Wave Machine

Introduction

Send a wave down a line of weighted rods.

Measure the speed of the wave.

Material

• Stopwatch
• Measuring tape

To Do and Notice

Move all the masses to the ends of the rods.

Move the end of a rod up and down and watch a wave move down the line of rods.

Measure the speed of the wave.

Math Root

First measure the length of the row of rods.

Then measure the time it takes for a pulse to travel the length of the rods.

The speed is the length divided by the time.

You can also allow the pulse to travel down the row of rods and reflect back travelling twice the distance in twice the time.

Going Further

Move the masses close to the center of each rod.

Measure the speed of the wave again.

Compare the two speeds.

What's Going On?

Each rod is connected to its neighbor by wires which can exert a twisting force or torque on the neighboring rods.

The position of the masses on each rod control its moment of inertia.

When a torque is exerted on a rod with a high moment of inertia, a rod with the weights far from the center, that rod accelerates slowly. When the same torque is exerted on a rod with a lower moment of inertia, when the masses are near the center the rod accelerates more rapidly.

The speed of the wave along the line of rods with the masses far from the center is slower than the speed when the masses are near the center of each rod.

 Scientific Explorations with Paul Doherty © 2005 8 May 2005