Archimedes’ Principle
THE BUOYANT
FORCE
Buoyancy
HOW BIG IS THE
BUOYANT FORCE?
Buoyancy
1m
1m
1m
THE CASPER BOX
Buoyancy
- no mass
- just holds stuff
1m
How big is the buoyant
force?
WATER
1m
THE CASPER BOX
1m
Buoyancy
- no mass
- just holds stuff
a
1m
How big is the buoyant
force?
STEEL
1m
THE CASPER BOX
1m
Buoyancy
- no mass
- just holds stuff
a
1m
How big is the buoyant
force?
STYROFOAM
1m
1m
Archimedes’ Principle:
The buoyant force acting on an object
is equal to the weight of the fluid:
displaced by the object, or
that would fill the “Casper Box”
If an object has greater density than
the fluid it will sink.
If an object has smaller density than
the fluid it will float.
How can a steel boat float?
Where will they “float?”
Object A: ρA = 0.8 g/cc
Object B: ρB = 0.95 g/cc
Object C: ρC = 1.1 g/cc
oil
ρ = 0.9 g/cc
Object D: ρD = 1.3 g/cc
Object E: ρE = 4.0 g/cc
fresh water
ρ = 1.0 g/cc
salt water
ρ = 1.2 g/cc
The bouyant force is greatest on a
ten ton ship if it is floating in:
A) fresh water
B) salt water
C) mercury
Buoyancy
Gravity
D) all the same
What does change?
Which block experiences the
largest buoyant force?
Archimedes’
Principle
balsa
oak
Newton’s
2nd Law
What should I remember?
Archimedes’ Principle always applies,
whether an object sinks or floats.
buoyant force is always equal to weight of
fluid displaced.
If an object floats, the buoyant force is
also equal to the object’s weight.
Problem
A cube of wood 20 cm on each side is
placed in a tub of water and is found to
sink so that only 6 cm of the block is
above the water. What is the density of
the wood?
6 cm
20 cm
6 cm
Buoyancy
20 cm
Gravity
Buoyant Force = Water Inside Box
Hard problem:
You are floating in a boat in your
very own swimming pool. There are
six people in your boat. You ask 3
people to get out of the boat and stand
on the side of the pool. Does the
water level of the pool go up or down
(or stay the same)?