FLUIDS REVIEW



FLUIDS REVIEW

1. List some examples of fluids

Any liquid or gas

2. How are liquids and gasses different?

Liquids have a definite volume

3. A solid rubber ball with a density of 0.98 g/cm3 has a radius of 1.0 cm. When the ball is placed in water, what buoyant force acts on the ball? (Vsphere = 4/3πr3, ρw = 1.00 g/cm3)

Since density of ball is less than water, the ball will float. If the ball floats, the buoyant force is equal to the weight of the ball.

Fb=Wball

Fb=mballg

Fb=ρballVballg

Fb=(0.98)[ 4/3π(1)3](9.81) = 40.3 N

4. The density of gold is 19.3 g/cm3. If a 1000 g gold bar was placed in a container of water, what volume of water would have been displaced?

V=m/ρ

V=1000/19.3 =51.8cm3

5. How do the densities of an object and a fluid affect whether or not the object will float in that fluid?

If an object’s density is less than the fluid, it will float; if it greater than the fluid it will sink

6. If an object is floating, what do you know about the buoyant force and the weight of the object?

They are equal

7. What is the relationship between apparent weight, weight and buoyant force?

Apparent Weight = Weight – Buoyant Force

8. What are units of pressure?

Pascal (Pa), N/m2, atm

9. What does Pascal’s principle state?

pressure applied to a fluid in a closed container is transmitted equally to every point of the fluid and to the walls of the container

10. Each of the tires on a bicycle has an area of 0.005 m2 in contact with the ground. The weight of the bicycle and rider is 800 N. What is the pressure in the tires?

Since there are 2 tires, the force is divided equally among them.

P = F/A

P = 400/.005 = 80000 Pa

11. You apply a 100 N force to a hydraulic jack piston with area 0.00001 m2. If the piston on the other side of the jack has an area of 0.001 m2, what is the maximum weight car your jack can lift?

F1/A1 = F2/A2

100/.00001 = F2/.001

F2 = 10000 N

12. How much pressure is exerted on a submarine at a depth of 8.50 km in the Pacific Ocean? (The density of sea water = 1.025 × 103 kg/m3, and the atmospheric pressure at sea level = 1.01 × 105 Pa.)

P = P0 + ρgh

P = 1.01 × 105 + (1.025 × 103)(9.81)(8500)

P = 8.56 × 107 Pa

13. Water at a pressure of 3.00 × 105 Pa flows through a horizontal pipe at a speed of 1.00 m/s. The pipe narrows to one-fourth its original diameter. What is the speed of the flow in the narrow section?

16.0 m/s

14. A 2.0 cm diameter faucet tap fills a 2.5 × 10−2 m3 container in 30.0 s. What is the speed at which the water leaves the faucet?

2.7 m/s

15. The time required to fill a glass with water from a large container with a spigot is 30.0 s. If you replace the spigot with a smaller one so that the speed of the water leaving the nozzle doubles, how long does it take to fill the glass?

30.0 s, because the volume rate of flow is constant

16. A fluid has a flow rate of 5.0 m3/s as it travels at a speed of 7.5 m/s through a pipe. What is the cross-sectional area of the pipe?

[pic]

17. A fluid flows through a pipe whose cross-sectional area changes from 2.00 m2 to 0.50 m2. If the fluid’s speed in the wide part of the pipe is 3.5 m/s, what is its speed when it moves through the narrow part of the pipe?

[pic]

18. What happens when a breeze blows between two foam plastic balls that are hung by strings 5 cm apart?

The balls move toward each other.

19. Use Bernoulli’s principle to explain why pressure in a fluid decreases within a pipe as its radius decreases.

From the continuity equation, the speed of a fluid in a narrowing pipe increases. This increase in speed indicates that the fluid is being accelerated forward by a net force, and that this force arises from a difference in pressure within the fluid. Because the force is in the forward direction, the pressure at the front of the fluid must be lower than the pressure behind it. Thus pressure within the fluid decreases as the fluid’s speed increases.

20. Use Bernoulli’s principle to explain how an airplane achieves lift by moving forward at high speed.

As the airplane moves forward with greater speed, the flow of air around the airplane wing reduces the pressure of the air. The wing is shaped so that air flows more rapidly over the wing than under it, and therefore causes the pressure above the wing to be lower than the pressure below it. The greater pressure under the wing raises the wing, causing the airplane to lift.

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