Density and Buoyancy for Grade 8 Science Class

2. Why do
ships float?

3. Archimedes
Principle
States that the upward buoyant
force that is exerted on an object
immersed in a fluid, whether fully
or partially submerged, is equal to
the weight of the fluid that the
object displaces
Buoyant Force (N) =
Weight of Liquid Displaced =
(Mass Liquid x 9.8m/s^2)

4. Buoyancy
• Definition: The upward
supportive force on an object in
a fluid
• The force of gravity acting
downward on a floating object
must be equal to the buoyant
force of the water acting
upwards
• If an object sinks, the force of
gravity is greater than the
buoyant force

5. Sinking vs Floating
Floating object:
Mass of Object (kg) = Mass of Liquid
Displaced (kg)
Weight of Object (N) = Weight of
displaced liquid (N)/Buoyant force (N)
Sinking object:
Mass of Object (kg) > Mass of Liquid
Displaced (kg)
Weight of Object (N) > Weight of
displaced liquid (N)/Buoyant force (N)

7. Example: If 100 kg of water is displaced by a floating object, the
buoyant force is equal to the weight of the water.
If the weight of the water displaced is 980N, then the
buoyant force is also 980N
Weight Water = Mass x Gravity (F = m x g)
= 100kg x 9.8m/s^2 = 980N

8. Shape & Buoyancy
• The ball and the ship have the
same weight – but the ball
sinks & ship floats – why?
• Because the shape of the ship
displaces more water than
the ball (the weight of the water
displaced is greater), more
upward buoyant force is
applied allowing the ship to
float

9. A boat has a weight of
1000N. When the boat is in
water, it can displace up to
500kg of water.
1. Will this boat sink or float?
2. What is the buoyant Force?
3. What is the maximum amount
of weight this boat can carry
before sinking?

10. Buoyancy
Applications:
Oil & Water
• Oil is less dense than water – so it
floats on water
• Difference in density helps during
oil spills
• Can scoop, suck or soak up much
of the oil on top of the water

11. Buoyancy
Applications: Fish
• Swim Bladder: a controllable, balloon-like
chamber that allows fish to alter their
buoyancy
• By changing amount of oxygen in bladder, fish
alter their density and ultimately how much
water they displace (changing their buoyancy)
• More oxygen in bladder = less dense =
higher float
• Less oxygen = less dense = more sinking

12. Buoyancy Applications:
Submarines
• Ballast Tanks:
Compartments in a ship or
submarine that take in
water to keep the ship
stable or help a submarine
dive below the surface

13. Boat
Challenge
Objective: Students will design and build
a boat using aluminum foil that can carry
the most weight while floating in water.
Materials: Aluminum foil
Best Design: Awarded to the boat with
the most innovative, aesthetically
pleasing, and well-executed design.
Carry the Most Weight: Awarded to the
boat that successfully carries the greatest
amount of weight while remaining afloat.