Page 326 - NS-2 Textbook

PHYSICAL SCIENCE 321

are collectively called Einstein's theory of relalivity, which of its energy back into work, thus losing energy. There is,
he formulated early in the last cenhlTy. therefore l an equivalence betv-.reen ""ork and energy, in-
dicated by the fact that they have the same lmitS: foot-
pounds in the English system and joules in the metric
FORCES system.
TIle rate at which work is done or energy is gained or
Let us take another look at Newton's second law of mo-
expended is power. In the English system the unit of
tion. How do ,,;e define acceleration? Acceleration is the
pm.ver is the horsepower, and in the metric system it is
change in velocity per unit of time, normally one second.
the 'vatt. TIlere are 746 watts in one horsepuwer. For ex-
Velocity is the rate of motion in a given direction. In the
ample, if 10 newtons of force acted over a distance of 10
example above, the velocity was expressed in terms of
meters for 5 seconds, the power generated would be (10
miles per hour. Other widely used units are feet per sec-
x 10)/5, or 20 watts (equivalent to about .013 horse-
ond, and in the metric systen1, meters per second or kilo-
power). A larger unit of power called the kilowatt (KW),
meters per hour. Force can be defined as power or energy
equivalent to 1,000 watts, is commonly used, especially
exerted against a body in a given direction. Its units are
in cOlmection with electrical power consumption. If the
pOlmds in the English system and newtons (N) in the
United States ever fully adopts the metric system, all
metric system. Mass is the quantity of material contained
products such as automobile engines will be rated in
in a body. In the metric system the unit of mass is the
kilowatts instead of horsepower. Most American-built
kilogram. In the English system the pound is often erro-
marine engines used on boats already use kilowatts as
neously used to indicate mass, although it is really a
their standard power units.
force unit.
The weight of a body is an expression of the amount
of the force of gravity on it at a given location. Thus, the THE PHYSICS OF FLIGHT
weight can change, depending on location, while mass
does not. For example, the force of gravity on the Moon's The path of a body in flight is determined by Newton's
surface is about one-sixth that of Earth. Thus, a person laws of motion, according to the forces acting upon it.
weighing 120 pounds on Earth would weigh only 20 Some of these forces are naturat and others are man-
pounds on the Moon. While on the way to the Moon, in made. Various combinations of these forces produce dif-
space where gravity is minimal, the person would have ferent effects on the flight path. Aerodynamics is the sci-
almost no ,veight. The person's Inass, howevel~ would be ence that deals with the motion of bodies moving
the same at all three locations. through air and other gases. Missiles and aircraft use
One kind of force that must be dealt with for all mov- aerodynamic forces to maintain their flight path. The sur-
ing earthbOlmd objects is /riclion. Friction is caused by face of a body in flight is called its ailfoil. The aerody-
contact between the moving object and other substances namic forces acting on a moving airfoil are thrust, drag,
around it. The amolUlt of the friction is dependent on the gravity (weight), and lift. Other factors that can affect a
nature of the materials in contact, the force behveen body in flight are the angle of attack between the airfoil
them, and sometimes their velocity relative to each other. and the airstream, and in the case of a body flying in a
Solids moving against each other generate can loci friction. clU'ved path, centripetal force.
Bodies moving through fluids such as water or air (con-
sidered a fluid in these cases) generate fluid /riction,
which increases with the speed of travel through the lift
fluid. This type of friction is often called dmg. Friction al-
ways acts in a direction opposite to the direction of mo- V
tion. The energy generated by friction is usually dissi-
pated in the form of heat. drag
thrust == ->
<=

WORK, ENERGY, AND POWER
~
When a force acts through a distance, work is said to have
been done. In the English system the units of work are
foot-pounds, and in the metric systen1, ne'wton-meters. gravity
One n€vvtOI1-rneter is called a joule. Doing \vork on a
body increases its level of energy. It may gain height, ve- The aerodynamic forces acting on a moving airfoil such as a missile
are thrust. drag, gravity, and lift. The combination of these forces
locity, or temperature, or sometimes all tlu'ee. Under the produces a resultant force that will determine where the missile
proper conditions a body may be able to transform some goes.