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If you want to move something, and there
is some resistance or resistive force to that movement,
then work is required to move the object. Work against a
resistive force is the force required to move an object
times the distance you move it. Another type of work is
simply expending energy.
Questions you may have about it include:
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What is a resistive force?
-
What is work against a force?
-
What is the other type of work?
Resistive forces
A resistive force is a force that causes
a moving object to slow down or tends to prevent a
stationary object to move. The resistive force acts in a
direction opposite to the one that you want to move the
object.
One example of several resistive forces
in action is the reason a moving car slows down when you
let off the gas pedal. It is coasting, and you are not
touching the brakes, but yet it slows down. Its
deceleration is caused by resistive forces.
Some examples of resistive forces are:
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Friction
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Gravity
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Air or water resistance
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Spring resistance
They all can slow down or decelerate a
moving object.
Friction is a force of resistance to
anything that is moving or sliding along a surface or
material. For example, a book sliding along the floor is
slowed down due to the force of friction at the area of
contact between the object and the floor. Also, your car
has brakes which use friction to slow down the vehicle.
An object moving through the air or
through water is also slowed down. A small amount of
this is due to the force of friction on the surface, but
most of the resistance is caused by the work required to
move the air or water out of the way.
You can decrease air or water resistance
by streamlining the object. In this way it pushes
through the fluid easier.
Airplanes use flaps, as well as reverse
their engines to increase the air resistance when want
to slow down during a landing.
Gravity can be considered a resistive
force, when an object is moving against the pull of
gravity. An object thrown up in the air slows down until
it reaches its peak, at which time it changes direction
and starts to speed up again.
Work against a resistance
The definition of work is the amount of
force required to move an object against resistance or
resistances for a given distance (W = F x d). For the
sake of clarity, we should always refer to work in terms
of work against a resistance.
Work is measured in foot-pounds or
Newton-meters. (It is not sure why they put the distance
unit first in the English measurement of work, instead
of the force unit. Perhaps it just sounds better. Or
maybe they want to confuse you.)
If you want to lift a box from the floor
to a table, you must work against the force of gravity.
The amount of work you must do is the weight of the
object times the height you are lifting it.
Work = Weight x Height.
Weight is measured as pounds or Newtons
and height as feet or meters, thus you get foot-pounds
or Newton-meters for the work done.
If you push a heavy box along the floor,
you must work against the force of friction.
Work = Friction force x Distance moved.
The friction force is equal to the
weight of the object times a number called the
coefficient of friction. This number is dependent on the
materials involved.
Other type of work
When you are thinking or holding a heavy
object, you are also doing work. But it is a different
type of work than is defined in most Physics books. This
type of work is expending energy, while the official
definition is moving an object against a force.
When you hold a heavy object, your body
is expending energy as you apply a force against gravity
to hold the object up. But you aren't moving the object,
so you aren't really doing work against a resistive
force. Still, in everyday language, you are doing "work"
by holding the object off the floor. It is necessary to
distinguish between these different uses of the word.
When you are thinking or trying to
figure out a problem, you are using mental energy. If it
is a struggle, people call it work. It is similar to
holding up the heavy object.
Thus, the definitions for work are:
Be sure you specifically define what you mean.
Work is required to act against
resistive forces, such as friction, air resistance, and
gravity. Work is force times distance against
resistance. The definition of work for humans is simply
expending energy trying to do something. |
