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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