Lesson 1:
Oscillatory Motion
Types of motion:
1. Transitional motion changes the initial position of the object. An example is
the motion of the car on the road.
2. Periodic motion is regularly repeated in equal periods of time.
An example is the motion of the pendulum & water waves.
Motion
Transitional Motion Periodic Motion
Oscillatory Motion Wave Motion
Ex. The motion of a car
Ex. Pendulum Ex. Water waves
Activity 1
Aim: To define oscillatory motion.
Tools: A pencil , a 30 cm long thread & a quarter pound coin.
Steps
1. Tie the coin with one end of the string & tie the other end of the thread around the
middle of the pencil to setup a simple pendulum.
2. Hold the pencil with your left hand, and pull the coin to the right. Then, let it go
according to the figure.
Observation
The oscillating body (The coin) moves forward & backward.
1
Conclusion
1. The coin oscillates around a point of rest (equilibrium) where the velocity
& kinetic energy of the coin is maximum just before it reaches the point of
rest (A).
2. As the coin moves away from the equilibrium point , its velocity & kinetic
energy decrease.
Oscillatory motion Circular motion
Tuning fork Swing Stretched string Rotary bee
The Microwave oven
Microwaves cause the vibration of water molecules inside food .
Water molecules collide with each other producing a lot of heat energy to coo
the food faster.
Pots made of pottery, ceramics or glasses are used in the microwave. Metal cooking
pots arent used inside a microwave because metal reflects the microwaves back
to the source.
Activity 2:
Aim: To represent oscillatory motion by a graph
Tools : Smooth paper tape winding around two rolls weight nailspringpencil.
Steps:
Use these tools to set the following apparatus.
2
Pull the spring downwards then let it go. While winding the paper roll regularly.
Observation:
The graph represents simple harmonic motion (oscillatory motion).
Properties of oscillatory motion
1. The Amplitude is the maximum displacement achieved by an oscillating body
away from its point of rest. It is measured in meter (m).
2- The complete oscillation is the movement made by the oscillating body when it
passes a single point in its path of motion, two consecutive times in the same
direction. The pendulum completes one oscillation when it covers the distance: B-
A-C-A-B
A complete oscillation consists of 4 amplitudes.(4
displacements)
The periodic time is the time needed by an oscillating body
to make a complete oscillation
The periodic time is denoted by (T).Periodic time is measured in seconds (s),
andis determined by the relation:
Periodic Time=
Time (s)
No. of complete oscillations
A solved example: What is the periodic time for the pendulum oscillating in the
figure?
3
Periodic time = x4 =2s
Frequency (F) is the number of complete oscillations in one
second. The unit of frequency is Hertz (Hz).
A simple pendulum makes 50 complete oscillations in 10 seconds ,its frequency is 5
Hertz. Frequency is the reciprocal of the periodic time.
No. of complete 1
vibrations
Frequenc Frequenc Periodic
y= Time (s) y= Time
Solved exercise
An oscillating body makes 120 complete oscillations in one minute. Calculate the
periodic time and frequency & periodic time.
F = 120 complete oscillations 60 s
= 2 Hz
T = 1/F
=1/2
= s
Multiples of the hertz
4
1 kilohertz = 1000 hertz
1 Megahertz = 1x 106 hertz
1 Gigahertz = 1 x 109
