HOMEWORKPage: 115Problems: all even
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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 thesame magnitude (3rd Law). The sports car
experiences the larger acceleration because it has a smaller mass, and acceleration is
inversely proportional to mass.
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Investigation 4-3Part 1 and Part 3
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Practice Problems
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4 (Jack)6 (Volunteer)
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