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Fbuoy
THE BUOYANT FORCE
Fbuoy
HOW BIG IS THE BUOYANT FORCE?
1 m1 m
1 m
Fbuoy
THE CASPER BOX
- no mass
- just holds stuff
1 m1 m
1 m WATER
WE,W = 10,000 N
How big is the buoyant force?
= 10,000 N
Fbuoy
THE CASPER BOX
- no mass
- just holds stuff
1 m1 m
1 m STEEL
WE,S = 78,000 N
How big is the buoyant force?
= 10,000 N
a
Fbuoy
THE CASPER BOX
- no mass
- just holds stuff
1 m1 m
1 m STYRO-FOAM
WE,S = 1,000 N
How big is the buoyant force?
= 10,000 N
a
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 water it will sink
• If an object has smaller density than water it will float
Where will they “float?”
salt water
= 1.2 g/ml
fresh water
= 1.0 g/ml
oil
= 0.9 g/ml
Object A: A = 0.8 g/ml
Object B: B = 0.95g/ml
Object C: C = 1.1 g/ml
Object D: D = 1.3 g/ml
Object E: E = 4.0 g/ml
How can a steel boat float?
The bouyant force is greatest on a ten ton ship if it is floating in:
• A) fresh water
• B) salt water
• C) mercuryWE,b
F(buoy)W,ba = ?
a = 0
• D) all the same
What does change?
THREE BLOCKS
• Which block has the greatest buoyant force?
Which block experiences the largest buoyant force?
WE,1
WE,2
Archimedes’Principle
Fw,1 Fw,2
oak
balsa
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
Hard problem:
You are floating in a boat in your very own swimming pool (you are rich). There are six gold bricks sitting on the floor of the boat (you are rich). You throw them into the pool. Does the water level of the pool go up or down (or stay the same)?
