic energy. Since no spring was involved and the cart moved along a level surface, there is no change in potential energy. How does the work you did compare to the change in kinetic energy?
25、Here, since the initial velocity is zero, KE = mv2 where m is the total mass of the cart and any added weights, and v is the final velocity. Record your values in the data table.2. In Part IV you did work to accelerate the cart. In this case the work went to changing the kinetic energy. This time th
26、ere is a change in potential energy. How does the work you did compare to the change in kinetic energy? Here, since the initial velocity is zero, KE = mv2 where m is the total mass of the cart and any added weights, and v is the final velocity. Record your values in the data table. How close is the difference between work and the change in Kinetic energy to the change in Gravitational Potential Energy?
