The ticker-tape car

By Marcus Wilson 28/05/2013


Somewhere in the Cambridge / Hamilton vicinity is a car with no oil in it. I know this because on the way in to work this morning there was a trail of oil on the road.  The damp road surface led to it being very prominent. A splash of oil, being less dense than water, will sit as a thin film on the water surface and show some colourful patterns due to interference of light reflected from the top and bottom surfaces.  

What was also clear was that these splashes of oil were not placed at equal intervals. They were closer together at intersections, and well spaced along the main road (to the point that I couldn’t follow them at times). A reasonable conclusion is that the oil was dripping at a roughly constant rate (a roughly constant time between each drips). When the car was travelling fast, there was a long time between splashes. When the car was travelling slowly, they were close together (I could see that the car had clearly stopped at the roundabout in the centre of Cambridge, for example). On the assumption that the car was travelling at approximately the speed limit on most roads, I could have estimated the rate of dropping by measuring the distance between the drops. I might find physics fun, but I don’t find it so fun as to stop on the side of SH1 and get out a long tape measure with heavy traffic roaring past, so I’m afraid I don’t have an answer to this. Perhaps more worryingly for the car driver, the car was leaving behind evidence of whether it had really stopped at stop signs. 

The pattern of splashes reminded me a lot of our experimental introduction to kinematics at school. We used a ticker-tape machine. We had a cart that was placed on a ramp, and the cart pulled a stream of ticker-tape behind it. The tape went through a machine that stamped it with dots at a constant rate. If the dots were close together, it was because the cart was moving slowly; if they were far apart, it was because the cart was moving fast. By analyzing the dots afterwards we could work out the velocity of the cart at any point on its descent of the ramp and its acceleration. 

Nowadays you can do the same experiment with a camera and a bit of computer software to do all the calculations for you. It might be more efficient, but its probably not as constructive as ticker-tape in getting a student’s head around what distance, velocity and acceleration are. And it’s definitely not as fun as ticker-tape was.