FORCES AND FLUIDS
Some things to bring out of the recesses of your mind:
Fluids: any matter that has the ability to flow = liquid
gas
Force: any push or pull exerted on matter
Pressure: force exerted on matter by air molecules
more air molecules=more pressure
less air molecules = less pressure
Let’s not forget DENSITY!!!
Density is the amount of matter in a given volume
DOES NOT CHANGE WITH CHANGES IN SHAPE
OR SIZE!!!
Density of water is 1 g/ml
The ability of an object to float is dependent on the
Object’s density with respect to the density of the
Fluid it is in.
If an object sinks, its density would be greater than the
Density of the fluid. In this case, greater than 1 if we
Are placing the object in water.
Styrofoam
D=0.75g/ml
Buoyancy is the force exerted on an object
by a fluid and it usually acts against gravity.
This is why objects feel lighter in water.
Some of the force of gravity (weight) is
cancelled out by the force of buoyancy. If
the buoyant force is stronger than the force
of gravity, the object will float in the fluid.
Gravity
Buoyant Force
Archimedes’ Principle states that the
buoyant force of a fluid is equaled to the
weight of the fluid displaced by the object. If
an object can displace (push away) enough
fluid to equal its weight, it will float. This is
why a tremendous ship made of iron can
float but a cannonball would sink. The ship’s
shape allows it to push enough water away
to equal the weight of the ship.
5.0
Pounds
If the object can push
away an equal weight of
fluid as its own weight, it
will float in that fluid.
0.0 lbs
5.0
10.0
Pounds
If the object pushes away
a lower weight of the fluid
than its own weight, it will
sink in that fluid.
0.0 lbs
6.0
The amount of water that was displaced by the
floating boat weights the same as the boat itself.
According to Archimedes’ Principle, this is why
the boat floats and doesn’t sink.
Displaced Water
If the rock is thrown overboard, how will the
water level in the tub change?
Sinkable
Rock
Here’s why the water level lowered:
Inside the boat the rock (10 lbs) is floating.
That means the rock is causing the boat to
displace 10 lbs of water to support it.
Outside the boat the rock would sink
(weighs 10 lbs but only can displace 6 lbs).
The rock sinks because it cannot displace
an amount of water equal to its weight.
Because 10 lbs of water takes up more
space than 6 lbs of water, the water level
lowers. Remember, when something
floats, it is able to displace the same
weight of fluid as its own weight. If it
sinks, it means it weighs more than the
fluid it is able to displace.
WOW, that was a mouthful.
Time for me SPINACH!
Press to view Buoyancy
BRAINPOP movie