Why Do Boats Float?
Why Do Boats Float?
Archimede’ss Principle
Archimede
• Any
Any object, wholly or object wholly or
partially immersed in a fluid is buoyed up
a fluid, is buoyed up by a force equal to the weight of the
the weight of the fluid displaced by the object (Archimedes
object. (Archimedes of Syracuse, 212 BC)
Remember…
• Weight
Weight is the force of is the force of
gravity pulling objects down toward the earth (Newtons)
• Mass (grams) is roughly equivalent to weight on Earth’s surface, BUT
• Mass is constant throughout the universe.
Density
• How
How “heavy”
heavy something is, compared to how
compared to how “big” it is
• Volume is how Volume is how “big”
big something is (l x h x w)
• Density = D i
mass/volume
So how do we know if something will float?
fl
• If the density of t e de s ty o
the object is LESS than the density of water it displaces, d l
it will float.
• Water’s density is W t ’ d it i
1 g/cm3
• If an object
If an object’ss density is less than 1, it will float!
,
But ships are made of steel, and steel sinks!
k
• A
A ship has hollow areas in ship has hollow areas in
its hull.
• If the ship
If the ship’ss AVERAGE AVERAGE
density is less than water’ss density, it will water
density it will
float.
• The more full the hull is, The more full the hull is
the lower the boat sits in (
)
the water (draft).
Ballast
• Ships
Ships are often more are often more
stable if they ride lower in the water
lower in the water.
• Ships that do not have cargo in their hulls may
cargo in their hulls may fill “ballast” tanks with water so they ride
water so they ride lower (more draft).
Taking on iron ore without sinking!
Taking on iron ore without sinking!
• Iron
Iron ore pellets ore pellets
(taconite) are loaded carefully and evenly
carefully and evenly until the ship is at its desired draft (depth)
desired draft (depth) in the water.
Bouyant force keeps it afloat!
force keeps it afloat!
• The
The weight of air in weight of air in
hollow spaces in the hull added to the
hull, added to the iron ore’s weight, average less than 1
average less than 1 g/cm3 (less than water).
water)