Experiment 7
NEWTON'S SECOND LAW

PROCEDURE

1.  Level the air track using the adjustable feet at the supply end of the track. The track is level when, with the air turned on, the glider will remain nearly motionless at any point along the track. Open the Worksheet and fill in the header information.

2.  Place the seven small masses (brass washers) on a balance and record their total mass m. We will assume the mass of each washer mw to be one seventh of the total. Record these values in the Worksheet in cells G9 and G11.

3.  Place the seven washers over one of the pegs on the glider and determine the total mass of the glider plus washers (M+m). Record this value in the Worksheet in cell G13.

Figure 7-3: Attaching Washer to the String

4.  Use the procedure illustrated in Figure 7-3. Attach a washer to one end of the string, run the string over the pulley, and loop the other end of the string over the center screw of the glider. The string should be long enough for the washer to just hang free of the pulley when the glider is at the far end of the air track.

5.  In this experiment, you will use the smart pulley to measure the linear acceleration of the system. Make sure the pulley is connected to the interface box and turn it on. Open the Smart-Pulley link.

6.  Hit the Collect button in the LoggerPro window. The data will automatically be collected once the motion starts. Turn on the air valve, allowing the glider to start from rest at the air supply end. When the washer has hit the floor, hit the Stop button.

The Graph Window will have the Velocity vs. Time data plotted. In order to determine the linear acceleration of the system, you need to find the slope (rate of change) of the Velocity vs. Time curve, which should be a straight line. The application will do this job for you, but we need to be selective about what points it should use.

7.  After the mass hits the floor, the data points that were taken should be thrown out since the system is not accelerating any more. Using the mouse, drag across the data that you want to keep (the straight-line portion).

8.  Select the ANALYZE...Linear Fit menu. An information box will appear that gives the slope and the intercept of the best straight line through the selected points. The fitted line should pass through only the linear portion of the data.

9.  Record the value of the Slope as it appears at the bottom as the acceleration for run 1 in cell D18.

10.  Make a total of three measurements of the acceleration with a single washer providing the net force. Record the number of washers as 1, and the three accelerations for the 3 runs in the Worksheet in the table at cells C18:F18.

11.  Take one washer off of the glider and thread it onto the string and over the pulley. Repeat procedure steps 6 through 10 for this new net force, but record the data in the trial 2 row.

12.  Repeat procedure 11 until all of the washers have been added to the net force, for a total of seven trials of 3 runs each.

13.  Determine the average of each set of three accelerations, and record them in cells G18:G24 of the Worksheet.

HINT: Let the spreadsheet do the work for you: In cell G18, enter: "= (D18 + E18 + F18)/3". This formula simply adds the 3 accelerations together and takes one-third the sum. To complete the averaging for all the trials, select all the cells from G18 to G24, and select under EDIT....FILL....Down.