Demo #5: Rigidity and Velocity

Have as many students as possible stand side-to-side in a line, arms outstretched with elbows locked, palms touching their neighbors' palms. [This may require quite a linear distance, so feel free to have the line wind about the room.] Ask one volunteer to act as the official timer. Tell the students that you are going to give a push on the outstretched arm of the student at one end of the line. When each student feels the push, he or she is to transmit that push to the next student. Tell the timer when you are going to start, and ask him or her to stop timing when the push reaches the end of the line (the last student should yell "stop").

Next, repeat the experiment, but this time ask the students to relax their arms slightly so that their elbows are not locked. [This may require the line to be shortened slightly, but this change is not usually significant.] This time the pulse should take substantially longer to traverse the line. The reason you tell them is that they are less rigid with their elbows unlocked than they are with them locked. Of course, it also has to do with the fact that they now have to think to transmit the push rather than having it directly sent through their locked arms. [Thought can be a highly inelastic process.] You can estimate the length of your line by assuming approximately 2 meters per student (at least for college students). By dividing this distance by the times measured in each experiment, you can show that when the students are more rigid (elbows locked) the wave travels faster, and when they are less rigid (elbows unlocked) the wave travels slower.