Action Reaction Pairs

Action-Reaction Pairs

Multiple Choice

Which of Newton's laws is related to action-reaction pairs?

• Newton's 1st Law

• Newton's 2nd Law

• Newton's 3rd Law

• None

A set of two forces that act against each other. The result of this pair is a net movement or acceleration.

What is an action-reaction pair?

​Ex. 1 A wood block slides along an old table...

• The block and table are imperfectly shaped.

• Their imperfections create friction.

​The force that pushes the block is called an applied force. Use the symbol:
F_app

Friction is a force that opposes motion. Measured in newtons. Use the symbol:
F_f

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​F_app

​F_f

​Ex. 1 A wood block slides along an old table...

The harder you push, the more F_app. But also, the more friction fights your effort, so more F_f.

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​F_app

​F_f

​Ex. 1 A wood block slides along an old table...

If you push very little, then F_app is reduced, but so is F_f.

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​F_app

​F_f

​Ex. 1 A wood block slides along an old table...

If you stop pushing, then the block stops moving. So F_app is reduced to zero. The same happens to F_f.

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​F_app = F_f = 0 N

​Ex. 1 A wood block slides along an old table...

F_app is the action (because you pushed the block).
F_f is the reaction (because friction exists AFTER you pushed the block).
They are linked to each other.
They are an action-reaction pair!

Multiple Choice

A woman pulls on a rope that is connected to a crate. The crate then drags against the ground and experiences friction with the grass and dirt. What was the action?

• The friction of the crate against the ground.

• The tension on the rope from the woman's pull.

Multiple Choice

A woman pulls on a rope that is connected to a crate. The crate then drags against the ground and experiences friction with the grass and dirt. What was the reaction?

• The friction of the crate against the ground.

• The tension on the rope from the woman's pull.

The turning of the wheel is the action that is needed to create the reaction (friction) that gets the car going.

Without friction, a car can't move...

• In cars, we call friction "traction".

• Traction is what moves the car.

• When it rains or snows, the car loses traction.

Multiple Choice

If the car spins out of control, it's because it lost friction with the ground. What did it lose?

• Inertia

• Traction

• Action

• Net Force

In the rain...

Cars require special tires that increase the friction with the ground by allowing water to be pushed out the side-walls.

​Rain & snow reduce friction, so the reaction is less. The reaction provides the traction for the car to drive safely. Lower reaction force means lower traction. This means dangerous driving conditions.