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# Mass, Pressure and Area

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A force of 1.05x 10^3 N is required to lift an object that is under the influence of an acceleration due to gravity of 7.76ft/sec^2. What is the object's mass in kilograms?

A waterbed mattress has the following dimensions: 210 cm X 160 cm X 23 cm. The density of water is 62.4 lb/ft^3. What is the weight of the bed in pounds? If the mattress is placed on the floor (no bed frame), how much pressure does it exert on the floor in pounds per square inch?

A stack of bricks weighs 174 KN (Kilo-newtons). The stack exerts 200 KPa (Kilo-pascals) of pressure on the ground. What is the area upon which this pressure is exerted (in square ft)?

The Hubert H. Humphrey Metrodome in Minneapolis opened in 1982 at a cost of \$55 million. To prepare for the footings, et cetera, 3.00 x 10^5 cubic yards of dirt were removed. The Metrodome's inflatable roof covers 10.0 acres and its interior volume is 60.0 x 10^3 cubic feet. During construction, 40.0 x 10^3 cubic yards of concrete, 11.9 x 10^3 tons of reinforcing steel, and 5.00 x 10^2 tons of structural steel were utilized. The roof material includes an outer layer of Teflon-coated glass and an inner layer of woven fiberglass. Given this information, answer the following: If removed evenly over the 10.0 acres, how deep a hole would be formed as a result of the excavation? In yards ? If the two-layer fiberglass roofing material has a mass of 0.667 lb_m/ft^2, what is the total mass of the roof? In lb m ?If 10.0 inches of wet snow collected on the roof (this happened in 1982), what is the added mass on the roof? (Assume 1 inch of water equals 10 inches of snow).

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#### Solution Preview

1. To lift an object a minimum force equal to its weight is required to be applied on the object in the upward direction.

Minimum force required to lift the object = F = Weight of the object =Mass of the object x Acceleration due to gravity = mg

Given: F = 1.05 x 10^3 N, g = 7.76 ft/sec^2 =7.76 x 0.3048 = 2.37 m/s^2 (as 1 ft = 0.3048 m)

Mass of the object m = F/g =1.05 x 10^3/2.37 = 443 kg

2. Dimensions of the mattress are given in cms; first these need to be converted into ft.

1 centimeter = 0.3937 inches and 1 inch = 1/12 ft. Hence, 1 cm = 0.3937/12 = 0.0328 ft

Hence, dimensions of the mattress in ft are: (210 x 0.0328)ft x (160 x 0.0328)ft x (23 ...

#### Solution Summary

Mass, pressure and areas are examined. The areas of pressures exerted are provided.

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## Fluids, Archimedes' Principle and Equation of Continuity

**Please consider significant figures and draw visual graphics **
Conceptual Questions:
 A person could not balance her entire weight on the pointed end of a single nail, because it would penetrate her skin. However she can lie safely on a "bed of nails" consists of many nails driven through a sheet of wood so that the pointed ends form a flat array. What is the "nail of beds" trick safe?

 As you climb a mountain, you ears "pop" because of the changes in atmospheric pressure. In which direction does your eardrum move (a) as you climb up and (b) as you climb down? Give your reasoning.

 Put an ice cube in a glass, and fill the glass to the brim with water. When the ice cube melts and the temperature of the water returns to its initial value would the water level drop, remain the same, or rise (causing the water to spill out)? Explain what would you observe in terms of Archimedes principle.

 As a person dives toward the bottom of a swimming pool, the pressure increases noticeably. Does the buoyant force also increase? Justify your answer. Neglect any change in water density with depth.

 The cross-sectional area of a stream of water becomes smaller as the water falls from the faucet. Account for this phenomenon in terms of equation of continuity. What would you expect to happen to the cross-sectional area when the water is shot upward, as it is in a fountain?
Problems
Mass Density
 One end of a wire is attached to a ceiling, and a solid brass ball is tied to the lower end. The tension in the wire is 120 N. What is the radius of the brass ball?

 Accomplished silver workers in India can pound silver into a incredibly thin sheets, as thin as 3.00 x 10-7 m (about one hundredth of the thickness of this sheet of paper). Find the area of such a sheet that can be performed from 1.00 kg of silver.
Pressure
 An airtight box has a removable lid of area 1.3 x10 -2 m2 and negligible weight. The box is taken up a mountain where the air pressure outside the box is 0.85 x 105 Pa. The inside of the box is completely evacuated. What is the magnitude of the force required to pull the lid off the box?

 A glass bottle of soda is sealed with a screw cap. The absolute pressure of the carbon dioxide inside the bottle is 1.80 x 105 Pa. assuming the top and the bottom surfaces of the cap each have an area of 4.10 x 10-4 m2, obtain the magnitude of the force of the screw thread exerts on the cap in order to keep it on the bottle. The air pressure outside the bottle is one atmosphere.
Pressure and Depth in a Static Fluid, Pressure Gauges
 The main water line enters a house on the first floor. The line has a gauge pressure of 1.90 x 105 Pa. (a) A faucet on the second floor, 6.50 m above the first floor, is turned off. What is the gauge pressure at this faucet? (b) How high could the faucet be before no water would flow from it, even if the faucet were open?

 The Mariana trench is located in the floor of the Pacific Ocean at a depth of about 11000 m below the surface of the water. The density of seawater is 1025 kg/m3. (a) If an underwater vehicle were to explore such a depth, what force would the water exerts on the vehicle observation window (radius = 0.10 m)? (b) For comparison, determine the weight of a jetliner whose mass is 1.2 x 105 kg.

 The human lungs can function satisfactorily up to a limit where the pressure difference between the outside and the inside of the lungs is one-twentieth of an atmosphere. Is a diver uses a snorkel for breathing, how far below the water can she swim? Assume the diver is in salt water whose density is 1025 kg/m3.
Archimedes' Principle
 A duck is floating on a lake with 25% of its volume beneath the water. What is the average density of the duck?

 A 0.10 m x 0.20 m x 0.30 m block is suspended from a wire and is completely under water. What buoyant force acts on the block?

 The density of ice is 917 kg/m3, and the density of sea water is 1025 kg/m3. A swimming polar bear climbs onto a piece of floating ice that has a volume of 5.2 m3. What is the weight of the heaviest bear that the ice can support without sinking completely beneath the water?

 A paperweight, when weighed in air, has a weight of W = 6.9 N. When completely immersed in water, however, it has a weight of Win water = 4.3 N. Find the volume of the paperweight.
The Equation of Continuity
 Oil is flowing with a speed of 1.22m/s through a pipeline with a radius of 0.305 m. How many gallons of oil (1 gal = 3.79 x 10-3 m3 flow in one day?

 A patient recovering from a surgery is being given fluid intravenously. The fluid has a density of 1030 kg/m3, and 9.5 x 10-4 m3 of it flows into the patient every six hours. Find the mass flow rate in kg/s.

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