Title EASA Part 66 - Module 2 - Physics 2.4 MB 98
##### Document Text Contents
Page 49

Issue 1 – 20 August 2001 Page 4-7

JAR 66 CATEGORY B1

MODULE 2

PHYSICS

engineering

uk

4.11 CHANGES IN MOMENTUM

What causes momentum to change? If the initial and final velocities of a mass
are u and v,

then change of momentum = mv - mu

= m (v - u).

Does the change of momentum happen slowly or quickly?

The rate of change of momentum = m
(v - u)

t

Inspection of this shows that force F (m.a) = m
(v - u)

t
, so, a force causes a

change in momentum.

The rate of change of momentum is proportional to the magnitude of the force
causing it.

Suppose a mass A overtakes a mass B, as shown below in illustration (a). On
impact, (b), the mass B will be accelerated by an impulsive force delivered by A,
whilst the mass A will be decelerated by an impulsive force delivered by B.

Fig 4.1 Conservation of Momentum

In accordance with Newton's Third Law, these impulsive forces, F , will be equal
and opposite and must, of course, act for the same small period of time. After the
impact, A and B will have some new velocities, v

a
and v

b
. By calculation, it can

be proven that the momentum before the impact equals the momentum after the
impact.

Page 50

Issue 1 – 20 August 2001 Page 4-8

JAR 66 CATEGORY B1

MODULE 2

PHYSICS

engineering

uk

4.12 GYROSCOPES

This topic covers both gyroscopes and the allied subject, that of balancing of
rotating masses. Both of these topics have direct application to aircraft
operations.

Gyroscopes are rotating masses (usually cylindrical in form) which are
deliberately employed because of the particular properties which they
demonstrate. (note, however, that any rotating mass may demonstrate these
properties, albeit unintentionally).

Basic concepts can be gained by reference to a hand-held bicycle wheel.

Imagine the wheel to be stationary; it is easy to tilt the axle one way or another.

There are two reasons why we must understand the basic principles of
gyroscopes.

Gyroscopes are used in several flight instruments, which are vital to the safety of

Secondly, there are many different components that will not operate correctly if
they are not perfectly balanced. For example, wheels, engines, propellers,
electric motors and many other components must run with perfect smoothness
and without vibration.

The gyroscope, (gyro) is a rotor that has freedom of motion in one or more planes
at right angles to the plane of rotation. With the rotor spinning, the gyro will
possess two fundamental properties:

Gyroscopic rigidity or inertia

Gyroscopic precession

The figure shows a gyro with freedom of
movement about two axes, BB and CC,
which are at 90 degrees to the axis of
rotation AA.

Fig 4.2 Gyroscope

Page 97

Issue 1 – 20 August 2001 Page 8-5

JAR 66 CATEGORY B1

MODULE 2

PHYSICS

engineering

uk

8.5.2 FREQUENCY CALCULATION

Using the same value as for the wavelength calculation, the frequency of the
approaching and receding source can be calculated using the formulae:

source

source

f
vv

v
obsevedfrequency 

 (for an approaching source)

source

source

f
vv

v
obsevedfrequency 

 (for a receding source)

The frequency of an approaching source will be higher and so the pitch of the
sound will be higher.