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College Physics (Urone)
Found in: Page 426

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Short Answer

Look back to Figure 12.4. Answer the following two questions. Why is P0 less than atmospheric? Why is P0 greater than Pi ?.

The air between the two vehicles has more K.E. than the air which is near the outer side of the vehicle and atmospheric air.

See the step by step solution

Step by Step Solution

Step 1: Pressure

Pressure is the physical force imparted to an object.

Step2: Concept of drop in pressure

When fluid passes through a narrow channel, the kinetic energy of the fluid increases. This energy has to come from somewhere. This energy comes from work done on the fluid. However, the total energy of the system must remain constant and it remains constant at the expense of a fall in random molecular energy i.e. pressure drop. As a result of this, the pressure of the fluid decreases.

Step 3: P0 is less than atmospheric pressure

When a car or truck is moving, it pushes the surrounding air outward. We can assume that the air surrounding to car or truck(region 2 in figure 1) is passing through a narrower cross-sectional area. As per Bernoulli's equation, this results in a decrease in the pressure of the surrounding air. The air which is far away from a car or truck (region 1 in figure 1) is unaffected. Thus, we can say Po is less than atmospheric pressure.

Figure 1: Pressure difference in the air between truck and car

Step 4: P0 is greater than Pi

When both the vehicles are moving, the air which is passing through the narrow channel formed between the two vehicles (region 3 in figure 1) has comparatively more kinetic energy. As per Bernoulli's equation,more kinetic energy will result in more decrease in pressure. The air which is passing near to outer sides of vehicles (region 2 in figure 1) has lower kinetic energy means comparatively greater pressure.Hence, we can say P0 is greater than Pi.

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