Chapter 13 Notes
A.
Pressure – the result of a force distributed over an area a.
To calculate the pressure, divide the force by the area over which the force acts. b.
Force is in Newtons (N) and area is in square meters (m 2 ). Dividing the two results in a unit known as a pascal (Pa)
Pressure = Force ÷ Area
Example: Mike sits on a desk chair with a force of 900 Newtons. If the chair has an area of 0.12 square meters, what is the resulting pressure? (7500 Pa)
Example: Kim sits on a bicycle seat with a force of 800 Newtons. If the chair has an area of
0.016 square meters, what is the resulting pressure? (50,000 Pa)
B.
Pressure in Fluids a.
Fluid – a substance that assumes the shape of its container. Both liquids and gases are fluids.
1.
Fluid pressure depends on the type of fluid
2.
Fluid pressure increases as depth increases (but volume does not matter) b.
The weight of Earth’s atmosphere exerts a pressure of about 101,325 Pascals at sea level.
1.
The atmosphere is exerting more than 1000 Newtons of force on the top of your head. Fortunately, the inside of your body also exerts pressure that balances the air pressure outside.
2.
Air pressure decreases as the altitude increases
What do you experience when traveling to different altitudes?

C.
Pascal’s Principle - A change in pressure at any point in a fluid is transmitted equally and unchanged in all directions throughout the fluid a.
Hydraulic system – a device that uses pressurized fluid acting on pistons of different sizes to change a force.
1.
An input force is applied to a small piston, which pushes against the fluid sealed in the hydraulic system and transmits the force to a large piston. The pressure pushing against the large piston acts on a much larger area. An increased output force is produced because a constant fluid pressure is exerted on the larger area of the output piston
D.
Bernoulli’s Principle – as the speed of a fluid increases, the pressure within the fluid decreases a.
Lift – an upward force due to a pressure difference between the top and bottom of a wing.
1.
The air traveling over the top of a wing moves faster than the air passing underneath – this creates a low-pressure area above the wing.
E.
Buoyancy a.
Buoyancy – the ability of a fluid to exert an upward force on an object placed in it
1.
Buoyancy results in the apparent loss of weight of an object in a fluid.
2.
Buoyant force – an upward force acting on an object in a fluid
3.
Because water pressure increases with depth, the forces pushing up on the bottom of the object are greater than the forces from pressure pushing down on top of the object (because the bottom of the object is deeper in the fluid)