HIGHWAY SEISMOGRAPH

As you ride down a highway, you can use your extended arm to model the operation of a seismograph.

Before this activity, you might want to read about how earthquakes are measured. You can also look at pictures of a real seismogram.

What do I need?
• A car, a driver, and a bumpy highway
• A felt-tipped pen
• A pad of lined paper

What do I do?
1. While driving down the highway as a passenger in a car, hold your arm straight out in front of you near, but not touching, the dashboard.

2. Notice that, each time you hit a bump, your hand moves up and down relative to the dashboard.

3. To record the motion, use one hand to hold the paper against the dashboard. Hold the pen in the other hand, stretched out at arm's length. As you drive along, move the pen slowly, tracing a line from left to right across the paper. Notice that, when you hit a bump, the pen moves up and down, making a seismogram-like recording.

What's going on?
When the car hits a bump it accelerates upward. Since your body is firmly attached to the car (you've got your seat belt on, of course), you accelerate along with it. Your hand and arm, however, are less firmly attached to the car, so their mass and inertia cause them to lag behind. Your arm seems to move down relative to the motion of the car. In actuality, your arm stays at the same height above the earth, but the dashboard moves up.

Earthquakes are measured on a logarithmic scale. We can create a similar logarithmic scale for car bumps—called the Pothole Scale—using these steps:

1. First, measure the maximum amplitude of each recorded bump.

2. Then, convert the measurement to micrometers.

3. Finally, take the base 10 logarithm of the amplitude in micrometers to get the bump rating on our new Pothole Scale. A 1 cm amplitude bump, for instance, is 104 micrometers, or a "4" on the Pothole Scale. A Pothole 5 bump is 10 times larger than a Pothole 4, so the pen motion for a pothole 5 is 10 cm. A Pothole 6 bump is big enough to destroy your car!

So what?
The Richter scale measures earthquakes. The Richter magnitude takes the amplitude in micrometers of the pen motion on a Wood-Anderson seismograph and takes the base 10 logarithm of this amplitude. (The magnitude is adjusted to make it what it would have been if the earthquake were 100 km away.)