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Another common force is friction.

When two surfaces slide past each other, the interaction between them produces a force of friction.

In this diagram, the book is moving to the right across the table as shown by the red arrow.

Diagram showing interaction pairs: The book is moving to the right and this produces friction on the table in the same direction as the book is moving (to the right) and friction  on the book in the opposite way to the direction the book is moving (to the left). The two friction forces are an interaction pair.

Interaction pair

Interaction pair

The blue and green arrows show the interaction pair of friction forces.

The book experiences a backwards force. This will tend to slow it down.

The table experiences a forwards force. This will tend to move it forwards with the book.

When you push backwards on the floor with your foot, the friction between your foot and the floor exerts a backwards force on the floor. The other force of the interaction pair is the floor pushing your foot forwards.

The result is that you move forwards, but the floor stays still.

A common experiment is to show the change in friction with different surfaces. In this experiment, a block of wood is pulled along a surface with a force metre. The greater the forces needed to pull the block, the higher the friction.

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