You have probably seen lots of advertisements for cars that talk about "Anti-lock Braking System" or "ABS". In this activity we will look at what ABS does.
You will need:
A toy car
A ramp of some sort that the car can roll down. (You may want to use a tabletop with one end propped up, the top of a cardboard box or a plank.)
Some blutak
What to do
Set up the ramp so it is fairly steep.
Roll the car down the ramp. It should go straight down.
Use a small lump of blutak to lock the back wheels of the car, so they won't spin. Put the blutak at the top of the wheels, so it won't scrape on the ground.
Try running the car down the ramp again. When I tried this, the car ended up spinning around and driving backwards down the ramp.
Try locking up the front wheel, or all four wheels. What happens?
Whats happening?
You should have found that when the rear wheels were locked, the car spun around as it rolled down the hill. This is because the rear wheels were sliding against the ground, while the front wheels were rolling. Rolling and sliding are actually very different.
If you follow a point on the edge of a rolling wheel you will see that it goes down, touches the ground and then goes up again. It doesn't slide against the ground. A rolling wheel will go forwards or backwards easily, but will not go sideways unless it starts sliding.
When the wheel is locked, it slides instead of rolling. It takes the same force to make it go in any direction, so a locked-up wheel will slide in any direction.
When your car is going down the ramp with the rear wheels locked, the rolling front wheels will always try to move in the direction the car is pointed. The sliding back wheels always try to go downhill, no matter which way the car is pointing. If the front wheels are pointed a tiny bit to the left, the front will try to go to the left, but the back will keep going straight downhill. This will twist the car around until the back wheels are at the front. When the wheels at the front lock up, they just go straight down the ramp and just drag the other wheels behind them.
If the back wheels are locked (red) they will try to slide straight down the ramp. If the front wheels are rolling (blue), they will try to go in the in the direction they are facing. This can make the front wheels go off to one side while the back wheels go straight down, spinning the car around.
If the front wheels lock (red) they will try to slide straight down the hill. If the rear wheels are rolling (blue), they will be pulled in behind the front wheels, so the car will go straight downhill. It doesn't spin, but it also can't steer.
In a real car, the wheels can lock up if you brake very hard. Locking the wheels in a car is bad for two reasons. The first is that you can't steer the car and it will probably spin, just like your toy car. The other problem with locking the wheels is that a sliding wheel takes longer to stop than a rolling wheel, because a rolling wheel can apply more force to the ground.
Applications
An Anti-lock Braking System (ABS) has a computer that monitors what the wheels are doing and can actually reduce the pressure on the brakes. If the driver tries to brake hard, the computer will notice if the wheels are about to lock up and reduce the pressure of the brakes. This means the wheel will almost, but not quite, lock up, which gives you the greatest braking force and lets the driver steer.
Some cars only have ABS on the rear wheels, to stop the car going into a spin if the driver brakes hard. If the front wheels lock up the driver can't steer the car, but at least it won't spin. Most new cars with ABS have it on all four wheels, which gives the best stopping distance and control.
Try to make your ramp reasonably steep
Locking the rear wheels with the blutak
Locked rear wheels make the car go into a spin as it goes down the ramp. (This images was made from several separate images of the car on the ramp.)
