My Visit With Galloping Gertie

One of the teaching units I enjoyed most in my physics classroom was on wave motions. Almost everything is capable of some sort of waving motion, or oscillation. The motions come in a wide range of frequencies and amplitudes dependent upon the object. Smaller objects tend to have high frequencies and small amplitudes. Large objects tend to have low frequencies and large amplitudes of motion. It is a fascinating field of study.

Some objects respond to an input of energy of some specific frequency and begin oscillating with the same frequency as the source. Their motion can grow in amplitude as the source of energy continues. A simple example is a pendulum with a child on a swing. Pushing the child at the right time inputs energy to drive the amplitude larger. The energy of drawing a violin bow across the strings of a violin sets some of the strings into vibrations that are large and produce sound. This response to the driving action goes by the name of resonance. A previous post of mine on resonance can be found here.

The resonance response can be so strong that large vibrating objects reach amplitudes that can be easily seen or felt. Between July and November 1940, the new suspension bridge across Tacoma Narrows responded to various strength winds by doing some gentle waving motions as motorists crossed it. Some called the bridge Galloping Gertie.

On 7 November 1940, a half mile long section of the suspension bridge was set into a waving motion none had seen before by a steady 42 mph wind from the side. It collapsed and fell into the narrows. The video of that event is a classic one used by physics teachers every year to dramatically demonstrate the effect of resonance.

Wear Your Helmet

by Jim and Melanie

It was a little past 7 pm. Dinner was finished. The three of us decided to go to the waterfront park for the evening to enjoy the bay and watch people.

“I rode 65+ miles yesterday morning with the bicycle group. My legs are tired from that hard ride. I could use a slow ride to stretch and relax. You two go ahead to the park and find a bench in the usual place. I’ll follow up and meet you a few minutes later on my bicycle.”

We drove down the steep hill to the waterfront. I could see him farther back in the distance in the rear-view mirror going slowly down the rough streets. We reached the parking area and crossed the street to get onto the walkway. It was a beautiful evening. Many people and some other bikers were out. Cars were going along the street, some entering and leaving driveways to restaurants. One car leaving in front of us backed up a few feet to allow us to walk past. We waved our thanks.

We were perhaps 20 feet past that driveway when we heard a strange crashing noise behind us. A bicyclist was sprawled face down on the pavement where we had just walked. He was not moving. The car that allowed us to pass was still sitting there, waiting to see what he would do. Then the realization hit us. That was our son on the pavement!

Fresnel Lenses | How They Work

Basics of Converging Lenses

The converging, or convex lens, is able to bring parallel rays of light toward a focal point. As a child, I played with a magnifying glass lens to burn leaves, grass, and other things.

The lens can also be used in a different way to project light rays parallel to each other in a beam. Simple projectors work on this basic principle. A lighthouse is designed to do this.