Physics 105

February 12, 2003

Today we will focus in more detail on how graphs are used in science to make scientific predictions and/or to test theoretical predictions. A detailed discussion on the use of scientific graphs can be found by following the following links:

We will apply the scientific method to a number of examples:

We will apply the scientific method in by measuring the acceleration of certain objects and determine how the measured acceleration depends on variables like mass, force, etc. An object accelerates when its velocity changes (see lecture notes on acceleration). We have already experimented with the sign of the acceleration (remember the measurements with the motion sensor). In most cases, the laws of physics predict the acceleration of an object as function of parameters like time, position, etc. If two observers, moving with constant velocity with respect to each other, look at the same object, they will observe the same acceleration, but not the same velocity and position. However, since the laws of physics provide us with predictions on the acceleration of an object, both observers will conclude that the same physics laws are satisfied in both observation frames.

We believe the laws of physics if their predictions can be verified by experiment. We illustrate this process by carrying out an experiment in which the acceleration of a cart is measured. Theory predicts that the observed acceleration is a constant g times the sin of the angle of inclination of a track. Theory predicts that the acceleration does not depend on the mass of the cart.

• Carry out experiment P04 to measure the gravitational acceleration. Compare the measured value with the accepted value! Do they agree? What are the errors in your measurement?

In this experiment we compare the results of a series of measurements with a theory, which has one unknown, the gravitational acceleration g. The theory predicts the functional dependence of the acceleration of the cart, which can be verified by changing for example the angle of inclination.

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").

Just a reminder:

The lab report that is due on Monday 2/17 should focus on velocity. The report should contain the following items:

• General discussion on velocity: when it it positive, when is it negative, how do you measure it? Describe the different methods we have used to measure velocity.
• Comparison of two techniques used to measure the velocity of a cart (motion sensor and photo gate). What are the advantages and disadvantages of each technique? Do both techniques give consistent results (note: make sure that you address the questions listed in the "Notes" window).
• Describe the measurements of the average velocity and the instantaneous velocity. What is the difference between these two velocities? Can you use the average velocity to estimate the instantaneous velocity?
• Describe the video analysis technique, and summarize your conclusions of the video analysis of the launch of the space shuttle and the launch of the lunar module.

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

Last updated on Monday, February 10, 2003 10:46