1. Pressure

2. Solids, Liquids, and Gases
Normal matter is made of atoms.
Atoms can interact to form three types of matter.
Solid
Incompressible
Subject to shear force
Liquid
Incompressible
Not subject to shear force
Gas
Compressible
Not subject to shear force

3. Density Matter has mass and takes up volume.
The ratio of the mass to the volume is the density.
Incompressible materials maintain their density.
Salt (solid): x 103 kg/m3
Water (liquid): x 103 kg/m3
Nitrogen (gas): kg/m3

4. Pressure The unit of pressure is the pascal.
Matter can exert the same force in all directions.
The ratio of the force exerted to the area is the pressure.
It’s a scalar, not a vector
The unit of pressure is the pascal.
1 Pa = 1 N/m2
Same type as stress

5. Volume Stress
Pressure from all sides can change the volume of a solid.
Volume stress measures the pressure.
Volume strain measures the change in area. P DV V
A (surface area)

6. Bulk Modulus Pressure changes volume, not length.
Bulk modulus B relates changes in pressure and volume.
The negative sign represents the decrease in volume with increasing pressure. P DV V
A (surface area)

7. Under Pressure
Steel has a bulk modulus of B = 60 GPa. A sphere with a volume of 0.50 m3 is constructed and lowered into the ocean where P = 20 MPa.
How much does the volume change?
Use the relation for bulk modulus.
B = -(DP) / (DV/V)
DV= -V DP / B
Substitute values:
(-0.50 m3)(2.0 x 107 Pa) / (6.0 x 1010 Pa)
DV = -1.6 x 10-4 m3

8. Surface Force Liquids and gases are fluids.
Any area in the fluid experiences equal forces from each direction.
This is just the law of inertia.
All forces are balanced
Forces are still vectors
Any arbitrary volume in the has balanced forces.

9. Fluids at Depth Vertical pressures are not the same.
Pressure below is higher
Pressure above is less
Include the weight Ft Fg Fb

10. Deep Sea Force The deepest point in the ocean is 11.3 km.
What is the pressure at that depth?
P0 = x 105 Pa
Water r = kg/m3
The pressure increases linearly with depth.
Solve for P = P0 + rgh.
h is positive with depth.
Substitute values:
rgh = 1.11 x 108 Pa.
This is much larger than P0.
P = 1.11 x 108 Pa
This is 1100 atm!
1 bar = 1.00 x 105 Pa
1 atm = 1013 millibar
1 atm = 14.7 psi

11. Surface Tension Liquid molecules attract each other.
Cohesion to self
Adhesion to other material
At the surface this force is the surface tension.
Force per length parallel to the surface: g
Constant for different liquids
insect leg on water

12. Capillary Action Liquid surfaces adhere to adjacent solids.
The contact angle q depends on the liquid and solid.
The surface tension can support a weight of fluid.