Density
• Density is a comparison of how much
matter there is in a certain amount of
space.
• Which square is more dense?
Calculating Density
• Density = mass
volume
– solids: d = grams/cubic centimeters (g/cm3)
– liquids: d = grams/milliliters (g/ml)
ALWAYS
REMEMBER THE
UNITS!
To get the mass just weigh the object
Volume
Length
Height
Width
•
for regular shapes
– volume = length x width x height
e.g.
= 10 cm3
• for irregular shapes
– submerge object in a liquid
– measure the volume of liquid
that is displaced in ml
– e.g. key displaced 3 ml of liquid
therefore its volume is 3 ml
• for liquids – measure directly
in ml
– e.g. 50 ml
Buoyancy and Buoyant
Forces
When you are swimming in
water, there are two forces
that
work against each other and
affect the motion of your body.
• the force of gravity is pulling
you down
• the water is also pushing
Bouyancy
• Buoyant force, or buoyancy, is the
upward force on objects submerged in
or floating on fluids.
• A buoyant force pushes away from the
centre of Earth.
Buoyancy
• BUOYANCY is the tendency of an object
or substance to float
Gravity – downward force of
gravity pulling on the object
Buoyant Force – upward force of
fluid pushing on the object
Bouyancy
• if gravity > buoyant force
→ object sinks
• neutral buoyancy gravity
= buoyant force object stays
suspended
• if gravity < buoyant force
→ object rises or floats
Bouyancy
• an object will float if its buoyant force,
when fully immersed, is greater than its
weight (gravitational force)
• it will sink if its weight is greater than the
buoyant force
• it will float when the buoyant force is
equal to its weight (or the force of gravity)
Archimede’s Principle
This
principle
explains
why some
objects float
in water and
others sink.
Salt vs. Fresh water
• Seawater (salt water) has a density of 1.03 g/mL
and fresh water has a density of 1.00 g/mL.
• Therefore, one litre of salt water weighs more
than one litre of fresh water.
• That is, salt water can support more weight per
volume than fresh water, so it is easier to float in
salt water.
Average Density
The average density of an object is the total mass
of
all substances that make up the object divided by
the
total volume.
• Average density results in objects that would
normally sink being able to float.
• Examples of technologies that have been
developed because of our understanding of
density and buoyancy include:
1. Ships
Ships can be built of steel
because their hollow hull
ensures that the average
density of the ship is less
than that of water.
2. Personal Floatation devices
(Life jackets)
• Personal flotation
devices (ex.Life jackets)
are filled with a
substance of very low
density.
• This way, a life jacket
lowers a person’s
average density,
allowing the person to
float.
3. Submarines
o By allowing water to flow
in or out, a submarine
can rise or sink in the
water.
o The submarine floats
when its weight is equal
to the buoyant force and
it sinks when its weight
is greater than the
buoyant force.
4. Hot Air Balloons
When the air inside a hot-air
balloon is heated, the air
particles:
• gain energy and
• spread out (forcing some of
the particles out of the
balloon)
The air inside the balloon
becomes
less dense than the air
surrounding
Average Density
So:
• an object will float if its average density is less
than the fluid in which it is immersed
• an object will sink if its average density is
denser than the fluid in which it is immersed
• when the object’s density is the same as the
medium, an object will neither sink nor float; it
is said to be neutrally buoyant.
Sink or Float?
• wooden boats vs. a water logged stick
• metal block vs. metal boats
• a sealed, empty plastic bottle vs. a plastic
bottle full of water
How are Density and Buoyancy
Related?
How Can Very Dense Objects
Float?
• Their average density is less than the fluid
they float in...
• Because they have air holes or are spread
out over a large area and filled with air
How can dense things float?
• the density of steel is 9.0 g/cm3
• the density of water is 1.0 g/ml and the
density of sea water is 1.03 g/ml
• how can a ship made of steel float?
Archimedes discovered that:
A ship will float when the weight of the water it displaces
equals the weight of the ship.
AND
Anything will float if it is shaped to displace its own weight
of water before it reaches the point where it will submerge.
Average Density
• average density – the total mass of ALL
the substances that make up an object
divided by the total volume of the object
• humans, fish, ships, submarines, etc can
float because of average density
– they have lots of air or water spaces inside them
Archimedes Principle
• Buoyant Force = weight in air – weight in fluid
e.g. 7 lbs – 4 lbs = buoyant force of 3 lbs
and the weight of the water displaced by the object is also 3 lbs
Don’t forget air is also a fluid…
• so the principles that apply to having
something float in water also apply to
having something float in air!
Hydrometer
• uses buoyancy to measure
density directly
• calibrated with markings to
show density in g/ml,
• a denser liquid holds the
hydrometer up higher
• a less dense liquid allows the
hydrometer to sink more