Physics 105

February 19, 2003

We believe the laws of physics if their predictions can be verified by experiment. We will be illustrating this process by carrying out a series of experiments in which the acceleration of a cart is measured under various circumstances. For each setup we have a theory predicts that the observed acceleration in terms of various paramateres that are varied during the experiment (e.g. the sin of the angle of inclination of a track, the mass of the cart, the mass of the puley). Today we will finish the first two experiments we carried out to compare experiment and theory, and finish analyzing the results. We will work on the following two experiments:
• In experiment P04 we already measured the gravitational acceleration. Compare the measured value with the accepted value! Do they agree? What are the errors in your measurement?
• In experiment P09 wewill determine how the acceleration of an object changes when you change the force acting on it. In this experiment we will be using a smart pulley to measure the velocity of the cart.

In these experiments we compare the results of a series of measurements with a theory. Consider the following questions when you look at the results of your experimens:

• Experiment P04:
• At each height you made several measurements of the acceleration. At each height, calculate the average value of the acceleration and the error in your measurement.
• Make a graph of sin(theta) vs acceleration using Excel. Fit the data with a line that goes through the origin. The slope of this line is your best estimate of the gravitational acceleration g.
• Estimate the upper limit of your measured g by using as data points the upper limits of your data (a + da).
• Estimate the lowerlimit of your measured g by using as data points the lower limits of your data (a - da).
• Combine the upper and lower limits to extract your best estimate of g and the error in this estimate.
• Experiment P09:
• In this experiment, the measured acceleration is predicted to depend on the two variables m and M: a = (m/(m+M))g.
• The theory suggests a number of different ways to veridy the predicted correlation between (m, M) and a:
• Vary m and M such that m + M remains constant: the measured acceleration should be proporional to m!
• Keep m fixed and vary M: the measured acceleration should be proportional to 1/(m+M)!

The motion we observe is always a result of forces acting on the objects in motion. Sometimes these forces are clearly present, like for example a horse pulling a cart, while in other cases we do not observe the forces directly but instead only observe their effect, like for example the force of friction. Experiments have shown that the acceleration of an object is directly proportional to the force acting on the object (please refer to the note "Force and Motion").

Note: there will be no lab report due next week.

© Frank L. H. Wolfs, University of Rochester, Rochester, NY 14627, USA

Last updated on Tuesday, February 18, 2003 9:18