Английский язык. Ильичева Н.А - 24 стр.

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meaning of the word “work” is quite different from the common meaning. A
student standing at rest and holding several books is doing no work although he
or she will feel tired after a time. Technically speaking, work is accomplished
only when a force acts though a distance.

1
       As a physical concept work is the result of the action of a force. We
define the work done by the force F as the product of the magnitude of the
force and the parallel distance through which the force acts. This concept
may be written as
                           work = force ⋅ parallel distance
                                          or
                                       W = Fd
Hence work requires not only a force but also motion or the movement of an
object through a distance. For example, when a person is mowing the lawn, only
the force component parallel to the lawn (the horizontal force) is used to
accomplish work. The vertical component of the force is doing no useful work,
since the force is tending to push the lawn mower into the ground. Since work is
the product of a force and a distance, the units of work are the units of force
times length. In the SI system the units of work are the newton – meter (N ⋅ m,
force ⋅ length). This unit is given the special name of joule.

2
       When a force is applied to change the velocity or direction of an object,
work is done against inertia. If we lift an object at a constant velocity there is no
net force on it because it is not accelerated. The weight of the object mg presses
down, and we push up with an equal and opposite force. The distance parallel
to the applied upward force is the height h that we lift the object. Thus the work
done against gravity is
                                 W = Fd, W = mgh
if we lift a 10 – kg mass a distance of 2 m, the work done is
                      W = mgh = 10kg ⋅ 9.8 m/s2 ⋅ 2m = 196 J
Friction always opposes motion. Hence to move something, we have to apply a
force. The work done by the applied force against friction is W = Fd.

3
      The rate of doing work is called power. Power is calculated by
dividing the work done by the time required to do it. Power can be written as
                                     work          W
                            power =       ,   P=
                                     time            t
Because work is the product of force and distance (W = Fd), power may be
written in terms of these quantities