s8pe-10303-ca
12/8/05
6:08 PM
MAZER
Page 88
When your force is greater than this limit, the forces on the chair are
no longer balanced, and the chair moves. The frictional force remains
at a new lower level once the chair is moving.
The harder two surfaces are
pushed together, the more difficult it is for the surfaces to slide over
each other. When an object is placed on a surface, the weight of the
object presses on that surface. The surface exerts an equal and opposite reaction force on the object. This reaction force, the normal force,
is one of the factors that determines how much friction there is.
Force Pressing the Surfaces Together
If you push a chair across the floor, there will be a certain amount
of friction between the chair and the floor. Increasing the weight of
the chair increases the normal force. The force of friction between the
chair and the floor is greater when a person is sitting in it than when
the chair was empty.
Friction depends on the total force pressing the surfaces together,
not on how much area this force acts over. Consider a rectangular
cardboard box. It can rest with its smaller or larger side on the floor.
The box will have the same force from friction regardless of which
side sits on the floor. The larger side has more area in contact with
the floor than the smaller side, but the weight of the box is more
spread out on the larger side.
check your reading
What factors influence frictional force? Give two examples.
Friction and Weight
Less Weight
More Weight
weight
weight
applied
force
applied
force
friction
The force of friction depends on the total force pushing the surfaces together. Here the weight of the
chair is the force pressing the surfaces together.
88 Unit 1: Motion and Forces
friction
The weight of the chair increases when someone sits
in it. The force of friction is now greater than when
the chair was empty.
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