1. Density
Density is a comparison of how much matter there is in a certain amount of space.
Which square is more dense?

2. ALWAYS REMEMBER THE UNITS!
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

3. Volume for regular shapes volume = length x width x height
e.g. = 10 cm3

4. for liquids – measure directly in ml
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

5. 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 you up with a buoyant force

6. 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.

7. 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

8. Bouyancy

9. neutral buoyancy gravity = buoyant force object stays suspended
if gravity > buoyant force → object sinks
neutral buoyancy gravity = buoyant force object stays suspended
if gravity < buoyant force → object rises or floats

10. 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)

11. Archimede’s Principle
This principle explains why some objects float in water and others sink.

12. 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.

13. 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:

14. 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.

15. 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.

16. 3. Submarines
By allowing water to flow in or out, a submarine can rise or sink in the water.
The submarine floats when its weight is equal to the buoyant force and it sinks when its weight is greater than the buoyant force.

17. 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
it, so it rises.

18. 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.

19. 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

20. How are Density and Buoyancy Related?

21. 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

22. 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.

23. 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

24. 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

25. 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!

26. 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