Friction
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| FRICTION |
What is Friction?
Friction, force that opposes the
motion of an object when the object is in contact with another object or
surface. Friction results from two surfaces rubbing against each other or
moving relative to one another. It can hinder the motion of an object or
prevent an object from moving at all. The strength of frictional force depends
on the nature of the surfaces that are in contact and the force pushing them
together. This force is usually related to the weight of the object or objects.
In cases involving fluid friction, the force depends upon the shape and speed
of an object as it moves through air, water, or other fluid.
Friction occurs to some degree in almost all
situations involving physical objects. In many cases, such as in a running
automobile engine, it hinders a process. For example, friction between the
moving parts of an engine resists the engine’s motion and turns energy into
heat, reducing the engine’s efficiency. Friction also makes it difficult to
slide a heavy object, such as a refrigerator or bookcase, along the ground. In
other cases, friction is helpful. Friction between people’s shoes and the
ground allows people to walk by pushing off the ground without slipping. On a
slick surface, such as ice, shoes slip and slide instead of gripping because of
the lack of friction, making walking difficult. Friction allows car tires to
grip and roll along the road without skidding. Friction between nails and beams
prevents the nails from sliding out and keeps buildings standing.
When friction affects a moving object, it turns the
object’s kinetic energy, or energy of motion, into heat. People welcome the
heat caused by friction when rubbing their hands together to stay warm.
Frictional heat is not so welcome when it damages machine parts, such as car
brakes.
CAUSES OF FRICTION
Friction occurs in part
because rough surfaces tend to catch on one another as they slide past each
other. Even surfaces that are apparently smooth can be rough at the microscopic
level. They have many ridges and grooves. The ridges of each surface can get
stuck in the grooves of the other, effectively creating a type of mechanical
bond, or glue, between the surfaces.
Two surfaces in contact
also tend to attract one another at the molecular level, forming chemical bonds.
These bonds can prevent an object from moving, even when it is pushed. If an
object is in motion, these bonds form and release. Making and breaking the
bonds takes energy away from the motion of the object.
Scientists do not yet fully understand the details
of how friction works, but through experiments they have found a way to
describe frictional forces in a wide variety of situations. The force of
friction between an object and a surface is equal to a constant number times
the force the object exerts directly on the surface. The constant number is
called the coefficient of friction for the two materials and is
abbreviated µ. The force the object exerts directly on the surface is called
the normal force and is abbreviated N. Friction depends on this force
because increasing the amount of force increases the amount of contact that the
object has with the surface at the microscopic level. The force of friction
between an object and a surface can be calculated from the following formula:
F = µ × N
In this equation, F is the force of
friction, µ is the coefficient of friction between the object and the
surface, and N is the normal force.
Scientists have measured the coefficient of friction for
many combinations of materials. Coefficients of friction depend on whether the
objects are initially moving or stationary and on the types of material
involved. The coefficient of friction for rubber sliding on concrete is 0.8
(relatively high), while the coefficient for Teflon sliding on steel is 0.04
(relatively low).
The normal force is the force the object
exerts perpendicular to the surface. In the case of a level surface, the normal
force is equal to the weight of the object. If the surface is inclined, only a
fraction of the object’s weight pushes directly into the surface, so the normal
force is less than the object’s weight.
KINDS OF FRICTION
Different kinds of motion give rise to
different types of friction between objects. Static friction occurs between
stationary objects, while sliding friction occurs between objects as they slide
against each other. Other types of friction include rolling friction and fluid
friction. The coefficient of friction for two materials may differ depending on
the type of friction involved.
Static friction prevents
an object from moving against a surface. It is the force that keeps a book from
sliding off a desk, even when the desk is slightly tilted, and that allows you
to pick up an object without the object slipping through your fingers. In order
to move something, you must first overcome the force of static friction between
the object and the surface on which it is resting. This force depends on the
coefficient of static friction (µs) between the object and
the surface and the normal force (N) of the object.
A book sliding off a desk or brakes
slowing down a wheel are both examples of sliding friction, also called kinetic
friction. Sliding friction acts in the direction opposite the direction of
motion. It prevents the book or wheel from moving as fast as it would without
friction. When sliding friction is acting, another force must be present to
keep an object moving. In the case of a book sliding off a desk, this force is
gravity. The force of kinetic friction depends on the coefficient of kinetic
friction between the object and the surface on which it is moving (µk)
and the normal force (N) of the object. For any pair of objects, the
coefficient of kinetic friction is usually less than the coefficient of static
friction. This means that it takes more force to start a book sliding than it
does to keep the book sliding.
Rolling friction hinders the motion
of an object rolling along a surface. Rolling friction slows down a ball
rolling on a basketball court or softball field, and it slows down the motion
of a tire rolling along the ground. Another force must be present to keep an
object rolling. For example, a pedaling bicyclist provides the force necessary
to the keep a bike in motion. Rolling friction depends on the coefficient of
rolling friction between the two materials (µr) and the
normal force (N) of the object. The coefficient of rolling friction is
usually about t that of sliding friction. Wheels and other round objects will
roll along the ground much more easily than they will slide along it.
Objects moving through a fluid
experience fluid friction, or drag. Drag acts between the object and the
fluid and hinders the motion of the object. The force of drag depends upon the
object’s shape, material, and speed, as well as the fluid’s viscosity.
Viscosity is a measure of a fluid’s resistance to flow. It results from the
friction that occurs between the fluid’s molecules, and it differs depending on
the type of fluid. Drag slows down airplanes flying through the air and fish
swimming through water. An airplane’s engines help it overcome drag and travel
forward, while a fish uses its muscles to overcome drag and swim. Calculating
the force of drag is much more complicated than calculating other types of
friction. (see Aerodynamics)
EFFECTS OF FRICTION
Friction helps people
convert one form of motion into another. For example, when people walk,
friction allows them to convert a push backward along the ground into forward
motion. Similarly, when car or bicycle tires push backward along the ground,
friction with the ground makes the tires roll forward. Friction allows us to
push and slide objects along the ground without our shoes slipping along the
ground in the opposite direction.
While friction allows us to convert one form of motion
to another, it also converts some energy into heat, noise, and wear and tear on
material. Losing energy to these effects often reduces the efficiency of a
machine. For example, a cyclist uses friction between shoes and pedals, the
chain and gears, and the bicycle’s tires and the road to make the bicycle move
forward. At the same time, friction between the chain and gears, between the
tires and the road, and between the cyclist and the air all resist the
cyclist’s motion. As the cyclist pedals, friction converts some of the
cyclist’s energy into heat, noise, and wear and tear on the bicycle. This
energy loss reduces the efficiency of the bicycle. In automobiles and
airplanes, friction converts some of the energy in the fuel into heat, noise,
and wear and tear on the engine’s parts. Excess frictional heat can damage an
engine and braking system. The wearing away of material in engines makes it
necessary to periodically replace some parts.
Sometimes the heat that friction produces is
useful. When a person strikes a match against a rough surface, friction
produces a large amount of heat on the head of the match and triggers the
chemical process of burning. Static friction, which prevents motion, does not
create heat.
REDUCING FRICTION
Reducing the amount of
friction in a machine increases the machine’s efficiency. Less friction means
less energy lost to heat, noise, and wearing down of material. People normally
use two methods to reduce friction. The first method involves reducing the
roughness of the surfaces in contact. For example, sanding two pieces of wood
lessens the amount of friction that occurs between them when they slide against
one another. Teflon creates very little friction because it is so smooth.
Applying a lubricant to a surface can also reduce
friction. Common examples of lubricants are oil and grease. They reduce
friction by minimizing the contact between rough surfaces. The lubricant’s
particles slide easily against each other and cause far less friction than
would occur between the surfaces. Lubricants such as machine oil reduce the
amount of energy lost to frictional heating and reduce the wear damage to the
machine surfaces caused by friction.
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