The pressure applied to the index card by the water is less than the pressure applied by the air. In other words, the air is pushing up on the card more than the water is pushing down on it.
Therefore, the air pressure is holding the paper snug against the cup.
Recall what we know about pressure
Explain why the water remains in the cup.
http://wow.osu.edu/experiments/gases/indexcard.html
http://co5.collegeonline.com/sumner_huckaby/applydevice.002.doc Fig 1
In this drawing, two pistons (red) fit into two glass cylinders filled with oil (light blue) and connected to one another with an oilfilled pipe. If you apply a downward force to one piston (the left one in this drawing), then the force is transmitted to the second piston through the oil in the pipe. Since oil is incompressible, the efficiency is very good
almost all of the applied force appears at the second piston.
The neat thing about hydraulic systems is that it is very easy to add force multiplication (or division) to the system. If you know how gears work, then you know that trading force for distance is very common in mechanical systems. In a hydraulic system, all you do is change the size of one piston and cylinder relative to the other, as shown here:
Basic Hydraulics Pascal’s Law
In the seventeenth century. Pascal, a French scientist. discovered the hydraulic lever. In laboratory experiments he proved that force and motion could be transferred by means of a confined liquid. Experimenting with weights and pistons of varying size. Pascal also found that mechanical advantage or force multiplication could be obtained in a pressure system and that the relationships between force and distance were exactly the same as with a mechanical lever. From the data Pascal collected, he formulated a law, which states:
Pressure on a confined fluid is transmitted equally in all directions and acts with equal force on equal areas.
So the basic idea behind any hydraulic system is very simple: Force that is applied at one point is transmitted to another point using an incompressible fluid. The fluid is almost always an oil of some sort. The force is almost always multiplied in the process. The picture below shows the simplest possible hydraulic system:
Fig. 2
Hydraulic multiplication. The piston on the right has a surface area nine times greater than the piston on the left. When force is applied to the left piston, it will move nine units for every one unit that the right piston moves, and the force is multiplied by nine on the righthand piston.
http://covenantchristian.org/bird/links/mathphysc/pascal.pps
Title: Feb 23 8:16 AM (2 of 7)
Applications of Fluid Principles
by Ron Kurtus (revised 22 August 2004)
There are several major applications of the special properties of fluids. The pressure of fluids can be amplified through the use of hydraulic mechanisms.
Changes in pressure with the velocity of the fluid allow airplanes to fly. Fluids are also used to reduce friction.
Questions you may have include:
* How can pressure by amplified through hydraulic mechanisms?
* How do airplanes fly?
* How is friction reduced?
This lesson will answer those questions. There is a miniquiz at the end of the lesson.
Hydraulics
Hydraulics is the scientific field that concerns fluidsusually liquidsthat are in partially enclosed containers, such that you can apply pressure in one area. An example is a cylinder with a piston.
Pressure from single piston
If you have a cylinder filled with a liquid and apply a force to a piston on one end of the cylinder, the pressure (P) on the walls of the cylinder equal the force (F) divided by the area (A) of the piston in the cylinder.
P = F / A Pressure on second piston
Now, if the first cylinder was connected to a second cylinder of larger diameter, the pressure inside that cylinder would be the same P, but the force F2 applied to the larger piston would now be:
P = F2 / A2 Pressure is the same The pressure for both is the same. Thus, F2 / A2 = F / A
or F2 = (F x A2) / A
For example, if F = 100 pounds and A = 5 square inches, then P = 20 pounds/square inch. P is the same on both pistons.
Force greater on large piston
If the larger piston had an area of A2 = 25 square inches, and the pressure remained at P = 20 pounds/square inch, then the resulting force on that piston would be F2 = (F x A2) / A = (100 x 25) / 5 = 500 pounds.
This is a mechanical advantage, similar to that seen with levers.
Used in brakes
Hydraulic mechanisms are used in the brakes in your car. The force applied on the brake pedal is multiplied on the brake drums. Another use is to jack up a heavy item, like a truck.
Velocity reduces pressure
The scientist Bernoulli discovered that the air pressure in a tube goes down when the velocity of the air in the tube increases. This discovery became known as Bernoulli's Principle.
Used by airplanes
The greatest application of this principle is used in airplanes. The wing of an airplane is usually curved on top and flat on the bottom. This shape is called the airfoil. When the air moves over the curved top portion of the wing or airfoil, it speeds up because of the shape. This lowers the pressure with respect to the bottom part of the wing. Lower pressure on the top results in the lift required to keep the airplane aloft.
The principle is so simple, but not very obvious.
Flying upsidedown
But if the airfoil gives lift, how can an airplane fly upside down?
If the airplane is going fast enough, other factors influence the lift. When the plane is upsidedown, it is really flying at a slight angle, so it is going slightly upward to compensate for the loss of lift.
Some airplanessuch as an airlinercan have great difficulty flying upside down. Usually only smaller stunt planes and military craft can do this maneuver.
Friction reduced
Solids can have rough surfaces. Even microscopic roughness can result in a substantial resistive force of friction when two solids are rubbed together, as well as wear on the parts.
Fluids offer little resistance
On the other hand, a fluid does not have a rough surface and rubbing a solid along a fluid results in little resistive force. Instead of friction, the resistance is due to the thickness or viscosity of the liquid, which affects its ability to move and change its shape.
Used as lubricants
The reduction of friction of two solids can then be achieved by separating them by a layer of a fluid, so the solid surfaces are not in direct contact. This is called lubrication. Water can be used as a lubricant, but it also evaporates quickly. Oils are typically used to lubricate parts and prevent friction, as well as excessive wear from the friction. In some small, highspeed parts, such as the harddrive of your computer, air is used as a lubricant.
In conclusion
Hydraulics use fluid pressure to create the same mechanical advantage as a lever. The Bernoulli Principle allows airplanes to fly from the lift created by reduced air pressure on the top of their wings. Fluids also can be used to reduce friction.
See Reader Questions and Feedback on this topic Jump to Where can you go from here? section Apply what you know
Miniquiz to check your understanding
1. If the area of the larger piston in an hydraulic device doubles, what happens to the force applied to that piston?
It stays the same It doubles