Forces in Fluids

7:1 Fluids and Pressure

Terms Fluid~ any material that can flow and takes

the shape of its containerPressure~the amount of force exerted on a

given areaPascal~SI unit for pressureAtmospheric pressure~ pressure cause

by the weight of the atmosphereDensity~the amount of matter in a certain

volumePascal’s Principle~ A change in pressure

at any point in an enclosed fluid will be transmitted equally to all parts of that fluid

All Fluids Exert Pressure•Pressure = Force ÷ Area

•1 N/m2=1 pascal•Force of the air particles hitting the inner surface of the tire creates pressure•Why are bubbles round?

–fluids exert pressure evenly in all directions–bubble expands in every direction–helps to create a sphere

Atmospheric Pressure• Atmospheric pressure is exerted on

everything on Earth• Exerts approx. 101,300 N on every

square meter or (10 N per square centimeter)

• The fluids inside your body exert pressure which works against atmospheric pressure

• Atmospheric pressure varies– Low at the top of atmosphere because

there is very little atmosphere to press down

Atmospheric Pressure (cont.’d)• Pressure depends on depth

– Pressure decreases as you go higher into the atmosphere

– Fluids in your body must adjust as you go between higher and lower points• Ears “popping”• Air behind eardrum expanding or

contracting• “pop” due to air being released

Water Pressure• Water pressure increases as depth

increases• Pressure depends on the depth of

the fluid not the total amount of fluid present

• Density makes a difference– Water is more dense then air– Water has a greater mass for a certain

volume– Waters exerts more pressure than air

Fluids Flow from High Pressure to Low Pressure

•When drinking through a straw, you remove air from inside the straw•Pressure inside the straw is reduced•Atmospheric pressure on surface of liquid remains the same•Pressure outside the straw is higher pressure then inside the straw so the liquid is pushed up the straw

PASCAL’S PASCAL’S PRINCIPLEPRINCIPLE

Chapter 11 Section 3Chapter 11 Section 3

Lets Review…Lets Review…What is pressure? What is pressure?

What is the equation for pressure? What is the equation for pressure?

What causes fluid pressure?What causes fluid pressure?When you catch a deep-sea fish, why

does its eyes pop-out?Why do your ears pop on an airplane

or up in the mountains?

Pascal’s PrinciplePascal’s PrincipleWhen force is applied to a confined When force is applied to a confined

liquid, the change in pressure is liquid, the change in pressure is transmitted equally to all parts of the transmitted equally to all parts of the

fluid.fluid.

Draw a bottle of water with arrows to Draw a bottle of water with arrows to illustrate the regular exerted pressure. illustrate the regular exerted pressure.

Then draw a water bottle that you squeeze. Then draw a water bottle that you squeeze. What happens to the pressure? What What happens to the pressure? What

happens if you open the top?happens if you open the top?

How does Pascal’s Principle explain what How does Pascal’s Principle explain what happens if you squeeze a water bottle?happens if you squeeze a water bottle?

A force applied to one section A force applied to one section of an enclosed liquid at rest of an enclosed liquid at rest

will be transferred to the will be transferred to the entire liquid with the same entire liquid with the same

amount of force.amount of force.

Hydraulic SystemsHydraulic SystemsA force applied to one piston increases A force applied to one piston increases

the fluid pressure throughout the the fluid pressure throughout the fluid.fluid.

If the second piston has a larger If the second piston has a larger surface area, the force is surface area, the force is multipliedmultiplied!!

Pressure = Force/AreaPressure = Force/Area

Force/Area = Pressure = Larger Force/Area = Pressure = Larger Force/Larger AreaForce/Larger Area