Page 328 - NS-2 Textbook

PHYSICAL SCIENCE 323

cause the opposing drag force increases steadily. When rises to 838 mph, reverses, and falls to 693 mph at the top
the nose rises off the nmway, the angle of attack also in- of the stratosphere. III
creases, adding still more upward force. Soon the total
lift force overcomes the downward weight produced by
gravity, and the plane lifts off. Shortly after takeoff the
plane's forward acceleration may fall off to zero as in-
creasing drag matches thrust. The plane continues at a CRITICAL THINKING
steady speed by Newton's first law, until it decreases
1. In the first part of this chapter the motion of a car was
drag by raising the landing gear plus its wing flaps. Now
used to illustrate how Newton's Laws of Motion op-
the plane can accelerate once again as it climbs to cruis-
erate. Do the same thing for a guided missile being
ing altitude and attains cruising speed.
fired from the grOlmd at an airborne target.
Should the propulsive force be decreased for any rea-
2. Discuss some of the ways that drag can be reduced for
son, such as to slow down for landing, the force of drag
a body moving through air.
will exceed the tJuust. The plane will slow down until
the forces are again in balance.
In aerodynmnics, acceleration is often measured in
terms of the standa.rd acceleration of gravity, abbreviated
by the letter "g". A freely falling frictionless body is at- Study Guide Questions
tracted to the Earth by a force equal to its weight, with
1. A. According to ancient Greek philosophers, what
the result that it accelerates at a constant rate of approxi-
keeps an object in motion?
mately 32 feet (9.8 meters) per second each second (writ- B. According to Isaac Newton, what keeps an object
2
2
ten as 32 ft/sec [9.8 m/sec ]). Its acceleration while in
in motion?
free fall is thus said to be 1 g. Bodies making rapid turns
2. State Newton's three laws of motion.
will experience other accelerations, and therefore forces,
3. A. How fast is light speed?
due to centripetal force (which can be many times that of
B. What theory describes the motion of bodies with
gravity), often expressed as multiples of g's. As an exam-
speeds nea.r light speed?
ple, if a jet fighter making a tight turn experiences a side-
4. What are the unlts of force
ways force of 5 g's, there is a side,vays force on it equal
A. In the English system of unlts?
to 5 times its weight.
B. In the meh·ic system?
A missile or aircraft is designed to withstand only a
5. A. Is the weight or the mass of an object subject to
certain number of g's, and if that is exceeded, damage to change?
its structure, its payload, or its instruments ll1ay occur. In B. How is the weight of an object determined?
the case of manned aircraft, its pilot may black out (be- 6. What kind of force must be reckoned with for all
come unconscious). earthbound objects in motion?
7. What are the four aerodynamic forces on bodies in
flight?
MACH NUMBERS
8. According to Bernoulli's theorem, how is lift devel-
Speeds of missiles and high-performance aircraft are oped by a curved wing?
9. What is meant by g-forces?
often expressed in terms of Mach numbers, rather than in
10. How are Mach numbers derived?
miles per haUl" or knots. The Mach number is the ratio of
the body's speed to the speed of sound in that particular
part of the atmosphere. For example, if an aircraft is fly- Vocabulary
ing at a speed equal to one-half the local speed of sound,
Newton's lmvs of motion centripetal force
it is said to be flying at Mach 0.5. If it moves at twice the
Ne,vtonian motion vector quantity
local speed of sOlmd, its speed is Mach 2.
acceleration free fan
The speed expressed by the Mach munber is not a
net force velocity
fixed quantity, because the speed of sound in air varies
resultant force Mach numbers
directly with the square root of air temperature. Because mehic system g force
of air temperature changes in the atmosphere with in- mass friction
creasing altitude, the speed of sound decreases from 770 drag aerodynamics
mph (344 meters per second) at sea level on an average lift thrust
day when the air is 68 degrees F (20 degrees C), to 661 angle of attack work
mph at the top of the troposphere. The speed of sound re- power energy
mains constant between 55,000 feet and 105,000 feet, then joules watts