1. Buoyancy

2. Pressure in a Fluid
• Objects in a fluid
experience an
buoyant
force which can
make them float.
• Liquids and gases are fluids—materials
that can flow and have no definite shape.
3.2 Pressure and the Buoyant Forces

3. Pressure and Depth
• The water more water above
you, the more pressure you feel.
3.2 Pressure and the Buoyant Forces

4. Pressure in All Directions
• Horizontal pressures cancel each other out.
3.2 Pressure and the Buoyant Forces

5. Buoyant Force and Depth
• The bottom of the fish is deeper, so there is more
force pushing up on it – a buoyant force.
3.2 Pressure and the Buoyant Forces

6. What causes buoyant force?
• Buoyant force is the force on an object
exerted by the surrounding fluid.
• When an object pushes water, the water pushes
back with as much force as it can.
• If the water can push back as hard, the object
floats (boat). If not, it sinks (steel).
3.2 Pressure and the Buoyant Forces

7. Archimedes’ Principle
• states that the buoyant force is equal to the
weight of the fluid the object displaces.
• 1 cm3 of steel weighs more than the 1 cm3 of water
it displaces, so it sinks. 1 cm3 of ice weighs less and
so it is pushed up.
Newton’s 3rd
Law – an
object can
push back
with no
more force
than it can
push.

8. How to measure the buoyant force
• Weight of object in air - weight of object
in water = upward buoyant force.
• The buoyant force is equal to the weight
of the displaced waterObject’s weight in air -
object’s weight in water
= Buoyant Force
= Weight of displaced water

9. Pressure is the ____ per unit area.
A mass
B volume
C force
D pascal
3.2 Pressure and the Buoyant Forces
A
B
C
D
0% 0%0%0%
1. A
2. B
3. C
4. D

10. Which direction does the buoyancy force
act on an object?
A upward
B downward
C to the left of the object
D to the right of the object
3.2 Pressure and the Buoyant Forces
A
B
C
D
0% 0%0%0%
1. A
2. B
3. C
4. D

11. Where is the pressure the greatest on
an object in a fluid?
A top
B bottom
C sides
D none of the above
3.2 Pressure and the Buoyant Forces
A
B
C
D
0% 0%0%0%
1. A
2. B
3. C
4. D