HOMEWORK
Page: 115
Problems: all even
1
4.3 Newton’s Third law
TODAY’s OBJECTIVES
• Learn about The Newton’s Third
Law
• Identify action-reaction pairs
• Identify friction
THE NEWTON’S THIRD LAW
Forces always exist in pairs
• The basis for the Newton’s Third Law
THE NEWTON’S THIRD LAW
If two objects interact, the magnitude of the
force exerted on object 1 by object 2 is equal to
the magnitude of the force simultaneously
exerted on object 2 by object 1, and these two
forces are opposite in direction
For every ACTION, there is an equal and
opposite REACTION
THE NEWTON’S THIRD LAW
ACTION - REACTION pair = forces that
objects exert on each other
ACTION - REACTION pair = DO NOT cancel
each other
THINK ABOUT IT…
(on a worksheet)
A child causes a wagon to accelerate by pulling it with a horizontal
force. Newton’s third law says that the wagon exerts an equal and
opposite force on the child. How can the wagon accelerate?
(Hint: Draw a free – body diagram for each object.)
THINK ABOUT IT…
(on a worksheet)
A child causes a wagon to accelerate by pulling it with a horizontal
force. Newton’s third law says that the wagon exerts an equal and
opposite force on the child. How can the wagon accelerate?
(Hint: Draw a free – body diagram for each object.)
The reaction force (from
the wagon) acts on the
child, not on the wagon
itself, so there is still a net
force on the wagon
Example 3
(on a worksheet)
identify the action-reaction pairs in the following situations:
a. A person takes a step.
b. A snowball hits someone in the back.
c. A baseball player catches a ball.
d. A gust of wind strikes a window
Example 4
(on a worksheet)
If a small sports car collides head-on with a massive truck, which vehicle
experiences the greater impact force? Which vehicle experiences the
greater acceleration? Explain your answers.
Example 4
(on a worksheet)
If a small sports car collides head-on with a massive truck, which vehicle
experiences the greater impact force? Which vehicle experiences the
greater acceleration? Explain your answers.
Each impact force has the
same magnitude (3rd Law). The sports car
experiences the larger acceleration because it
has a smaller mass, and acceleration is
inversely proportional to mass.
Classwork
Page: 117
Problems: Quick Check
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Classwork
Page: 118 - 119
Problems:
Investigation 4-3
Part 1 and Part 3
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HOMEWORK
Page: 120
Problems: all
16
Practice Problems
Page: 112
Problems: 3 (Nathan)
4 (Lauryn)
18
Practice Problems
Page: 115
Problems: 2 (Larissa)
4 (Jack)
6 (Volunteer)
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Practice Problems
Page: 120
Problems: 2 (Larissa)
4 (Jack)
6 (Volunteer)
20