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Physics of Hearing

- Explain the terms in Bernoulli’s equation.
- Explain how Bernoulli’s equation is related to conservation of energy.
- Explain how to derive Bernoulli’s principle from Bernoulli’s equation.
- Calculate with Bernoulli’s principle.
- List some applications of Bernoulli’s principle.

${W}_{\text{net}}=\frac{1}{2}{mv}^{2}-\frac{1}{2}{{mv}_{0}}^{2}\\$

.There are a number of common examples of pressure dropping in rapidly-moving fluids. Shower curtains have a disagreeable habit of bulging into the shower stall when the shower is on. The high-velocity stream of water and air creates a region of lower pressure inside the shower, and standard atmospheric pressure on the other side. The pressure difference results in a net force inward pushing the curtain in. You may also have noticed that when passing a truck on the highway, your car tends to veer toward it. The reason is the same—the high velocity of the air between the car and the truck creates a region of lower pressure, and the vehicles are pushed together by greater pressure on the outside. (See Figure 1.) This effect was observed as far back as the mid-1800s, when it was found that trains passing in opposite directions tipped precariously toward one another.

Hold the short edge of a sheet of paper parallel to your mouth with one hand on each side of your mouth. The page should slant downward over your hands. Blow over the top of the page. Describe what happens and explain the reason for this behavior.

$P+\frac{1}{2}\rho v^{2}+\rho gh=\text{constant}\\$

,
where ${P}_{1}+\frac{1}{2}{{\rho v}_{1}}^{2}+\rho {gh}_{1}={P}_{2}+\frac{1}{2}{{\rho v}_{2}}^{2}+\rho {gh}_{2}\\$

.
Bernoulli’s equation is a form of the conservation of energy principle. Note that the second and third terms are the kinetic and potential energy with $\frac{1}{2}{\rho v}^{2}=\frac{\frac{1}{2}{\text{mv}}^{2}}{V}=\frac{\text{KE}}{V}\\$

So $\frac{1}{2}{\rho v}^{2}\\$

is the kinetic energy per unit volume. Making the same substitution into the third term in the equation, we find$\rho {gh}=\frac{mgh}{V}=\frac{{\text{PE}}_{\text{g}}}{V}\\$

,
so Conservation of energy applied to fluid flow produces Bernoulli’s equation. The net work done by the fluid’s pressure results in changes in the fluid’s KE and PE_{g} per unit volume. If other forms of energy are involved in fluid flow, Bernoulli’s equation can be modified to take these forms into account. Such forms of energy include thermal energy dissipated because of fluid viscosity.

*P*_{1 }+ *ρ**gh*_{1 }= *P*_{2 }+ *ρ**gh*_{2}.

${P}_{2}={P}_{1}+\rho {gh}_{1}\\$

This equation tells us that, in static fluids, pressure increases with depth. As we go from point 1 to point 2 in the fluid, the depth increases by $\Delta{\text{PE}}_{\text{g}}= mgh\\$

) Bernoulli’s equation includes the fact that the pressure due to the weight of a fluid is ${P}_{1}+\frac{1}{2}{{\rho v}_{1}}^{2}={P}_{2}+\frac{1}{2}{{\rho v}_{2}}^{2}\\$

.
Situations in which fluid flows at a constant depth are so important that this equation is often called
In Example 1 from Flow Rate and Its Relation to Velocity, we found that the speed of water in a hose increased from 1.96 m/s to 25.5 m/s going from the hose to the nozzle. Calculate the pressure in the hose, given that the absolute pressure in the nozzle is 1.0 × 10^{5} N/m^{2} (atmospheric, as it must be) and assuming level, frictionless flow.

**Strategy**

Level flow means constant depth, so Bernoulli’s principle applies. We use the subscript 1 for values in the hose and 2 for those in the nozzle. We are thus asked to find *P*_{1}.

**Solution**

Solving Bernoulli’s principle for *P*_{1} yields

**Discussion**

This absolute pressure in the hose is greater than in the nozzle, as expected since *v* is greater in the nozzle. The pressure *P*_{2} in the nozzle must be atmospheric since it emerges into the atmosphere without other changes in conditions.

${P}_{1}={P}_{2}+\frac{1}{2}{{\rho v}_{2}}^{2}-\frac{1}{2}{{\rho v}_{1}}^{2}={P}_{2}+\frac{1}{2}\rho \left({{v}_{2}}^{2}-{{v}_{1}}^{2}\right)\\$

Substituting known values,$\begin{array}{c}{P}_{1} = 7 1.01\times 10^{5} \text{ N/m}^{2} +\frac{1}{2}\left(10^{3}\text{ kg/m}^{3}\right)\left[\left(25.5 \text{ m/s}\right)^{2}-\left(1.96 \text{ m/s}\right)^{2}\right]\\ = 4.24\times {10}^{5}\text{ N/m}^{2}\end{array}\\$

For a good illustration of Bernoulli’s principle, make two strips of paper, each about 15 cm long and 4 cm wide. Hold the small end of one strip up to your lips and let it drape over your finger. Blow across the paper. What happens? Now hold two strips of paper up to your lips, separated by your fingers. Blow between the strips. What happens?

${P}_{1}+\frac{1}{2}{{\rho v}_{1}}^{2}={P}_{2}+\frac{1}{2}{{\rho v}_{2}}^{2}\\$

becomes ${P}_{1}={P}_{2}+\frac{1}{2}{{\rho v}_{2}}^{2}\\$

.$\frac{1}{2}{{\rho v}_{2}}^{2}\\$

, and so the fluid in the manometer rises by $h\propto \frac{1}{2}{{\rho v}_{2}}^{2}\\$

(Recall that the symbol ∝ means “proportional to.”) Solving for ${v}_{2}\propto \sqrt{h}\\$

.
Figure 4(b) shows a version of this device that is in common use for measuring various fluid velocities; such devices are frequently used as air speed indicators in aircraft.- Bernoulli’s equation states that the sum on each side of the following equation is constant, or the same at any two points in an incompressible frictionless fluid:

${P}_{1}+\frac{1}{2}{{\rho v}_{1}}^{2}+\rho {gh}_{1}={P}_{2}+\frac{1}{2}{{\rho v}_{2}}^{2}+\rho {gh}_{2}\\$

.
- Bernoulli’s principle is Bernoulli’s equation applied to situations in which depth is constant. The terms involving depth (or height
*h*) subtract out, yielding

${P}_{1}+\frac{1}{2}{{\rho v}_{1}}^{2}={P}_{2}+\frac{1}{2}{{\rho v}_{2}}^{2}\\$. - Bernoulli’s principle has many applications, including entrainment, wings and sails, and velocity measurement.

1. You can squirt water a considerably greater distance by placing your thumb over the end of a garden hose and then releasing, than by leaving it completely uncovered. Explain how this works.

2. Water is shot nearly vertically upward in a decorative fountain and the stream is observed to broaden as it rises. Conversely, a stream of water falling straight down from a faucet narrows. Explain why, and discuss whether surface tension enhances or reduces the effect in each case.

3. Refer to Figure 1. Answer the following two questions. Why is *P*_{o} less than atmospheric? Why is *P*_{o} greater than *P*_{i} ?

4. Give an example of entrainment not mentioned in the text.

5. Many entrainment devices have a constriction, called a Venturi, such as shown in Figure 5. How does this bolster entrainment?

6. Some chimney pipes have a T-shape, with a crosspiece on top that helps draw up gases whenever there is even a slight breeze. Explain how this works in terms of Bernoulli’s principle.

7. Is there a limit to the height to which an entrainment device can raise a fluid? Explain your answer.

8. Why is it preferable for airplanes to take off into the wind rather than with the wind?

9. Roofs are sometimes pushed off vertically during a tropical cyclone, and buildings sometimes explode outward when hit by a tornado. Use Bernoulli’s principle to explain these phenomena.

10. Why does a sailboat need a keel?

11. It is dangerous to stand close to railroad tracks when a rapidly moving commuter train passes. Explain why atmospheric pressure would push you toward the moving train.

12. Water pressure inside a hose nozzle can be less than atmospheric pressure due to the Bernoulli effect. Explain in terms of energy how the water can emerge from the nozzle against the opposing atmospheric pressure.

13. A perfume bottle or atomizer sprays a fluid that is in the bottle. (Figure 6.) How does the fluid rise up in the vertical tube in the bottle?

14. If you lower the window on a car while moving, an empty plastic bag can sometimes fly out the window. Why does this happen?

1. Verify that pressure has units of energy per unit volume.

2. Suppose you have a wind speed gauge like the pitot tube shown in Example 2 from Flow Rate and Its Relation to Velocity. By what factor must wind speed increase to double the value of *h *in the manometer? Is this independent of the moving fluid and the fluid in the manometer?

3. If the pressure reading of your pitot tube is 15.0 mm Hg at a speed of 200 km/h, what will it be at 700 km/h at the same altitude?

4. Calculate the maximum height to which water could be squirted with the hose in Example 2 from Flow Rate and Its Relation to Velocity if it: (a) Emerges from the nozzle. (b) Emerges with the nozzle removed, assuming the same flow rate.

5. Every few years, winds in Boulder, Colorado, attain sustained speeds of 45.0 m/s (about 100 mi/h) when the jet stream descends during early spring. Approximately what is the force due to the Bernoulli effect on a roof having an area of 220 m^{2}? Typical air density in Boulder is 1.14 kg/m^{3}, and the corresponding atmospheric pressure is 8.89 × 10^{4 }N/m^{2}. (Bernoulli’s principle as stated in the text assumes laminar flow. Using the principle here produces only an approximate result, because there is significant turbulence.)

6. (a) Calculate the approximate force on a square meter of sail, given the horizontal velocity of the wind is 6.00 m/s parallel to its front surface and 3.50 m/s along its back surface. Take the density of air to be 1.29 kg/m^{3}. (The calculation, based on Bernoulli’s principle, is approximate due to the effects of turbulence.) (b) Discuss whether this force is great enough to be effective for propelling a sailboat.

7. (a) What is the pressure drop due to the Bernoulli effect as water goes into a 3.00-cm-diameter nozzle from a 9.00-cm-diameter fire hose while carrying a flow of 40.0 L/s? (b) To what maximum height above the nozzle can this water rise? (The actual height will be significantly smaller due to air resistance.)

8. (a) Using Bernoulli’s equation, show that the measured fluid speed *v*_{ }for a pitot tube, like the one in Figure 4(b), is given by

* h* is the height of the manometer fluid,

*g *is the acceleration due to gravity. (Note that *v *is indeed proportional to the square root of *h*, as stated in the text.) (b) Calculate *v *for moving air if a mercury manometer’s *h *is 0.200 m.

$v={\left(\frac{2\rho′gh}{\rho }\right)}^{1/2}\\$

,
where$\rho′\\$

is the density of the manometer fluid, $\rho\\$

is the density of the moving fluid, and - Bernoulli’s equation:
- the equation resulting from applying conservation of energy to an incompressible frictionless fluid:
*P*+ 1/2*pv*^{2}+*pgh*= constant , through the fluid

- Bernoulli’s principle:
- Bernoulli’s equation applied at constant depth:
*P*_{1}+ 1/2*pv*_{1}^{2}=*P*_{2}+ 1/2*pv*_{2}^{2}

$\begin{array}{c} {P}&=&\frac{\text{Force}}{\text{Area}}, \\ (P)_{\text{units}}&=&\text{N/m}^{2}=\text{N}\cdot\text{m/m}^{3}=\text{J/m}^{3}\\ &=& \text{energy/volume}\end{array}\\$

3. 184 mm Hg

5. 2.54 × 10