Skidding downhill, in this Moment of Science.
Let a toy car roll down a long, steep wooden ramp. If all four wheels turn freely, the car will be going pretty fast when it gets to the bottom.
Now lock all four wheels, maybe with tape stuck to the wheels across the fenders. The car now skids, out of control, down the ramp.
Now, what happens if you lock ”only the rear wheels?" You might guess that the car will now go down the ramp front end first, since the rear wheels skid while the front wheels roll, and friction from the skidding wheels pulls the rear end back— right?
Well, try it—lock only the rear wheels, leave the front wheels free to turn, point the toy car down the ramp and let it go. You'll probably see an amazing result: the car spins around and goes down the ramp backwards—that is, with the locked wheels ahead and the freely rolling wheels trailing behind!
Rolling wheels have better contact with the road than skidding wheels because the bottom point of a rolling wheel is always at rest on the road.
Also, it's always harder to start something sliding, or skidding, over a surface than it is to keep something sliding once it's started: static friction is greater than sliding friction.
So, as the car goes down the ramp, static friction under the front wheels maintains good contact with the ramp. The front end becomes a pivot. Meanwhile, the much smaller sliding friction under the locked rear wheels allows the rear end to come around to the front.
This demonstrates one safety advantage of antilock brakes on a real car. As long as the wheels roll rather than skid, you're using static friction rather than sliding friction to bring your car to a stop. Also, you can steer better when you're rolling than when you're skidding.
This moment of science comes from Indiana University with production support from the Office of the Provost.
I'm Yaël Ksander.
