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College Physics for AP® Courses 2e

12.3 The Most General Applications of Bernoulli’s Equation

College Physics for AP® Courses 2e12.3 The Most General Applications of Bernoulli’s Equation

Learning Objectives

By the end of this section, you will be able to:

  • Calculate using Torricelli’s theorem.
  • Calculate power in fluid flow.

Torricelli’s Theorem

Figure 12.8 shows water gushing from a large tube through a dam. What is its speed as it emerges? Interestingly, if resistance is negligible, the speed is just what it would be if the water fell a distance hh from the surface of the reservoir; the water’s speed is independent of the size of the opening. Let us check this out. Bernoulli’s equation must be used since the depth is not constant. We consider water flowing from the surface (point 1) to the tube’s outlet (point 2). Bernoulli’s equation as stated in previously is


Both P1P1 and P2P2 equal atmospheric pressure (P1P1 is atmospheric pressure because it is the pressure at the top of the reservoir. P2P2 must be atmospheric pressure, since the emerging water is surrounded by the atmosphere and cannot have a pressure different from atmospheric pressure.) and subtract out of the equation, leaving


Solving this equation for v22v22, noting that the density ρ ρ cancels (because the fluid is incompressible), yields


We let h=h1h2h=h1h2; the equation then becomes


where hh is the height dropped by the water. This is simply a kinematic equation for any object falling a distance hh with negligible resistance. In fluids, this last equation is called Torricelli’s theorem. Note that the result is independent of the velocity’s direction, just as we found when applying conservation of energy to falling objects.

Part a of the figure shows a photograph of a dam with water gushing from a large tube at the base of a dam. Part b shows the schematic diagram for the flow of water in a reservoir. The reservoir is shown in the form of a triangular section with a horizontal opening along the base little near to the base. The water is shown to flow through the horizontal opening near the base. The height which it falls is shown as h two. The pressure and velocity of water at this point are P two and v two. The height to which the water can fall if it falls from a height h above the opening is given by h 2. The pressure and velocity of water at this point are P one and v one.
Figure 12.8 (a) Water gushes from the base of the Studen Kladenetz dam in Bulgaria. (credit: Kiril Kapustin; (b) In the absence of significant resistance, water flows from the reservoir with the same speed it would have if it fell the distance hh without friction. This is an example of Torricelli’s theorem.
Figure shows a fire engine that is stationed next to a tall building. A floor of the building ten meters above the ground has caught fire. The flames are shown coming out. A fire man has reached close to the fire caught area using a ladder and is spraying water on the fire using a hose attached to the fire engine.
Figure 12.9 Pressure in the nozzle of this fire hose is less than at ground level for two reasons: the water has to go uphill to get to the nozzle, and speed increases in the nozzle. In spite of its lowered pressure, the water can exert a large force on anything it strikes, by virtue of its kinetic energy. Pressure in the water stream becomes equal to atmospheric pressure once it emerges into the air.

All preceding applications of Bernoulli’s equation involved simplifying conditions, such as constant height or constant pressure. The next example is a more general application of Bernoulli’s equation in which pressure, velocity, and height all change. (See Figure 12.9.)

Example 12.5

Calculating Pressure: A Fire Hose Nozzle

Fire hoses used in major structure fires have inside diameters of 6.40 cm. Suppose such a hose carries a flow of 40.0 L/s starting at a gauge pressure of 1.62×106N/m21.62×106N/m2. The hose goes 10.0 m up a ladder to a nozzle having an inside diameter of 3.00 cm. Assuming negligible resistance, what is the pressure in the nozzle?


Here we must use Bernoulli’s equation to solve for the pressure, since depth is not constant.


Bernoulli’s equation states

P 1 + 1 2 ρv 1 2 +ρ gh1 =P2+ 12 ρv22 +ρ gh2 , P 1 + 1 2 ρv 1 2 +ρ gh1 =P2+ 12 ρv22 +ρ gh2 ,

where the subscripts 1 and 2 refer to the initial conditions at ground level and the final conditions inside the nozzle, respectively. We must first find the speeds v1v1 and v2v2. Since Q = A 1 v 1 Q = A 1 v 1 , we get


Similarly, we find

v2=56.588 m/s.v2=56.588 m/s.

(This rather large speed is helpful in reaching the fire.) Now, taking h1h1 to be zero, we solve Bernoulli’s equation for P2P2:

P2=P1+12ρ v 1 2 v 2 2 ρgh2.P2=P1+12ρ v 1 2 v 2 2 ρgh2.

Substituting known values yields

P2= 1.62 × 106 N/m2 + 12 (1000 kg/m3 )[ (12.434m/s )2 (56.588m/s )2] (1000 kg/m3 )(9.80 m/s2 )(10.0 m) 1800N/m2. P2= 1.62 × 106 N/m2 + 12 (1000 kg/m3 )[ (12.434m/s )2 (56.588m/s )2] (1000 kg/m3 )(9.80 m/s2 )(10.0 m) 1800N/m2.


This value is a gauge pressure, since the initial pressure was given as a gauge pressure. Thus the nozzle pressure is very close to atmospheric pressure, as it must because the water exits into the atmosphere without changes in its conditions.

Power in Fluid Flow

Power is the rate at which work is done or energy in any form is used or supplied. To see the relationship of power to fluid flow, consider Bernoulli’s equation:


All three terms have units of energy per unit volume, as discussed in the previous section. Now, considering units, if we multiply energy per unit volume by flow rate (volume per unit time), we get units of power. That is, (E/V)(V/t)=E/t(E/V)(V/t)=E/t. This means that if we multiply Bernoulli’s equation by flow rate QQ, we get power. In equation form, this is


Each term has a clear physical meaning. For example, PQPQ is the power supplied to a fluid, perhaps by a pump, to give it its pressure PP. Similarly, 12ρv2Q12ρv2Q is the power supplied to a fluid to give it its kinetic energy. And ρghQρghQ is the power going to gravitational potential energy.

Making Connections: Power

Power is defined as the rate of energy transferred, or E/tE/t. Fluid flow involves several types of power. Each type of power is identified with a specific type of energy being expended or changed in form.

Example 12.6

Calculating Power in a Moving Fluid

Suppose the fire hose in the previous example is fed by a pump that receives water through a hose with a 6.40-cm diameter coming from a hydrant with a pressure of 0.700×106N/m20.700×106N/m2. What power does the pump supply to the water?


Here we must consider energy forms as well as how they relate to fluid flow. Since the input and output hoses have the same diameters and are at the same height, the pump does not change the speed of the water nor its height, and so the water’s kinetic energy and gravitational potential energy are unchanged. That means the pump only supplies power to increase water pressure by 0.92×106N/m20.92×106N/m2 (from 0.700×106N/m20.700×106N/m2 to 1.62×106N/m21.62×106N/m2).


As discussed above, the power associated with pressure is

power = PQ = 0.920×106N/m2 40.0×103m3/s. = 3.68×104W=36.8kW . power = PQ = 0.920×106N/m2 40.0×103m3/s. = 3.68×104W=36.8kW .


Such a substantial amount of power requires a large pump, such as is found on some fire trucks. (This kilowatt value converts to about 50 hp.) The pump in this example increases only the water’s pressure. If a pump—such as the heart—directly increases velocity and height as well as pressure, we would have to calculate all three terms to find the power it supplies.

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