Buoyancy Notes 2013

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What is Buoyancy?Buoy = float (in

Latin)Archimedes

discovered this with the golden crown.

What is Buoyancy?Buoyancy= “the floating force”

Water is “heavier” than the object…so the object floats

Low density-more likely to float

What are the units for Buoyancy?Buoyant force is measured in Newtons (N)

What is displacement?Displace = push awayThe amount of water

displaced by an object tells you the volume of that object.

The weight of the displaced water is the buoyant force acting on that object.

How much fluid was displaced here?

What is buoyant force?What is buoyant force?

Buoyant force is the upward force of a fluid on an object.

What is a fluid?What is a fluid?

A fluid is any liquid or gas.

What are the types of buoyancy?Positive buoyancy:

Buoyant force is greater than weight so the object floats.

Neutral buoyancy:Buoyant force is equal to weight so the object is suspended in the fluid.

Negative buoyancy:Buoyant force is less than weight so the object sinks.

When does an object FLOAT?When does an object FLOAT?

An object will float if the buoyant force is greater than or equal to the weight of object.

WEIGHT

BUOYANT FORCE

When will an object SINK?When will an object SINK?

An object will sink if the buoyant force of the object is less than than the weight of the object.

WEIGHT

BUOYANT FORCE

How do you Calculate BF?

Buoyant Force= Weight of displaced fluid

BF = Wair - Wwater

Buoyant = Weight of - Weight of object

Force object in air in water

Buoyant Force =Buoyant Force =WeightWeight of Displaced water of Displaced water

•When the object is submerged in water, it displaces an amount of water that weighs 3 lbs.

•Therefore the buoyant force pushing up on the object is

Buoyant Force = Buoyant Force = Weight of object in air minus weight of Weight of object in air minus weight of

object in waterobject in water•Wair = 7 lbs

•Wwater = 4 lbs

•Buoyant Force

= 7-4 = 3 lbs

Volume of Volume of object=Volume of object=Volume of displaced waterdisplaced water

Challenge!Challenge!If we drop this object in the water will it sink or float?

Hint: compare Hint: compare the object’s the object’s

weight in air and weight in air and the buoyant forcethe buoyant force

Challenge!Challenge!If we drop this object in the water will it sink or float?

Hint: compare Hint: compare the object’s the object’s

weight in air and weight in air and the buoyant forcethe buoyant force

SINKSINK

6 cm3 7 lbs7 lbs3 lbs3 lbs

Why do things float?1. Things float if they are less dense

than the fluid they are in.

2. Things float if they weigh less than the

buoyant force pushing up on them.

3. Things float if they are shaped so their

weight is spread out.

Hot air balloons: heat to go up

Submarines: fill tanks with air to surfacefill with water to submerge

Lowest Mass, Lowest Density, Floats

Low Mass, Low Density, Neutral

High Mass, High Density, Sink

Sub’s carry compressed air tanks to re-fill their ballast tanks

Ballast tanks

Density

Air Density= 0.04 g/ml

Balloon Density = 0.045 g/ml

Density

Air Density= 0.04 g/ml

NOT MOVING UP OR DOWN

Density

DensityAir Density= 0.04 g/ml

ShapeHow can you get 5000 pounds of solid steel to float?

Ouch!Ouch!You know the You know the Buoyant Force.Buoyant Force.

OutputA rock has a mass of 76 g

and displaced 80 mL of water when it was submerged. What is the volume of the rock?

If it displaced a volume of 80mL, then the volume of the rock is 80mL!

OUTPUTA volume of 200 mL of water is

displaced by a ducky that weighs 10N. The weight of the displaced water is 20N. What is the buoyant force on the ducky and will it sink or float?

BF = 20NThe ducky will float because 20N

(weight of the displaced fluid) is greater than 10N (weight of the duck).

OUTPUTA little 27g fish displaced a volume of

water that weighed 25N. The weight of the fish was 30N. What is the buoyant force on the fish and will it sink or float?

BF = 25NThe fish will sink because the weight of the

fish is greater than the buoyant force.

OUTPUTA person weighs 250N while

swimming in the dead sea. When outside of the water they weigh 600N. What is the buoyant force acting on them? Will they sink or float?

BF = Wair – Wwater = 600 – 250 = 350N

The person will float because their weight in the water is less than the buoyant force.

OUTPUTA dog jumps into a

pool. The water holds him up because he only weighs 10N in water. On land, he weighs 45N. What is the buoyant force?

BF = 35N

OutputAn object has a weight of 7N in air. The

object displaces 16 mL of water that weighs 7N. What is the buoyant force? Will the object sink or float?

The weight of the displaced fluid is the buoyant force, so the BF = 7N. The object will be neutrally buoyant and float.

Observation Time!

Don’t forget your conclusion:

Buoyancy Notes 2013

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