Tennis Ball Impact on Eye

A blow to the eye by a tennis ball can cause more damage than one might expect because of the transmission of the pressure to the back of the eye.

A doctor I knew used an orange to demonstrate the dangers of a blow to the eye. He would hold the orange in one hand and strike it sharply with the palm of his other hand on one side. The peeling of the orange on the side opposite the blow would split. By Pascal's principle, we know that pressure exerted on one part of an enclosed static fluid will transmit undiminished to all parts of the fluid. The pressure transmitted through the orange to its back side was enough to rupture the peeling, while the side on which the blow occurred suffered no apparent damage. By this example he sought to communicate the danger to the retina from a frontal blow on the cornea of the eye.

Ah! Pascal!

An air mattress takes advantage of Pascal's principle to protect you from the bumpy ground, rocks, and limbs. The pressure is equal on all the parts of your body which are in contact with the air matress. The same principle applies to a water bed since Pascal's principle applies to enclosed static fluids. The principle is used in clinical beds which reduce the incidence of ulcers on patients confined to bed for a lengthy period.

Another variation for hospital beds used for long-term care is the use of very fine particles in the mattress. They have almost fluid properties and approximate Pascal's principle behavior, but can be agitated with a source of vibration to give a gentle massage to the patient's skin.

Hydraulic Brakes

The wheel cylinder of hydraulic drum brakes acts as a double hydraulic press in that the pressure in the input line from the brake pedal is exerted in both directions on the movable wheel cylinder pistons. In both directions, this multiplies the force on the fluid by the ratio of the area of the cylinder to the area of the supply line.

Besides the muliplication of force achieved, Pascal's principle guarantees that the pressure is transmitted equally to all parts of the enclosed fluid system. This gives straight-line braking unless there is a fluid leak or something to cause a significant difference in the friction of the surfaces.

A practical problem with four-wheel hydraulic brakes is that the front of the car will tend to "dive" upon braking. This means that the front of the car tends to drop because the braking force on the front wheels in particular acts to produce a torque that rotates the front of the car downward. This tendency is alleviated somewhat with "anti-dive" brakes which have more braking force exerted by the rear wheels than the front. This is accomplished by making the rear wheel cylinders larger than those in the front wheels. This gives a larger force multiplication with those rear "hydraulic presses" with the same pressure, keeping the straight-line braking but exerting more of it with the rear wheels.