2002-11-03

# Projectile Motion Pictures

These works of geeky art are produced by setting my digital camera to a long exposure, turning out the lights, having an assistant (my ever patient girlfriend) trigger the shutter and then throwing a strobe across the room. The strobe is a DSE kit that I built long ago. I had it out with the intention to take it home and measure the speed of my trivial motor project, but got creative while I was still at the office.

Here is the strobe, even though it was being throw onto the couch, it did take a couple of hard knocks (ever tried throwing something in the dark with any kind of accuracy?), landing on the floor many times. It still works fine, no damage at all.

I can't remember what rate I had it set at, I just set what seemed like a reasonable flash rate and tried a few shots. The circuit was set on wide, but the velocities involved didn't smear out the pulses very noticeably. In the full-size pics you can see it, but it is not obtrusive.

The hardest part was making sure the strobe LED kept pointing roughly at the camera throughout the whole flight. This is rather hard after a few bounces. Also, after it lands and the shutter is still opened it often illuminated the area.

Anyway, onto the pictures. Our first is the long one on the right, it is a almost vertical linear drop with a small bounce. As you can see in the bounce the LED turned away from the camera for part of the path as the zippy box the strobe was is rotated.

The second on the left is a parabolic projectile motion path complicated at the end (left) with a few bounces. The strobe bounced off the back of the couch once, the off the arm twice before ending up on the floor and out of the frame.

The third picture shows the strobe lighting the couch after it stopped moving. This path is similar to the second one, but the parabolic path is not symmetric, I assume this is due to rotation of the strobe or air resistance? The shape reminds me of the resisted projectile motion paths I used to compute in high school 4 unit maths.

These pictures all show horizontal velocity is basically constant during flight. The drop shot also shows gravitational acceleration, if I had noted the timing I could have done some math with it and got some useful results. The parabolic paths are great for just plain proof that projectile paths are close to parabolic in the abscence of large resistance, they also demonstrate the drop in vertical velocity caused by gravity, followed by an increase in the other direction after the peek altitude is achieved.

Now a few muck-up shots. The first is of me waving the strobe around while sitting on the couch. Note how the colour of the "white" LED changes off-axis, this is not an exposure effect, the phosphor coating is not perfect in these early devices.

The second one shows my face illuminated by the strobe in the final few seconds of the exposure.

Fun stuff!