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
- 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
- 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
- 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
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