Buoyancy: How can a ship made of iron that
weighs about 200 million pounds float?
FOIL SHAPE
PREDICTION
OBSERVATION
Boat
Sink or Float
Sink or Float
Ball
Sink or Float
Sink or Float
Block
Sink or Float
Sink or Float
EXPLANATION
Aluminum Foil Boats
• Objective: To design/build two different
aluminum foil boats that will hold as many
pennies as possible without sinking.
• Procedure
1. Obtain two 20cmx20cm pieces of aluminum foil.
2. Build your boats.
3. Test them by adding pennies until they sink.
4. Dry the pennies off and return them.
5. Which boat in the class won? Why?
Shape and density determine if something will float.
Buoyant Force: When an object displaces
water the water pushes back on it.
What happens to the weight of an object when placed in water?
4
Intro to Forces: PHET Sim
• Force: a push or a pull
(unit: Newtons)
• If Force are balanced an
object will not move or
change its motion!
• Unbalanced forces lead
to changes in motion.
https://phet.colorado.edu/en/simulation/forces-and-motion-basics
These four cubes are resting on a table. Each cube is made of a different material.
Rank these cubes in terms of mass, weight and volume from largest to smallest.
(a)
(b)
(c)
(d)
Mass Highest to
[1]
Lowest
[2]
[3]
[4]
or
Not enough
information
Weight Highest
[1]
to Lowest
[2]
[3]
[4]
or
Not enough
information
Volume Highest
[1]
to Lowest
[2]
[3]
[4]
or
Not enough
information
These four cubes are resting on a frictionless surface. Each cube is pushed and
pulled by different forces. Rank each object from the highest to lowest net force
Force arrows show direction only, they are not drawn to scale.
70N
100N
50N
100N
35N
65N
80N
90N
10kg
10kg
10kg
10kg
(a)
(b)
(c)
(d)
[1]
Highest
[2]
[3]
[4]
Lowest
Explain:
These four cubes are resting on a frictionless surface. Each cube is pushed and
pulled by different forces. Rank each object from the highest to lowest net force
Force arrows show direction only, they are not drawn to scale.
40N
30N
10kg
10N
(a)
[1]
Highest
[2]
75N
65N
60N
10kg
10kg
(b)
(c)
[3]
[4]
Lowest
85N
10N
90N
10kg
(d)
Explain:
Buoyant Force (upward push of water)
1. If Weight is greater
than Buoyant Force
an object sinks.
2. If Buoyant Force is
greater than weight
the object rises and
then floats.
3. When Buoyant
Force equals weight
the object floats or
suspends.
Weight Force (heaviness of object)
Which is greater: the Weight or
Buoyant Force?
10
Floating: Buoyant Force = Weight
If a 100N boat is floating the
water must be pushing on it
with a force of 100N
F = 100N
F = 100N
If we add weight (50N) to the boat
it will sink down deeper and the
buoyant force will increase.
F = 150N
F = 150N
Buoyant Force (upward push of water)
Archimedes Principle:
Buoyant force is equal
to the weight of the
liquid displaced .
The more water an
object displaces the
more buoyant force
pushing on it.
This is why a block of
aluminum sinks but an
aluminum foil boat
floats.
Weight Force (heaviness of object)
Shape is important!
If you were to hold a basketball
under water it will push back very
hard and would fly upwards if
your were to release it.
Buoyant Force > Weight
If you were to hold a basketball
under water it will push back very
hard and would fly upwards if
your were to release it.
Buoyant Force > Weight
When floating the
buoyant force on ball
equals its weight.
Why is it easier to lift a person in water?
Weight (force)
Lift (force)
50N
50N
Why is it easier to lift a person in water?
The water helps you!
Weight (force)
Lift (force)
50N
Weight (force)
50N
Buoyant (force)
Lift (force)
Concrete Boats
• Heavy iron ships can float
because they are shaped
to displace a large
amount of water and
being hollow and filled
with air reduces their
overall density.
• Its easier to pick someone
up in water because the
water helps you (buoyant
force).