Locked Brakes

The difference between static and kinetic friction can cause the car to skid when braking.

What it shows:

The automobile is a radio-contolled car (the same one we use for the Reactionary Roadbed). When the front wheels are locked so that they can't turn, the car simply slides to a stop. This is because the static friction of the rolling rear wheels is greater than the kinetic friction of the sliding front wheels. If instead the rear wheels are locked, the car goes into a dramatic tailspin as it comes to a stop. In this case the lesser friction of the rear wheels alllows the rear of the car to slide forward ahead of the front of the car. This is known as a tailspin.

How it works:

To prevent them from rolling and simulating "locking the breaks,", the wheels are secured to the body of the car with duct tape. The car is allowed to roll down a tilted 4'×8' sheet of styrofoam. The car rolls straight down without any brakes or if the front wheels are locked. In contrast, locking the rear wheels (but not the front) results in a 180 degree spin as it slides down the hill.

Setting it up:

Use aluminum sheathed styrofoam. One end rests on the floor and the other end on a stool.


The demo is a good opportunity to explain anti-locking brake systems found on all modern cars.