Behaviour of Fluids


It’s a beautiful summer day when you and your friends go outside to play volleyball, much like the kids in  Figure 1. There’s only one problem—the ball is flat.You pump air into the ball until it is firm. The firmness of the ball is the result of the motion of the air particles in the ball. As the air particles in the ball move, they collide with one another and with the inside walls of the ball. As each particle collides with the inside walls, it exerts a force, pushing the surface of the ball outward. A force is a push or a pull. The forces of all the individual particles add together to make up the pressure of the air. Pressure is equal to the force exerted on a surface divided by the total area over which the force is exerted.


When force is measured in newtons (N) and area is measured in square meters (m2), pressure is measured in newtons per square meter (N/m2). This unit of pressure is called a pascal (Pa). A more useful unit when discussing atmospheric pressure is the kilopascal (kPa), which is 1,000 pascals.



Figure 1

Force and Area

You can see from the equation on the opposite page that pressure depends on the quantity of force exerted and the area over which the force is exerted. As the force increases over a given area, pressure increases. If the force decreases, the pressure will decrease. However, if the area changes, the same amount of force can result in different pressure. Figure 2 shows that if the force of the ballerina’s weight is exerted over a smaller area, the pressure increases. If that same force is exerted over a larger area, the pressure will decrease.



Figure 2

Changes in Gas Pressure

In the same way that atmospheric pressure can vary as conditions change, the pressure of gases in confined containers also can change. The pressure of a gas in a closed container changes with volume and temperature.

Pressure and Volume

If you squeeze a portion of a filled balloon, the remaining portion of the balloon becomes more firm. By squeezing it, you decrease the volume of the balloon, forcing the same number of gas particles into a smaller space. As a result, the particles collide with the walls more often, thereby producing greater pressure. This is true as long as the temperature of the gas remains the same. You can see the change in the motion of the particles in  Figure 3. What will happen if the volume of a gas increases? If you make a container larger without changing its temperature, the gas particles will collide less often and thereby produce a lower pressure.



Figure 3

Float or Sink

You may have noticed that you feel lighter in water than you do when you climb out of it. While you are under water, you experience water pressure pushing on you in all directions. Just as air pressure increases as you walk down a mountain, water pressure increases as you swim deeper in water. Water pressure increases with depth. As a result, the pressure pushing up on the bottom of an object is greater than the pressure pushing down on it because the bottom of the object is deeper than the top. The difference in pressure results in an upward force on an object immersed in a fluid, as shown in Figure 4. This force is known as the buoyant force. If the buoyant force is equal to the weight of an object, the object will float. If the buoyant force is less than the weight of an object, the object will sink.



Figure 4

Archimedes’ Principle

What determines the buoyant force? According to Archimedes’ (ar kuh MEE deez)  principle, the buoyant force on an object is equal to the weight of the fluid dis- placed by the object.  In other words, if you place an object in a beaker that already is filled to the brim with water, some water will spill out of the beaker.

Pascal’s Principle

What happens if you squeeze a plastic container filled with water? If the container is closed, the water has nowhere to go. As a result, the pressure in the water increases by the same amount everywhere in the container—not just where you squeeze or near the top of the container. When a force is applied to a confined fluid, an increase in pressure is transmitted equally to all parts of the fluid. This relationship is known as Pascal’s principle.

 Source:  Glencoe Science-The Nature of Matter-SE_0078617650


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