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Self-Check
Can a frictionless surface exert a normal force. Can a frictional force exist
without a normal force.
If you try to accelerate or decelerate your car too quickly, the forces
between your wheels and the road become too great, and they begin
slipping. This is not good, because kinetic friction is weaker than static
friction, resulting in less control. Also, if this occurs while you are turning,
the car’s handling changes abruptly because the kinetic friction force is in a
different direction than the static friction force had been: contrary to the
car’s direction of motion, rather than contrary to the forces applied to the
tire.
Most people respond with disbelief when told of the experimental
evidence that both static and kinetic friction are approximately independent
of the amount of surface area in contact. Even after doing a hands-on
exercise with spring scales to show that it is true, many students are unwill-
ing to believe their own observations, and insist that bigger tires “give more
traction.” In fact, the main reason why you would not want to put small
tires on a big heavy car is that the tires would burst!
Although many people expect that friction would be proportional to
surface area, such a proportionality would make predictions contrary to
many everyday observations. A dog’s feet, for example, have very little
surface area in contact with the ground compared to a human’s feet, and yet
we know that a dog can often win a tug-of-war with a person.
The reason why a smaller surface area does not lead to less friction is
that the force between the two surfaces is more concentrated, causing their
bumps and holes to dig into each other more deeply.
Frictionless ice can certainly make a normal force, since otherwise a hockey puck would sink into the ice. Friction is
not possible without a normal force, however: we can see this from the equation, or from common sense, e.g. while
sliding down a rope you do not get any friction unless you grip the rope.
Section 5.2Classification and Behavior of Forces