Forces in Fluids Chapter 11

Why don’t you sink into the snow when you wear snow shoes? Because the size of the area over which the force is distributed has changed.

the force pushing on a surface. is equal to the force exerted on a surface divided by the total area over which the force is exerted. Pressure = F / A PRESSURE

Units for Pressure Force is measured in Newtons (N). Area is measured in square meters. SI unit for pressure is N/m 2 which is also called the Pascal (Pa). 1 Pa = 1N/m 2

Fluid Pressure A fluid is a substance that can easily flow. Fluids are substances in the liquid or gas state. Molecules in these states have a LOT of ENERGY! Fluids exert pressure on surfaces they touch.

Fluids are made of molecules. Molecules are constantly moving. All of the forces exerted by the individual molecules in a fluid add together to make up the pressure exerted by the fluid.

Fluid pressure = Total force exerted by the fluid divided by the area over which the force is exerted.

Fluid Pressure all around. Air Pressure or atmospheric pressure is the pressure exerted on us by the air.

How can we support the weight of the atmosphere? Each cubic meter of air has a mass of 1 kg. How can we support this and not feel a thing? Because in a fluid that is not moving, pressure at a given point is exerted equally in all directions.

Balanced Pressures As the atmosphere is pushing down on your hand – there is also atmospheric pressure pushing up on our hand.

Variations in fluid pressure Air pressure decreases as elevation increases. Remember air pressure at a certain point results from the weight of air above that point. The higher you go the less weight above you.

- Pressure

Why do your ears pop? As the air pressure outside your body adjusts the air pressure inside your body has to adjust too. The pressure inside is greater than outside so pressure is released and all is balanced again.

Pressure and depth Pressure increases as you descend – because there is more water above you and therefore more weight above you.

Air Pressure decreases as elevation increases.

11.1 HW Check 1. What 2 factors does pressure depend on? A. Force and depth B. Elevation and force C. Force and area D. Area and elevation

1b. Who exerts more pressure on the ground? A. A 50-kg woman standing in heels B. A 50-kg woman standing in work boots

2a. How do fluids exert pressure? A. Particles of fluids try to squeeze into the smallest spaces possible B. Particles of fluids exert a force when colliding with surfaces C. Particles of fluids can’t handle any forces being exerted on them

2b. Since most of the weight of the atmosphere is above you, why aren’t you crushed by it? A. The pressure is exerted equally in all directions B. There is more pressure pushing up on you than down on you C. pressure of the atmosphere only acts on solid substances

3a. How does atmospheric pressure change as you move away from the surface of the earth? A. It stays the same B. It increases C. It decreases D. It disappears

3b. Compare the change in atmospheric pressure with elevation to the change in water pressure with depth. A. Both increase with an increase in elevation or depth B. Air pressure increases when elevation increases, but water pressure decreases when depth increases. C. Air pressure decreases when elevation increases, but water pressure increases when depth increases

3c. Why must an astronaut wear a pressurized suit in space? A. It only provides needed oxygen to breathe B. It balances the pressure inside and outside the body C. It protects from deadly gases in space

When a force is applied to a confined fluid, the increase in pressure is transmitted equally to all parts of the fluid.

Transmitting Pressure in a Fluid When force is applied to a confined fluid, the change in pressure is transmitted equally to all parts of the fluid. Deals with Fluids! This is why water Squirts from a bottle When squeezed.

Hydraulic Devices In a hydraulic device, a force applied to one piston increases the fluid pressure equally throughout the fluid. Multiplies Force

The pressure exerted by a moving stream of fluid is less than its surrounding fluid.

Therefore, as the speed of the fluid increases its pressure decreases.

A non-spinning baseball or a stationary baseball in an airstream exhibits symmetric flow. A baseball which is thrown with spin will curve because one side of the ball will experience a reduced pressure. This is commonly interpreted as an application of the Bernoulli principle. The roughness of the ball's surface and the laces on the ball are important! With a perfectly smooth ball you would not get enough interaction with the air. Bernoulli’s and Baseball