Physics for Scientists and Engineers, 6e Chapter 14 – Fluid Mechanics

a large professional basketball player wearing sneakers Suppose you are standing directly behind someone who steps back and accidentally stomps on your foot with the heel of one shoe. Would you be better off if that person were a large professional basketball player wearing sneakers a petite woman wearing spike-heeled shoes? 1 2 3 4 5

Because the basketball player’s weight is distributed over the larger surface area of the shoe, the pressure (F / A) that he applies is relatively small. The woman’s lesser weight is distributed over the very small cross-sectional area of the spiked heel, so the pressure is high.

The pressure at the bottom of a filled glass of water (ρ = 1000 kg/m3) is P. The water is poured out and the glass is filled with ethyl alcohol (ρ = 806 kg/m3). The pressure at the bottom of the glass is smaller than P equal to P larger than P indeterminate 1 2 3 4 5

Because both fluids have the same depth, the one with the smaller density (alcohol) will exert the smaller pressure.

Several common barometers are built, with a variety of fluids Several common barometers are built, with a variety of fluids. For which fluid will the column of fluid in the barometer be the highest? mercury water ethyl alcohol benzene 1 2 3 4 5

All barometers will have the same pressure at the bottom of the column of fluid – atmospheric pressure. Thus, the barometer with the highest column will be the one with the fluid of lowest density.

easier equally difficult harder impossible You have invented a spacesuit with a straw passing through the faceplate so that you can drink from a glass while on the surface of a planet. Out on the surface of the Moon, you attempt to drink through the straw from an open glass of water. The value of g on the Moon is about one sixth of that on Earth. Compared to the difficulty in drinking through a straw on Earth, you find drinking through a straw on the Moon to be easier equally difficult harder impossible 1 2 3 4 5

Because there is no atmosphere on the Moon, there is no atmospheric pressure to provide a force to push the water up the straw.

impossible to determine An apple is held completely submerged just below the surface of a container of water. The apple is then moved to a deeper point in the water. Compared to the force needed to hold the apple just below the surface, the force needed to hold it at a deeper point is larger the same smaller impossible to determine 1 2 3 4 5

For a totally submerged object, the buoyant force does not depend on the depth in an incompressible fluid.

A glass of water contains a single floating ice cube as in the figure below. When the ice melts, the water level goes up goes down remains the same 1 2 3 4 5

The ice cube displaces a volume of water that has a weight equal to that of the ice cube. When the ice cube melts, it becomes a parcel of water with the same weight and exactly the volume that was displaced by the ice cube before.

whichever one is nearest your mouth the one of radius 3 mm You tape two different sodas straws together end-to-end to make a longer straw with no leaks. The two straws have radii of 3 mm and 5 mm. You drink a soda through your combination straw. In which straw is the speed of the liquid the highest? whichever one is nearest your mouth the one of radius 3 mm the one of radius 5 mm Neither – the speed is the same in both straws. 1 2 3 4 5

The liquid moves at the highest speed in the straw with the smaller cross sectional area.

They move toward each other. They move away from each other. You observe two helium balloons floating next to each other at the ends of strings secured to a table. The facing surfaces of the balloons are separated by 1 – 2 cm. You blow through the small space between the balloons. What happens to the balloons? They move toward each other. They move away from each other. They are unaffected. 1 2 3 4 5

The high-speed air between the balloons results in low pressure in this region. The higher pressure on the outer surfaces of the balloons pushes them toward each other.

Physics for Scientists and Engineers, 6e Chapter 14 – Fluid Mechanics

a large professional basketball player wearing sneakers Suppose you are standing directly behind someone who steps back and accidentally stomps on your foot with the heel of one shoe. Would you be better off if that person were a large professional basketball player wearing sneakers a petite woman wearing spike-heeled shoes? 1 2 3 4 5

Because the basketball player’s weight is distributed over the larger surface area of the shoe, the pressure (F / A) that he applies is relatively small. The woman’s lesser weight is distributed over the very small cross-sectional area of the spiked heel, so the pressure is high.

The pressure at the bottom of a filled glass of water (ρ = 1000 kg/m3) is P. The water is poured out and the glass is filled with ethyl alcohol (ρ = 806 kg/m3). The pressure at the bottom of the glass is smaller than P equal to P larger than P indeterminate 1 2 3 4 5

Because both fluids have the same depth, the one with the smaller density (alcohol) will exert the smaller pressure.

Several common barometers are built, with a variety of fluids Several common barometers are built, with a variety of fluids. For which fluid will the column of fluid in the barometer be the highest? mercury water ethyl alcohol benzene 1 2 3 4 5

All barometers will have the same pressure at the bottom of the column of fluid – atmospheric pressure. Thus, the barometer with the highest column will be the one with the fluid of lowest density.

easier equally difficult harder impossible You have invented a spacesuit with a straw passing through the faceplate so that you can drink from a glass while on the surface of a planet. Out on the surface of the Moon, you attempt to drink through the straw from an open glass of water. The value of g on the Moon is about one sixth of that on Earth. Compared to the difficulty in drinking through a straw on Earth, you find drinking through a straw on the Moon to be easier equally difficult harder impossible 1 2 3 4 5

Because there is no atmosphere on the Moon, there is no atmospheric pressure to provide a force to push the water up the straw.

impossible to determine An apple is held completely submerged just below the surface of a container of water. The apple is then moved to a deeper point in the water. Compared to the force needed to hold the apple just below the surface, the force needed to hold it at a deeper point is larger the same smaller impossible to determine 1 2 3 4 5

For a totally submerged object, the buoyant force does not depend on the depth in an incompressible fluid.

A glass of water contains a single floating ice cube as in the figure below. When the ice melts, the water level goes up goes down remains the same 1 2 3 4 5

The ice cube displaces a volume of water that has a weight equal to that of the ice cube. When the ice cube melts, it becomes a parcel of water with the same weight and exactly the volume that was displaced by the ice cube before.

whichever one is nearest your mouth the one of radius 3 mm You tape two different sodas straws together end-to-end to make a longer straw with no leaks. The two straws have radii of 3 mm and 5 mm. You drink a soda through your combination straw. In which straw is the speed of the liquid the highest? whichever one is nearest your mouth the one of radius 3 mm the one of radius 5 mm Neither – the speed is the same in both straws. 1 2 3 4 5

The liquid moves at the highest speed in the straw with the smaller cross sectional area.

They move toward each other. They move away from each other. You observe two helium balloons floating next to each other at the ends of strings secured to a table. The facing surfaces of the balloons are separated by 1 – 2 cm. You blow through the small space between the balloons. What happens to the balloons? They move toward each other. They move away from each other. They are unaffected. 1 2 3 4 5

The high-speed air between the balloons results in low pressure in this region. The higher pressure on the outer surfaces of the balloons pushes them toward each other.