Hydraulic brakes
The braking system of a motor vehicle is another case where the principle of transmitting liquid pressure applies — and this one we trust our lives to every day.
Figure to be added
From the pedal to the wheel
- The driver applies a force with the foot to the pedal.
- That force acts on the piston of the master cylinder.
- The piston produces a pressure in the oil inside the cylinder.
- The oil transmits that pressure to the slave cylinder near the wheel.
- The brake pads connected to the slave cylinder's piston are pressed against the brake drum or disc.
A force greater than the foot's
The cross-sectional area of the slave cylinder is larger than that of the master cylinder. Since the pressure is transmitted unchanged, the force the slave cylinder applies to the brake pads is greater than the force the driver applies with the foot.
Every wheel at once
One oil system connects the slave cylinders at all the wheels, so the pressure produced at the pedal reaches all of them together. Every wheel therefore brakes at the same moment.
Air in the brake line
If air gets into the brake lines the brakes go soft. The reason is simple — air can be compressed, so part of the force applied at the pedal is spent squeezing that air rather than transmitting pressure.