Pressure of a fluid or gas Atmospheric pressure: 14.70 lbs/in2= 1.013x105 N/m2 or Pa Demo: lungs, can

Density r­air = 1.29 kg/m3 r­water = 1000 kg/m3 r­mercury = 13500 kg/m3   Weight of an object with density r, in terms of dimensions.

Pressure vs depth in a static fluid Comes from the weight of water (and air) above some area A at depth h

Pascal’s principle: For a fluid at rest, the pressure in the fluid depends only on the depth, not the shape of the container. Hydraulic systems and force amplification

Hydraulic systems and force amplification Do the pistons move the same distance during the lifting?

Manometers

Straws and pumps: lifting fluids How high can we “pull” water with a vacuum? How do you get water from a very deep well?

Lifting fluids Barometers How high can we “pull” mercury with a vacuum? Barometers

P1. For a longer canyon behind the dam (red arrow length), the dam ____________ a) can be weaker b) must be stronger c) can be the same.

P2. On the moon, where gravity is less, and there is no atmosphere, if you pumped out the air at the top of a barometer, the mercury would rise ________ compared to on earth. higher less high the same not at all

Bouyancy: higher pressure on bottom of object…it’s deeper.

Bouyancy: water displaced moves upward … it “wants” to fall back down (to lower potential energy)

Archimedes’ principle The buoyant force is the weight of the fluid that the object is displacing

Floating vs sinking Competition between upward and downward forces Comes down to density of object vs density of the fluid

Floating objects rise out of the water until: Submarines and fish can adjust their density!

Two cubes of the same size made of steel and wood are placed in water Two cubes of the same size made of steel and wood are placed in water. The wood floats (some is above the water), and the steel sinks. After they come to rest,   P3. the bouyant force is greatest on which block? steel wood none…same

A block of wood of size 0. 5m x 8m x 2m has density 250kg/m3 A block of wood of size 0.5m x 8m x 2m has density 250kg/m3. It is held all the way under the water with a chain. Draw a FBD of the block Find the buoyant force and the tension in the chain

A 200-ton ship enters the lock of a canal A 200-ton ship enters the lock of a canal. The fit between the sides of the lock and the ship is tight so that the weight of the water left in the lock after it closes is much less than 200 tons. Can the ship float?