Physic 121: Phundamentls of Phyics I December 1, 2006 D. Roberts University of Maryland PHYS 121 Chapter 9 Solids and Fluids D. Roberts University of Maryland PHYS 121 Pressure The force exerted by a
fluid on a submerged object at any point if perpendicular to the surface of the object F N P in Pa 2 A m D. Roberts University of Maryland PHYS 121 Pressure and Depth Examine the darker region, assumed to be a fluid It has a cross-sectional area A
Extends to a depth h below the surface Three external forces act on the region D. Roberts University of Maryland PHYS 121 Pressure and Depth equation Po is normal atmospheric pressure 1.013 x 105 Pa = 14.7 lb/in2 The pressure does not depend upon the shape of the container
D. Roberts University of Maryland PHYS 121 Pascals Principle A change in pressure applied to an enclosed fluid is transmitted undimished to every point of the fluid and to the walls of the container. First recognized by Blaise Pascal, a French scientist (1623 1662) D. Roberts University of Maryland PHYS 121 Pascals Principle, cont
The hydraulic press is an important application of Pascals Principle F1 F2 P A1 A 2 Also used in hydraulic brakes, forklifts, car lifts, etc. D. Roberts University of Maryland PHYS 121 ?
A container is filled with oil and fitted on both ends with pistons. The area of the left piston is 10mm 2; that of the right piston 10,000 mm2. What force must be exerted on the left piston to keep the 10,000-N car on the right at the same height? 17% 17% 17% 17% 17% 17% 2 3 4 5 10 nt 8 in N
fo rm at io n N 6 10 su f fi ci e In 0 of 5 1 N
10 , 00 0 10 0 N 30 N 10 N 100 N 10,000 N 106 N 108 N Insufficient information
10 1. 2. 3. 4. 5. 6. Pressure Measurements: Manometer One end of the U-shaped tube is open to the atmosphere The other end is connected to the pressure to be measured Pressure at B is Po+ghghgh D. Roberts
University of Maryland PHYS 121 Pressure Measurements: Barometer Invented by Torricelli (1608 1647) A long closed tube is filled with mercury and inverted in a dish of mercury Measures atmospheric pressure as ghgh D. Roberts University of Maryland PHYS 121 Archimedes 287 212 BC
Greek mathematician, physicist, and engineer Buoyant force Inventor D. Roberts University of Maryland PHYS 121 Archimedes' Principle Any object completely or partially submerged in a fluid is buoyed up by a force whose magnitude is equal to the weight of the fluid displaced by the object. D. Roberts University of Maryland
PHYS 121 Buoyant Force The upward force is called the buoyant force The physical cause of the buoyant force is the pressure difference between the top and the bottom of the object D. Roberts University of Maryland PHYS 121 Buoyant Force, cont. The magnitude of the buoyant force always equals the weight of the displaced fluid The buoyant force is the same for a totally
submerged object of any size, shape, or density B fluid Vfluid g wfluid D. Roberts University of Maryland PHYS 121 Buoyant Force, final The buoyant force is exerted by the fluid Whether an object sinks or floats depends on the relationship between the buoyant force and the weight D. Roberts University of Maryland PHYS 121
Archimedes Principle: Totally Submerged Object The upward buoyant force is B=ghfluidgVobj The downward gravitational force is w=mg=ghobjgVobj The net force is B-w=(ghfluid-ghobj)gVobj D. Roberts University of Maryland PHYS 121 Totally Submerged Object The object is less dense than the fluid The object experiences a net upward force D. Roberts
University of Maryland PHYS 121 Totally Submerged Object, 2 The object is more dense than the fluid The net force is downward The object accelerates downward D. Roberts University of Maryland PHYS 121 Archimedes Principle: Floating Object The object is in static equilibrium The upward buoyant force is balanced by the
downward force of gravity Volume of the fluid displaced corresponds to the volume of the object beneath the fluid level D. Roberts University of Maryland PHYS 121 Archimedes Principle: Floating Object, cont The forces balance obj fluid D. Roberts Vfluid
Vobj University of Maryland PHYS 121 ? A ball floats on water, as seen in the photograph below, but will sink when put into mineral spirits. The water is dyed green with food coloring to distinguish it from the clear mineral spirits. Note how far the floating ball sinks into the water. When the mineral spirits is poured slowly onto the top of the water, it will remain on top because the two fluids are immiscible. If the mineral spirits is poured slowly on top of the water while the ball is floating on the water, what will happen to the level at which the ball floats? 3 4
5 at t.. . ll w ba at flo ill ll w ill ill
ba ll w e 2 Th 1 Th e 0 of 5 flo
flo at at hi gh . lo w .. 3. 33% 33% 33% ba 2.
30 e The ball will float lower in the water The ball will float higher out of the water The ball will float at the same level with respect to the surface of the water after the mineral spirits are poured on top of the water Th 1.
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