s8pe-10203-ca
12/6/05
8:51 AM
MAZER
Page 57
KEY CONCEPT
Forces act in pairs.
CALIFORNIA
Content Standard
Extension of 8.2.d
Students know how to
identify separately the
two or more forces
that are acting on a
single static object,
including gravity,
elastic forces due to
tension or compression
in matter, and friction.
BEFORE, you learned
NOW, you will learn
• A force is a push or a pull
• Increasing the force on
an object increases the
acceleration
• The acceleration of an object
depends on its mass and the
force applied to it
• How Newton’s third law
relates action/reaction pairs
of forces
• How Newton’s laws
work together
THINK ABOUT
How do jellyfish move?
VOCABULARY
Newton’s third law p. 57
Jellyfish do not have much control
over their movements. They drift
with the current in the ocean.
However, jellyfish do have some
control over their up-and-down
motion. By squeezing water out of
its umbrella-like body, the jellyfish shown here applies a force in one
direction to move in the opposite direction. If the water is forced
downward, the jellyfish moves upward. How can a person or an object
move in one direction by exerting a force in the opposite direction?
Newton’s third law relates action and
reaction forces.
COMBINATION NOTES
In your notebook, make an
outline and draw a diagram
about Newton’s third law.
Newton made an important observation that explains the motion
of the jellyfish. He noticed that forces always act in pairs.
Newton’s third law states that every time one object exerts a force
on another object, the second object exerts a force that is equal in
size and opposite in direction back on the first object. As the jellyfish
contracts its body, it applies a downward force on the water. The
water applies an equal force back on the jellyfish. It is this equal and
opposite force on the jellyfish that pushes it up. This is similar to
what happens when a blown-up balloon is released. The balloon
pushes air out the end, and the air pushes back on the balloon and
moves it forward.
check your reading
What moves the jellyfish through the water?
Chapter 2: Forces 57
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