General Physics I Lab. M1 The Atwood Machine

Transcription

1 Purpose General Physics I Lab In this experiment, you will learn the basic operation of computer interfacing and use it in an experimental study of Newton s second law. Equipment and components Science Workshop 750 Interface, Smart pulley, mass set, hanger (x2), stand and clamp, cushion, thread. Background Computer interfacing consists of three main components: sensors, an interface box (hardware) and a control program to be run in a host computer (software). Measuring devices with an electronic output are called sensors and they are used to measure and output the data to the interface box (Science Workshop 750). The interface box collects and converts the data from the sensors and output them to the host computer. To display, manipulate and analyze the data, a computer interface program (DataStudio) is used. Computer interfacing allows us to collect large amounts of data with high speed and accuracy. Nowadays, most physics research experiments use computer interfacing. In the following, we give a brief introduction to the hardware and software to be used in the experiment. Please refer to the Help menu of the DataStudio program for more details. Starting a new experiment In the experiment to be carried out below, the basic control program has been written up for you, and you may start the experiment by running the program right away. However, it is strongly recommended that you understand the basic operation of the computer interfacing. 1. Connect the Science Workshop 750 interface box to the computer. Turn on the interface and the computer. 2. Double-click the DataStudio icon to start the program. 3. Select Create Experiment in the welcome screen. A new experiment setup window will display as shown in Figure Physically connect the sensor(s) to the interface box by plugging the connector of the sensor(s) to the appropriate channel(s). There are two types of sensors: analogue and digital. 5. Click on the corresponding channel to add a sensor for measurement. 6. Click the sensor icon to view and setup the sensor properties, such as the sample rate, measurements, calibration, etc. of the sensor, as shown in Fig Double-click or drag the data display icon in the Displays list menu to display the measurement. 8. The setup is ready now. Click the Start icon to start data recording or select the Monitor Data from the Experiment menu to monitor the measurement without recording the data. M1-1/10

2 Figure 1 New experimental setup window Figure 2 Sensor property window M1-2/10

3 Display windows The following is a list of useful display windows with annotation, which may serve as a quick reference for you to select the desirable display. 1) Digits 2) Meter Figure 3 Digits Display Window 3) Table Figure 4 Meter Display Window Figure 5 Table Display Window M1-3/10

4 4) Graph Data export Figure 6 Graph Display Window You may export any data set in DataStudio to a text file (*.txt) by: 1. On the File menu, click Export Data. 2. Choose a measurement (data set) in the Export Data text box. Click OK. 3. Navigate to the desired folder and type a name. Click Save. Then the data will be saved in a text file (*.txt) for a later use. Graph export You may export the Graph Display in DataStudio to a bitmap file (*.bmp) by: 1. Click and select the Graph Display window that you want to export. 2. On the Display menu, click Export Picture. 3. Navigate to the desired folder and type a name. Click Save. Then the Graph display will be saved in a bitmap file (*.bmp) for a later use. M1-4/10

5 Procedure In this experiment, a Smart Pulley is used to measure the motion of two hanging masses as one moves up and the other moves down. The two masses have different weights and are connected by a thin string. The DataStudio program calculates the changing speed of the masses as they move. A graph of speed versus time reveals the acceleration of the system. Computer setup 1. Connect the Science Workshop 750 interface box to the computer. Turn on the interface and the computer. 2. Connect the stereo phone plug of the Smart pulley to Digital Channel 1 on the interface box. 3. Open the M1 program in the course folder. The program will open with a graph display of Velocity (m/sec) versus Time (sec). Experimental setup (as shown in Figure 7) 1. Place the stand and clamp near the edge of the table. Mount the Smart pulley in the clamp so that its holding rod is horizontal. 2. Put a cushion (foam or plastic air bag will be provided) on the floor under the Smart pulley. 3. Use a piece of thread about 10 cm longer than the distance from the top of the pulley to the floor. Place the thread in the groove of the pulley. 4. Fasten a mass hanger to each end of the thread by wrapping the thread four or five turns around the notched area of the mass hanger. 5. Place ~100 grams of mass on one mass hanger and record the total mass as M 1 in Table 1. Be sure to include the mass of the mass hanger in the total mass. Place slightly more than 100 grams on the other hanger. Record this total mass as M 2. Figure 7 Experimental setup M1-5/10

6 6. Move the heavier mass (M 2 ) upward until the lighter mass (M 1 ) almost touches the floor. Hold the heavier mass to keep it from falling. Rotate the pulley so that the infrared light beam from the photogate is unblocked. In this case, the red LED (light-emitting diode) indicator on the photogate is off (no light). Data recording 1: Constant total mass 1. Click the Start button. Let the heavier mass fall. Data recording will begin when the infrared light beam from the photogate is blocked and the red LED indicator is on. 2. Click the "Stop" button to end data recording just before the heavier mass reaches the floor. Make sure that the upward moving mass does NOT hit the Smart Pulley! NOTE: The two masses may collide, as one descends and the other rises. Please have another data run if this happens. Run #1 will appear in the Data list. 3. Determine the experimental acceleration, a exp, by using the curve fit tool of the graph display. Record its value in Table Export and print the velocity versus time graph together with the curve fitting and paste it in the lab report. NOTE: Resize the graph to a suitable size before printing them. 5. Change the ratio of M 1 to M 2 by removing a mass from one hanger and adding it to the other. This allows you to change the net force without changing the total mass. 6. Repeat steps 1-3 for two more different values of M 1 /M 2. Record the values of M 1 and M 2 for each combination. Change the net force each time but keep the total mass constant. Record all the experimental values and complete the calculated values in Table 1. Data recording 2: Constant net force 1. Arrange the masses as they were at the beginning of Data recording 1. Now, change the total mass but keep the net force the same as that in the first run of Data recording 1. To do this, you need to add exactly the same amount of additional mass to both mass hangers. Make sure that the difference in mass remains the same as it was at the beginning of Data recording Record the new values of M 1 and M 2 in Table Take the data as in steps 1 3 in Data recording Repeat steps 2-3 for two more times with a different value of the total mass each time but keeping the net force constant. Record all the experimental values and complete the calculated values in Table Export and print the velocity versus time graph together with the curve fittings (including all three sets of data) and paste it in the lab report. NOTE: Resize the graph to a suitable size before printing them. M1-6/10

7 Name Date Lab session (Day & time) Lab partner Lab Report A. Answer the following question BEFORE the lab session (15 pts) 1. The acceleration of an object depends on the net applied force and the object s mass. In an Atwood's machine, as shown below, the difference in weight between the two hanging masses determines the net force acting on the system of both masses. This net force accelerates both of the hanging masses; the heavier mass is accelerated downward, and the lighter mass is accelerated upward. In the free body diagram shown above, T is the tension in the string, M 1 is the lighter mass, M 2 is the heavier mass, and g is the acceleration due to gravity. Assuming that the pulley has no mass, the string has no mass and doesn t stretch, and that there is no friction, show that the acceleration a of the entire system is given by M a = g M 2 1 M + M 1 2 M1-7/10

8 Name LA ( ) B. Results and data analysis (64 pts) Table 1: Constant total mass (32 pts) Paste the velocity vs. time graph here. Trial M 1 (kg) M 2 (kg) a exp (m/s 2 ) F net (N) M 1 + M 2 (kg) a theory (m/s 2 ) Percent error* Run#1 Run#2 Run#3 Calculate the net force F net = gm ( 2 M1). M2 M 1 Calculate the theoretical acceleration atheory = g. M 1+ M 2 * Percent error = a exp a a theory theory 100% M1-8/10

9 Table 2: Constant net force (32 pts) Paste the velocity vs. time graph here. Trial M 1 (kg) M 2 (kg) a exp (m/s 2 ) F net (N) M 1 + M 2 (kg) a theory (m/s 2 ) Percent error Run#1 Run#2 Run#3 Calculate the net force F net = gm ( 2 M1). M2 M 1 Calculate the theoretical acceleration atheory = g. M 1+ M 2 TA signature: M1-9/10

10 C. Answer the following questions after the experiment (7 pts each) General Physics I Lab 2. Compare the values of the % error between the measured value of the acceleration and the theoretical value, as shown in Tables 1 and 2. What are the main sources of error in this experiment? 3. Why is a better result obtained when you use a very large net force? 4. In the calculation of the acceleration a, we have assumed that the pulley is frictionless. Can you find a simple way in the experiment to test whether this is true? If so, how to correct for it? M1-10/10