Action Reaction Pairs

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?

• 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

​F_app

​F_f

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

​F_app

​F_f

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

​F_app

​F_f

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

​F_app = F_f = 0 N

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!

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.

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.