Tuesday, August 30, 2011

Using physics (and skateboards) to minimize a three-car collision

You're at a stoplight. There is a car in front of you and you're both waiting for the light to turn green. In your rear view mirror, you see another car approaching the intersection at high speed, obviously unaware that the light is red. He is going to hit you. What should you do?

Should you just sit there and pray that you'll be alright? Should you stand on the brake pedal? Should you tap the car in front of you? To find out, we strapped a trio of accelerometers onto skateboards and crashed them together to see which scenario would result in the least acceleration for the car in the middle.

Buzz Skyline theorized that you could minimize your acceleration by inching up to the car in front of you, touching its bumper, and then applying heavy braking, turning yourself into the middle ball in an over-sized Newton's Cradle.

The classic executive office toy demonstrates the conservation of momentum with five balls suspended next to one another. Swinging the ball on one end into the row of other balls stops the first ball and transfers the momentum through the middle balls, which remain stationary, to the ball on the other end, which swings away. The idea is that by touching the car in front of you, you can transfer the momentum of the car crashing into you to that front car and you will move very little just like the middle balls in the cradle. To see if that would happen, we set up a skateboard collision center in our office hallway.

First, we tested a two-board collision to see what kind of acceleration the stationary middle board would feel on its own. Then, we added a third board in front of the middle board leaving an eight inch gap between the two. The middle board's acceleration, we knew, should be the same in both tests. Lastly, we tested a three-board collision with the two stationary boards touching as though bumper-to-bumper. We repeated the test scenarios three times, recording each board's accelerations with the accelerometers.

[The PhysicsBuzz team tests car collisions using skateboards in the APS hallway.]

To test the theory that adding brakes would reduce the middle car's acceleration, we used masking tape to secure the middle board's wheels, preventing them from spinning. We then repeated the three collision scenarios with the "brakes" on, again, three times over.

The graphs produced from the collisions showed that our accelerometers might not be up to the task of recording skateboard collisions. Several of the graphs showed higher accelerations for the second collisions when we know the first and second collisions should be the same for the middle board. It looks like the instruments aren't quick enough to record the very rapid accelerations that the middle board experiences in each test. To get around that, we averaged the tests.

For the no-brakes test, the middle board's acceleration for the two-board collision was 16 meters per second squared. The three-board with a gap collision averaged 17 meters per second squared. For the three-board collision with the bumpers touching, however, the middle board's acceleration was half those averages at eight meters per second squared.

With the brakes on, the results were almost exactly the same. The first two scenarios averaged an acceleration of 16 meters per second squared, but the final scenario, with the bumpers touching, gave us 11 meters per second squared. Though we expected to find smaller accelerations with the brakes on, the one thing that was consistent throughout the data sets was that the middle board's acceleration was always smallest during the bumper-on-bumper impact.

So there you have it. If you ever find yourself stopped at a red light with a car barreling toward your behind, consider pulling up and kissing the car in front of you. Whether or not you want to step on the brakes is up to you. Let us know how it turns out because until we do a full-scale test (anyone want to volunteer their cars?) we can't say for sure whether it will really keep you safe.


  1. A couple things that jump out at me right away: From the purely physical standpoint, the issue is that skateboards don't have crumple zones (and nor do the Newton's balls), so not touching the car in front of you (if you have left enough room) might actually allow you to not damage that vehicle and incur further damage to your own. On that note, if you do pull up and touch the other car, you might be liable for some of the damage to that vehicle since if you left room, the damage to it would probably be less or possibly none, especially if you're standing on your brakes.

  2. PS: Get the Mythbusters on this for the full-scale. ;)