Chapter 11 – Forces in Fluids• Pressure = force/area

• Examples of units for pressure:

newtons/m2 lbs/in2

• Fluid (liquid or a gas) exerts pressure in all directions.

• All the forces exerted by the individual molecules in a fluid add together to make up the total pressure of a fluid.

Atmospheric Pressure

• Average atmospheric pressure at sea level = 10.1 N/cm2

– In English units atmospheric pressure is 14.7 lbs/in2

– This is the weight of the column of air above us.

• Air pressure decreases with elevation (ears pop).– Water pressure increases with depth

• Pascal’s principle – When a force is applied to a confined fluid, an increase in pressure is transmitted equally to all parts of the fluid

• Hydraulic system can multiply force

Buoyancy

• Archimedes’s principle – the buoyant force of an object is equal to the weight of the water it displaces.

• If the buoyant force is greater than the gravitational force (weight) on the object, the object will float.

Density = mass/volume

Density of water = 1g/cm3 = 1g/mL

If the density of an object is greater than one, it will sink. If the density of an object is less than one, it will float **in water**

Bernoulli’s Principle – The faster air moves over a surface the less pressure it exerts on the surface

Examples: airplane wing

Wind goes faster over the top of the wing (less pressure)

Examples: wind over a surface

Wind

Paper

Ground

**Less pressure on the top of the paper **

Examples: Atomizer

Liquid

Air

Low pressure pulls liquid up

Example: Ball in a vertical air stream

Less pressure on right pulls the ball back into air stream