Standard 8
All objects experience a buoyant
force when immersed in a fluid.
Anticipatory Set
Why do you think these two liquids weigh differently
when they are the same amount?
Standard 8.8 (a-b)
A. Students know density is mass per unit
volume.
B. Students know how to calculate the density
of substances (regular and irregular solids
and liquids) from measurements of mass and
volume.
LANGUAGE OF THE
DISCIPLINE
• Density: the mass-to-volume ratio of a substance
• Volume: is the amount of space occupied by a 3D object
• Graduated Cylinder: container used to measure
volume of a liquid.
Density
• Which grouping is the
most dense below?
Density
• Density is a physical property of matter
• Density can be used to identify things
– Example: No matter what size Aluminum is, or
what shape, it will always be the same density
• The ratio of mass to volume will always be the same
Measuring Volume
Type of
substance
Tool/method
Liquid
Graduated cylinder
Regular
solid
Length * width * height
Irregular
solid
Water displacement test
V (solid)= V (water & solid) – V (water)
Water displacement
• Example: What is the
volume of the
dinosaur?
• V (solid)= V (water &
solid) – V (water)
5.6-4.8 = 0.8
Calculating Density
Check for Understanding
• If you want to know the density and only know
the mass what else do you need to find?
• volume
• What method should you use to find out the
volume of an irregular solid?
• Water displacement
• How do you figure out the volume of a regular
solid?
• Length x width x height
Practice & HW
• Guided Practice:
– Read through Guided instruction on page 63-64
and highlight important information
– Complete questions 1-4 on page 64
– Raise hand for a stamp
• Independent Practice:
– Complete questions 1-4 on page 65
• HW:
– Worksheet
Anticipatory Set
Why do some objects sink and some float?
Standard 8.8 (c-d)
C. Students know the buoyant force on an object in
a fluid is an upward force equal to the weight of
the fluid the object has displaced.
D. Students know how to predict whether an object
will float or sink.
LANGUAGE OF THE
DISCIPLINE
• Buoyant force: the upward force acting on a
submerged object
• Hydrometer: tube-like instrument used to
determine density of liquid compared to
water
• Archimedes principal: a substance in a fluid is
help up by a force equal to the weight of the
displaced fluid
• Naturally buoyant: an object that will not sink
Buoyant Force
• is equal to the weight of the volume of fluid
displaced by object.
– So BIGGER VOLUME= BIGGER BUOYANT FORCE
• Is measured in NEWTONS (since it is a force)
• This is why you feel lighter in water
Buoyant Force
• Buoyant force occurs in
fluids…
• FLUIDS CAN BE GASES
OR LIQUIDS
• Example: Helium
balloons rise because
there is a greater
buoyant force on it.
Archimedes’ principal
Pushed up by a force equal to
weight of water displaced
Sink or Float
• Sinking and floating all
has to do with density
and buoyant force
• Naturally buoyant
objects will not sink
• This is why oil floats on
water- it is less dense!
Sink or Float
• EXCEPTION: sometimes
very dense objects can
float.
– Example: Steel Boats
– They can float because
of they way they are
shaped so they do not
displace as much water.
• Hydrometer
– It will sink deeper in LESS
dense liquid
– Float higher in MORE
dense liquid
Check for Understanding
Page 69- Questions 2-5
Answers:
2. The plate would sink
3. The weight of the rock is greater than the weight of
the water displaced, so the rock will sink
4. The distance it sinks in hydrometer indicated the
density. The lower it sinks the less dense the liquid
5. A beach ball on top of water (since the beach ball is
full of a fluid- AIR)
Practice & HW
• Guided Practice:
– Read through Guided instruction on page 67-69
and highlight important information
– Complete the questions 6-8 on page 70
– Raise hand for a stamp
• Independent Practice:
– Complete questions 1-4 on page 71
• HW:
– worksheet.