Goals: To practice the picket fence method of measuring acceleration and to use the method to measure acceleration in several situations

Introduction: You have already learned two methods of measuring acceleration. The most common method we have used is to generate a velocity-vs-time graph and then find the slope. The picket fence method used in this lab does the same. A Plexiglas strip is divided into alternating dark and clear strips of equal width. When the strip moves through a photogate interfaced to a computer, time intervals are measured for the passage of successive dark strips through the gate. The raw data is then converted to a graph of velocity vs. time.

Method and Results:

In the first experiment, the air track should be reasonably level. Lay a picket fence along the groove on the top of a red glider and secure it with a strip of tape at the bottom on both sides. With the glider on the air track, position the photogate so that the picket fence can pass unobstructed through the gate. Be sure that the photogate beam is perpendicular to the plane of the strip.

The photogate must be plugged into port 1 of the interface box. We are using the PRECISION TIMER software which can be accessed from the Physics DosShell Menu on the computers.

A) If the software asks, select Game Port Interface (NOT Serial 6500 Interface).

2. Select PHOTOGATE STATUS to make sure the photogate is functioning properly. Slide the

and UNBLOCKED.

3. If the photogate reads correctly, select the MOTION TIMER mode. Turn on the air supply. You are now ready to collect data.

D) Press <ENTER>. A list of the time intervals will be displayed. Press <ENTER> again to bring up a menu of analysis options.

E) Select g - graph data.

6. Select V - Velocity vs. time. What does the slope of a velocity-vs-time graph represent in

this procedure) in your lab book.

7. Select C - Picket Fence.

(in order to do a linear least-squares fit and print the coefficients of the fit) before hitting <Enter>

I) Select automatic scaling option A for both x- and y-axes.

When the graph is drawn, the data and fit will be shown. The slope, M, and y-intercept, B, will be given, together with the uncertainty (the number following the +/-) in each. The correlation coefficient, R, is a measure of the probability that the variables are actually correlated in a linear relationship. A value of 0 means no correlation; 1 (or -1) means perfect correlation. Expect all the fits to be very good (R nearly ±1). If they are not, assume that you made a mistake (exception: if the value of the acceleration is near zero, the correlation coefficient may not be near 1 because the fit has a hard time deciding whether the acceleration is positive or negative; in this case don’t worry about the value of the correlation coefficient) and try again.

Experiment 1) In this experiment you will give the glider a push to send it through the gate; the glider will be a horizontal (untilted) airtrack. However, make sure to stop pushing before any portion of the fence enters the gate. Stop the glider & picket fence before it hits the end of the air track (but only after it has finished passing through the photogate.)

Before actually taking data with the glider on a horizontal track, predict approximately what the acceleration of the glider should be as it goes through the photogate; write down your prediction (and your reason for it)! Then run the experiment.

Now return to step D on the obverse to see how to get the value of acceleration ... write down the slope, intercept, and uncertainties of each for the glider on the horizontal track.

For example: acceleration for horizontal air track = 0.100 ± 0.002 m/s²

(make sure the type of experiment (e.g., "horizontal air track" ) is included in your answer)

Is the value what you expected, to within the uncertainty of the fit? Explain.

Experiment 2) Now tilt the air track with riser blocks. Measure the height of the blocks you used and the distance between the air track feet and record them to 3 sig figs. You may have to readjust the photogate to make sure the pickets pass through unobstructed. Predict whether the glider's acceleration will be positive, negative or zero if you release the glider just above the gate. Then release the glider and catch it before bouncing. Again repeat the steps in boldface above. Was your prediction correct ? If not, explain why not.

Experiment 3) In this part, you will give the glider a push from the bottom of the track so that the picket fence passes all the way through the gate (but not while you are pushing it). But once again, before taking data, predict the approximate value (this time, a number and its +/- sign) of the acceleration that you should obtain in this part (with a reason). Then run the experiment, and record the slope, intercept, and uncertainties of the fit.

a) Compare the slopes from Experiments 2 and 3. Were the results expected? Explain.

b) Compare the intercepts from Experiments 2 and 3. Were the results expected? Explain.

Experiment 4) In this part you will mount the picket fence on a blue glider, and then release the glider above the photogate as in Experiment 2 above. Keep the height of the riser blocks the same. Once again, before taking data, predict the approximate value of the acceleration that you should obtain in this part (with a reason). Then, run the experiment, and record the slope, intercept, and uncertainties of the fit.

Compare the slopes from Experiments 2 and 4. Are the results what you expected? Explain.

Experiment 5) Lay the photogate on its side so that it extends over the table. Put a foam pad or box on the floor below the gate. You will now allow the picket fence to drop through the photogate. Predict the acceleration you expect before trying it. Find a picket fence not attached to a glider and hold it just above the gate. Release it and measure the acceleration. Record slope, intercept, and uncertainties of the fit. Is the acceleration within 1% of the expected value? If not, try again. Record all attempts.

Don’t forget to include a SUMMARY