January 16, 2013

Work and Energy:Frequently Asked Questions (F.A.Qs)

38. What is work?

Work is the overcoming of resistance. The term implies
a change of position and is independent of the time

39. What may we take as a type of work?

The lifting of a bodv against the force of gravity, i, e,.
against the  pull  of the earth.

4a How may we measure such work ?

By considering both the weight of the body and the height
which it is raised.

41. How are work-units classified?

As gravitation units and absolute units, with two in each

42. What are the gravitation units of work ?

The work expended in lifting one pound one foot against
the force of gravity is called a loot-pound. The work
expended in lifting one kilogram one meter against the
same force is called a kilogrammeter.

43. What are the absolute units of work ?

The work done by one poundal in producing a displace-
ment of one foot is called a foot-poundal. The work
done by one dyne in producing a displacement of one
centimeter is called an erg.

44. What is the numerical relation between these units ?

A foot-poundal is equivalent to 421,402 ergs ; a foot-pound
is equivalent to 32.16 times that many ergs. Since a
force of one kilogram is equivalent to 9^,000 d3mes,
and a meter to 100 centimeters, a kilogrammeter is
equivalent to 98,000,000 ergs. In any case, the work
done is numerically represented by the product of the
number of utiits of force into the number of units of
displacement Multiply the number of weight-units by
the number of height units.

45. A laborer with his hod of bricks weighs 300 pounds. How
much work does he perform in carrying his load to the top
of a building ^o feet high?
300X50=15,000, the number of foot-pounds. ,

46. What is CLctivity ?
The activity of an agent is the rate at which it can do

47. Who/ is a horse-power?

It is the most common unit of activity, and represents the
ability to do 33,000 foot-pounds in a minute, or 550 foot-
pounds in a second.

48. How is horse- power computed ?

Multiply the number of pounds raised by the "number of
vertical feet through which it is raisea, and divide the
product by 33,000 times the number of minutes (or by
550 times the number of seconds) required to do the

50. What is energy ?

The power of doing work.

51. Name the two great classes of energy.
Kinetic and potential.

52. What is kinetic energy ?

Euer^ of motion ; i, e.^ the power of doing work that a
body has by virtue of its motion.

53. What is potential energy ?

Energy of position ; 1. e.g the power of doing work that a
body has by virtue of its position.

54. Illustrate kinetic energy.

The energy of running water, a falling pile-driver, a re-
volving fly-wheel.

55. Illustrate potential energy, ,.

The energy of a head of water, a coiled spring, a drawn

56. How are these varieties of energy related ?

They are mutually convertible. Either may be converted
into an equivalent amount of the other.

57. Illustrate this statement.

It requires a certain amount of work to wind up a clock.
When the clock is wound up, it has a store of potential
energy. When the pendulum is Fet in vibration, the
energy stored in the coiled spring or raised weight will
perform an amount of work equal to that performed in
winding up the clock.

58. Give a further illustration,

A ball is thrown vertically upwards with a certain velocity.
Its kinetic energy lifts it to a certain height and, when
the ball is at that point, its velocity is zero. It, there-
fore, has no kinetic energy. All that it had at the start
has disappeared, having been converted into an equiva-
lent amount of potential energy. For, at this moment,
the ball has a position of advantage from which it de-
rives a power of doing work; it may be used as a
weight to run machinery or in other ways. This energy
of position maj be reconverted into its original form as
energy of motion, for, if the body is permitted to fall,
it will regain the velocity with which it started. At
the middle point, going up or down, the energy of the
t>a^l is half kinetic and half potential, and at every
point of the path, the sum of the two energies is a con-
stant quantity.

59. Hoiv is kinetic energy tneasured in gravitation units?

We have the formula K. E. =1/2 mv2, in which w represents
the weight and z/ the velocity of the moving body, and ^
the acceleration due to gravity {i.e.y 32.16 feet or 9.8
meters). Substituting in this formula values measured
in feet and pouuds, we have the value of the kinetic
energy in foot-pounds; using meters and kilograms,
we have the value in kilogrammeters.

6O How  kinetic energy mensured in absolute units ?

We have the formula, K. E.=>1/2 mv2. Measuring mass in
pounds, and velocity in feet per second, this g^ves the
energy in foot-poundals. Measuring mass in grams,
and velocity in centimeters per second, gives the energy
in ergs.

61. What is meant by the conservation of energy?

When the universe was hurled into space bjr the hand of
the Creator, it was endovred with a certain amount of
energy. Like matter, energy may appear in many
difiPerent shapes. The sum total of all these different
forms of energy in the universe taken as a whole is a
constant quantity, for energy, like matter, is indestruct-


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