Physics is the branch of science that deals with matter, energy and the interaction between them. It covers subjects such as forces and motion as well as electricity and electronics. If it moves or has energy it is covered in Physics class.

Like many of our classes, the students’ interests determined the course of study for the recently completed third session of Physics This class for the older students culminated with the students photographing a high speed object (the BB from an air rifle) with a camera. Yes, you read that right- guns in school. More about the air rifle later.

The Idea
It all started when students expressed interest in the high speed video they see on television, specifically seeing things break or blow up in super slow motion. They expressed a strong desire to learn to make a video like this. Of course the school can’t afford $50,000 video cameras; the teacher had to find a compromise. Some research yielded methods to take high speed photographs using an ordinary flash unit in a darkened room. The teacher thought this would be a good goal for the six week class since it covered setting up repeatable experiments, velocity of objects, electrical circuits, properties of sound waves and kinetic energy. The students were excited and worked to learn this technique and its nuances in order to achieve their goals.

Capturing a photograph of a high speed event requires very little equipment and a lot of preparation. To record sharp photos of a fast moving object required that they capture an image over a very short amount of time, about 1/30,000th of a second. Since a fast camera can open and close their shutter in 1/8000th of a second a different approach was required. If you open the shutter of a camera in a dark room and then set off a flash unit to light up the room for 1/30,000th of a second you can freeze fast moving objects, like when a strobe light flashes on a darkened dance floor. The trick is to initiate the flash unit at the right time, since only one flash is required to capture the image on the camera.

The Preparations
The first concept the students had to understand was velocity and conversion of units. They started small, by timing ourselves in a 100 meter dash and using the times to determine their scalar velocities. They also practiced their ability to quickly convert SI (metric) units to English units and back again.

In order to build a device that would initiate the flash unit the students had to learn about another branch of physics, basic electronic circuits. They had to understand the function of resistors, capacitors and other components to understand how a sound initiator functioned. This type of device uses an amplified microphone connected to the flash unit to spark the flash. This sound initiator was used by the children to take photographs of balloons popping. They learned that the location of the initiator was essential to the success of the photo due to the speed that sound travels. They observed what photos looked like as the initiator’s microphone was moved farther from the balloon. After multiple attempts they were able to calculate how long it took for a balloon to pop based on the position of the microphone and the speed of sound.

The Next Step
This balloon photo was an excellent start, but being between 10 to 15 years of age, they really wanted to see something break apart more dramatically. This led to the cracker and the egg. One of the key factors in taking high speed photographs is consistency. Yes, a student could hit an egg with a hammer, but could they do it in the dark and be absolutely consistent so the result is measurable? Besides, eggs don’t make noise like balloons. The class talked about various methods they could rely upon to safely make something move fast and accurately while also being safe. One student suggested a firecracker, but that was immediately vetoed by the teacher as being completely unsafe and also illegal.

The next student offered the use of his BB gun. It took quite a while for the teacher to answer that offer. The response was based on many positive attributes: the ability to move an object(the BB) with enough velocity to make a dramatic photo but not so quickly as to hurt someone if something went wrong, moving the object at a consistent velocity to make the experiment repeatable and having the ability to secure it to eliminate the inconsistency of a human aiming a gun. The potential drawback was singular but huge, it was a gun. Yes, guns are weapons, but they are also really just tools, not unlike the knives in the kitchen or the ones used to carve wood in class. Based on the safety and control the students had demonstrated in Archery and the use of carving knives in Woodworking, the answer to the offer was eventually “yes”.

This turn of events required a whole new type of initiator, since a BB gun is too quiet for the sound initiator. A new flash initiator based on an infrared beam was built and tested. It was designed to send a signal to the flash a short time after the BB broke the beam of light. Varying the delay gave them the ability to calculate the velocity of the BB as well as the kinetic energy of the impact.

The Experiment
To ensure the safety of the group members the air rifle was placed in a vise to aim and secure it. Students were given a safety briefing and were required to stand in a safe area to the rear of the experiment set up. The BB’s were safely stopped by a piece of cloth after making the twelve inch trip from the end of the barrel to the target.

The students were able to take two photos, one of a cracker just at the moment of the BB’s impact (that copper colored ball is the BB traveling at 50 mph) and an egg (the BB has entered the egg but still hadn’t exited at the time of the photo). From these two photos, the students were able to see how the kinetic energy was transferred to the cracker as well as how the liquid in the egg absorbed the energy and emerged with just a few cracks.

The Lesson
In the end, the class accomplished their goal of taking a photo of something dramatic and to freeze time to find out what really happens when things break. They had to learn about velocity, electronic circuits, sound, microphones and kinetic energy before finally using a BB gun to capture their photographs. The world we live in is filled with an appalling amount of violence and guns, both in movies and real life. It’s actually a relief to learn that sometimes a gun can be used as the carrot rather than the stick.

This entry was posted on Thursday, January 21st, 2010 at 9:02 am and is filed under news. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.