s8pe-10403-ca
12/8/05
9:03 PM
MAZER
Page 116
KEY CONCEPT
The buoyant force acts
on objects in fluids.
CALIFORNIA
BEFORE, you learned
NOW, you will learn
Content Standards
• Objects immersed in water
displace some of the water
• Pressure acts in all directions
on objects immersed in a fluid
and varies with depth
• Density is mass divided by
volume
• About the buoyant force
• How fluid displacement relates
to buoyant force
• What determines if an object
sinks or floats
8.8.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.
8.8.d. Students
know how to predict
whether an object
will float or sink.
EXPLORE Forces in Liquid (8.8.c)
How does water affect weight?
PROCEDURE
1
VOCABULARY
buoyant force p. 117
Tie a piece of string to the middle of the
pencil. Tie 4 paper clips to a string on each
end of the pencil as shown.
2 Move the middle string along the pencil
until the paper clips are balanced and the
pencil hangs parallel to the table.
MATERIALS
• 3 pieces of
string
• pencil
• 8 paper clips
• cup full of
water
3 While keeping the pencil balanced, slowly lower
the paper clips on one end of the pencil into the
water. Observe what happens.
WHAT DO YOU THINK?
• How did the water affect the balance between
the two sets of paper clips?
• Did the water exert a force on the paper clips? Explain.
Fluids exert an upward force on objects.
If you drop an ice cube into a glass of water, the ice cube may sink a
little bit at first, but it will quickly rise and float at the surface of the
water. Ice isn’t the only thing that floats in water. Most people can
float in water. Plastic bags, cork, and many types of wood also float.
If you drop an ice cube, a plastic bag, a cork, or a piece of wood in
air, each of them will fall quickly to the ground. What causes an object
to fall in air but float in water? Gravity acts with the same force
whether the object is in air or in water. It pulls all objects downward.
When an object floats, a force pushes up on the object. The force acts
against the downward force of gravity. It is called the buoyant force.
116 Unit 1: Motion and Forces
PDF