10. A Set of First Day Demos

A Set of First Day Demos with Audience Appeal and Discussion

Some instructors like to give a "magic show" on the first day of class to illustrate some of the physics that will be covered during the quarter. Examples of mechanics demos with audience appeal are (see entry further in catalog for illustration and more description):

Unidentified Rolling Objects

Faith in Physics Pendulum

Monkey and Hunter

Turntable and Weights

Turntable and Bicycle Wheel

Another suggestion for the first day of class is to do a set of demos that review a little high school physics and lead into the subjects to be covered in the first two weeks. Here is a sample scenario:

Start with Drop Two Balls. Ask your students, "If I drop this steel ball and this wooden ball simultaneously from the same height, which will hit the ground first?" You will be surprised at the various answers, even from a Physics 8A class. Solicit explanations from several different students of their reasoning; this will give you an idea of the level of the class. If most of the students claim the balls will hit the ground at the same time ask, "What if I drop this ball and this sheet of paper?" This can then lead into a discussion of the effects of air resistance and what factors influence it, like the area fronting the wind. Lester Hirsch suggests: Borrow the two sheets of paper from two different students. Tell them not to tear the pages out of their notebooks; you are going to return the pages. Crumple one up into a ball and drop it simultaneously with a flat sheet. Here we have isolated the effect of air resistance by dropping two objects with the same mass, but different shapes. After the demonstration hand the flat sheet back to the first student, and carefully smooth out the crumpled sheet and hand it back to the second student. When he makes a face, tell the class that conservation has to start somewhere, and that he has to be the one this time!

To make this demonstration somewhat more quantitative, you may wish project a slide of a wooden ball, a steel ball, and a ping-pong ball falling, photographed at 1/20 sec. intervals. (See Three Balls Falling)

The Guinea and Feather Tube might well be used in this discussion also.

If you establish in a high level class that all of the students are pretty clear on the concepts of equal gravitational acceleration for different masses and effect of air resistance, try the following, "According to Newton's Third Law, what force is equal and opposite the weight of this steel ball I am holding in my hand?" You will get a variety of different answers to this question. After the discussion establishes that the third law pair to the weight of the ball acts on the entire planet Earth, ask, "So, according to Newton's Third Law, the steel ball exerts a force equal to its weight on planet Earth. As the two balls fall, doesn't the steel ball pull the Earth over towards it more than the wooden ball, so the steel ball must really strike the Earth first, even if we neglect air resistance?"

As a final first-day demonstration, drop the "happy" and "unhappy" balls to show that objects that look identical may have very different physical properties.