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Forces and Surfaces
If you look down from a great height, such as from the window of an
airplane, a flat field appears to be smooth. If you were to walk in the
field, however, you would see that the ground has many bumps and
holes. In the same way, a flat surface such as a piece of plastic may
look and feel smooth. However, if you look at the plastic through a
strong microscope, you see that it has tiny bumps and ridges. Friction
depends on how these bumps and ridges on one surface interact with
and stick to the bumps and ridges on other surfaces. There are several
factors that determine the friction between two surfaces.
Friction between two surfaces depends on the
materials that make up the surfaces. Different combinations of
surfaces produce different frictional forces. A rubber hockey puck
sliding across ice has a smaller frictional force on it than the same
puck sliding across a wooden floor. The friction between rubber and
ice is less than the friction between rubber and wood.
Types of Surfaces
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Learn more about
friction, forces, and
surfaces.
reminder
Remember that balanced
forces on an object do not
change the object’s motion.
You need a larger force to start something
moving than you do to keep something moving. If you have ever
tried to push a heavy chair, you may have noticed that you had to
push harder and harder until the chair suddenly accelerated forward.
Motion of the Surfaces
As you apply a force to push a chair or any other object that is not
moving, the frictional force keeping it from sliding increases so the
forces stay balanced. However, the frictional force has an upper limit.
Friction and Motion
Before Object Moves
While Object Moves
acceleration
applied
force
applied
force
friction
When an object is standing still, there is a maximum
force needed to overcome friction and start it moving.
Any force less than this will be exactly balanced by the
force of friction, and the object will not move.
friction
Once the object is moving, the frictional force remains
constant. This constant force is less than the maximum
force needed to start the object moving.
Chapter 3: Gravity, Elastic Forces, and Friction 87
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