Physics AP

Mr. Jean

September 27th, 2011

The plan:

• Pascal’s Law

• Typical AP pressure questions– A Car lift (jack)– A Pain in your ear!

Pascal Law’s:

• A change in the pressure applied to a fluid is transmitted undiminished to every point of the fluid and the walls of the container.

Pressure is CONSTANT!

• The increase in pressure is the same on the two sides of a pump. Thus a small force F1 creates a larger force F2.

• Small inputs can create large outputs!

A Car lift (jack)

A Pain in your ear!

• Pf = Final pressure reading (Pa)• Pi = Initial pressure reading (Pa)• ρ = density of matter (kg/m3)• g = gravity (m/s2)• h = total height (m)

Demo – Mini-jacks

Buoyant Forces:

Buoyant Force:

• The upwards force exerted by a fluid on any immersed object is called the buoyant force.

How to think about Buoyancy:

• Imagine a beach ball sized parcel of water beneath the water surface. – Since this parcel is at equilibrium the water

has a buoyant force which offsets the force of gravity.

– This upwards force’s magnitude is equal to the weight of the water in there parcel.

IMPORTANT FACT!

• The magnitude of the buoyant force on an object is always equal to the weight of the fluid displaced by the object.

• This is know as Archimedes’ principle.

Buoyant Force:

• B = pfluidgV

• B = buoyant force in newtons (haha Brody)

• p = coefficient of density of fluid.

• g = gravitation field (positive magnitude!)

• V = volume of water displaced.

Through question:• You are on a boat in a lake. You have a massive

steel anchor aboard. What happens with respect to the lakes water height along the shoreline when you drop the anchor in?

• A) The lake level gets higher with respect to the shoreline.

• B) The lake level remains the same...• C) The lake level gets lower...

Case #1: Object Totally Submerged

• B = pfluidgVobject

• Fg = Mg = pobjgVobject

• Net Force B – Fg = (pfluid + pobj)gVobj

• KEY POINT: The direction of motion of an object submerged in a fluid is determined only by the densities of the object and the fluid!

Case #2:Object Floating

• (Vfluid / Vobj ) = (pobj / pfluid)

• This equation shows that the fraction of volume of a floating object that is below the fluid surface is equal to the ratio of the density of the object to that of the fluid.

Archimedes Crown: