## A Laboratory Exercise in Indirect Measurement
Discussion: Modern physics depends heavily on indirectly determining physical properties of objects. The following activity may help convince students that indirect determinations are important methods of obtaining accurate information. This exercise can be used as an introduction to a discussion of the Rutherford model of the atom. This activity simulates an experiment in particle physics where a target material would be bombarded by high speed particles, and the collisions studied. It gives you a chance to use a "Monte Carlo" technique.
Use copies of the circle sets on the following pages. Place the circled paper on the floor, face down over a sheet of carbon paper. Working in pairs drop marbles or ball bearings from head height so that they hit the paper. The sphere must be caught after the first bounce. Repeat this at least 100 times. It may be more convenient to drop the marbles from just above the paper, however, one should then take care to distribute the hits as randomly as possible over the entire target area. It is "OK" to miss the paper from time to time. Those points will naturally be excluded form the data set. (Note the paper is 21.5 cm by 28.0 cm.)
Therefore, you can calculate the total area of all circles. From this you can calculate:
The area of one circle can be used to calculate the radius of a circle (area = pi*radius*radius). This calculated radius may then be compared with a direct radius measurement.
1. What is the radius of one circle?
Included in the Topics in Modern Physics, May 1990, and Catching the Sun, 1992, Fermilab. |