How can you predict if an object will sink or float in a fluid?

11-1/11-2: Pressure and Floating & SinkingHow can you predict if an object will

sink or float in a fluid?

Anticipatory Set

Why do some things float while others sink?

California StandardsScience Standard 8.8.d: Students know how to

predict whether an object will float or sink.Science Standard 8.9.f: Apply simple mathematic

relationships to determine a missing quantity in a mathematic expression, given the to remaining terms.

Science Standard 8.8.c: Students know the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object has displaced.

Inputpressure: equal to the force exerted on a

surface divided by the total area over which the force is exerted (Pressure = Force ÷ Area).

pascal: the unit of pressure (Pa), named for Blaise Pascal.

fluid: a material that can easily flow.barometer: an instrument used to measure

atmospheric pressure.

Inputdensity: mass per unit volume (Density =

Mass ÷ Volume).buoyant force: the upward force exerted by

a fluid on a submerged object.Archimedes’ principle: the buoyant force

acting on a submerged object is equal to the weight of the volume of fluid displaced by the object.

Input & ModelingWhat is Pressure?

The amount of pressure you exert depends on the area over which you exert a force.

Input & ModelingArea

The area of a surface is the number of square units that it covers. To find the area of a rectangle, multiple the length by the width.

Modeling

Practice Problem

Which has a greater area: a rectangle that is 4 cm X 20 cm or a square that is 10 cm X 10 cm?

The square has the greater area.4 cm X 20 cm = 80 cm2

10 cm X 10 cm = 100 cm2

Input & ModelingCalculating Pressure

Pressure = Force ÷ AreaPressure is measured in an SI unit called

a pascal (Pa): 1N/square meter = 1 Pa.

Example:Calculate the pressure produced by a force of 800 N acting on an area of 2.0 square meters.

P = F ÷ AP= (800N) ÷ (2.0 meters2)P = 400 N ÷ meters2

P = 400 Pa

Input & ModelingCalculating Pressure

The SI unit for pressure, the pascal, is named for French mathematician Blaise Pascal.

The SI unit for force, the newton, is named for English physicist Sir Isaac Newton.

Modeling

Calculate the pressure produced by a force of 450 N on an area of 3 square meters. P = F/A P = (450 N) / (3 square meters) P = 150 N / square meter P = 150 Pa

What is the pressure on 2.5 by 3 meter area being acted upon by a force of 6000 N? P = 6000 N / (2.5)(3) square meters P = 6000 N / 7.5 square meters P = 800 Pa

Input & ModelingCalculating Force from Pressure

Example:The pressure of a gas contained in a cylinder with a movable piston is 300 Pa. The area of the piston is 0.5 square meters. Calculate the force that is exerted on the piston.F = P x AF = (300 Pa) x 0.5 m2

F = 150 N

Modeling

The pressure of a gas contained in a cylinder with a movable piston is 700 Pa. The area of the piston is 0.2 square meters. What is the force that is exerted on the piston?

F = P x A

F = (700 Pa) (0.2 square meters)F = 140 N

Input & ModelingFluid Pressure

All of the forces exerted by the individual particles in a fluid combine

to make up the pressure exerted by the fluid.


Air pressureAir is a mixture of gases that make up Earth's

atmosphere. These gases press down on everything on Earth’s surface.

Air exerts pressure because it has mass.Balanced pressure

Balanced pressure explains why the tremendous air pressure pushing on you from all sides does not crush you.

Pressure from fluids inside your body balance the air pressure outside your body.


Atmospheric pressure decreases as your elevation increases.You measure atmospheric pressure with an

instrument called a barometer.Water pressure increases as depth

increases.

Input & ModelingVariations in Fluid Pressure

As your elevation increases, atmospheric pressure decreases.

Input & ModelingVariations in Fluid Pressure

Water pressure increases as depth increases.

Input & ModelingDensity

Changes in density cause a submarine to dive rise, or float.

Input & ModelingBuoyancy

The pressure on the bottom of a submerged object is greater than the pressure on the top. The result is a net force in the upward direction.


The buoyant force works opposite the weight of an object.


Archimedes’ principle states that the buoyant force acting on a submerged object is equal to the weight of the fluid the object displaces.


A solid block of steel sinks in water. A steel ship with the same weight floats on the surface.

Input & ModelingCalculating Density

Density = Mass ÷ VolumeThe density of a substance is its mass

per unit of volume.

Example:A sample of liquid has a mass of 24 g and a volume of 16 mL. What is the density?

D = M ÷ VD = (24 g) ÷ (16 mL)D = 1.5 g/mL

Modeling

A piece of metal has a mass of 43.5 g and a volume of 15 cm3. What is its density?

D = M ÷ V

D = (43.5 g) ÷ (15 cm3)

D = 2.9 g/cm3

Modeling

A block has a mass of 320 g and a volume of 80 cubic centimeters. What is its density?

D = 4 g/cm3

An orange has a mass of 250 g and a volume of 750 mL. What is its density?

D = .33 g/mL

C4UQuick Quiz PPT

Get out your whiteboard and marker!

HOMEWORK CONNECTION Read pages 416-429 in your Science textbook.

Complete the Section 11-1 & 11-2 Review & Reinforce

Write a detailed SUMMARY of the section and complete the UNANSWERED QUESTIONS section of your notes.

Choose two of the remaining Depth & Complexity ICONS in your notes and explain how they relate to this section.

How can you predict if an object will sink or float in a fluid?

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