lesson 6.1 physics f5

7/27/2019 Lesson 6.1 physics F5

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Introduction

A wave is produced by a source of vibration.Mechanical waves need a material medium or theirpropagation.Examples of mechanical waves include water

waves ,sound waves, shock waves as in earthquakesand waves in strings, springs and rods.Electromagnetic waves do not need a materialmedium for their propagation.Examples of electromagnetic waves are radio waves ,light waves , infra-red (IR) and ultra-violet ( UV).As a wave travels through a medium, there is no

transfer of matter but only transfer of energy from avibrating source.

Common terminology of vibrations

Vibration:

A periodic motion where the system performs arepeated to-and-fro motion about an equilibriumposition.

Equilibrium position:

The position of the object where is no resultant forceacts on the object.

One oscillation:

To-and-fro motion from the equilibrium position.

Amplitude :

The maximum displacement of the objects from theirequilibrium position.

Period, T

The time taken for one complete oscillation.

The S.I. unit is second (s)

t = time

n = number of oscillations

Frequency, f

The number of complete oscillations per second.The S.I. unit is Hertz (Hz)

Or

Graph displacement-time

Example 1

A pendulum makes 20 complete oscillations in 24.0s.Calculate

(a) the period (b) the frequency

Solution

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LESSON 1Vibrations and Waves

T = tn

f = 1T

T = nt


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Example 2

Based on he graph above, determine the

(a) amplitude (b) period(c) frequency

Solution

Natural Frequency:

The frequency of the free oscillation of a systemwithout any external forces are exerted to thesystem.

Factors affecting the natural frequency of thevibration systems.

Vibration system Factors affecting Formula

Oscillation of apendulum

Oscillation of aspring hang with amass

Oscillation of ajigsaw blade fixedwith plasticine ball

Transformation of energy in a vibration system.

What is meant by Resonance?

A phenomenon that occurs when the frequency offorced vibrations on an object matches the objectsnatural frequency and a dramatic increase in

amplitude results.

Experiment to show a phenomenon of resonance

The pendulum B ( driver pendulum) is pulled well

aside an released so that it oscillates in planeperpendicular.All the pendulums oscillate but with differentamplitudes.The pendulum D whose length equals that of thedriver (pendulum B) has the greatest amplitudebecause its natural frequency of oscillation is thesame as the frequency of the driving pendulum.

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1 The figure shows a bob of a simple pendulum isswinging between point A and point C.

One oscillation is counted when it oscillatingfrom

A A to BB A to CC A to C and back C to BD A to C and back C to A

2 The figure shows a bob of a simple pendulum isswinging between point P and point Q. It takes13.8 s to swing from P to Q and back to Ptwenty times

What is the period of the pendulum?

A 0.69 s B 1.38 sC 1.45 s D 27.6 sE 276 s

3 The figure shows a pendulum is swinging.

The time taken from the rest position M to itsmaximum displacement N is 2.0 s.

What is the period of the pendulum?

A 2.0 s B 4.0 sC 6.0 s D 8.0 sE 10.0 s

4 A child on a swing makes 20 complete swingsin 30 seconds.What is the frequency of the oscillation?

A 1 Hz B 1 Hz 3 2

C 2 Hz D 3 Hz 3 2

E 2 Hz

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Base on the figure above, which distancerepresents the amplitude?

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6 The diagram shows how displacement varieswith time.

Which of the following is true?

Amplitude/ m Period / s Frequency / Hz

A 0 .1 0.50 2

B 0.2 0.50 1C 0.1 0.25 4D 0.2 0.50 2

7 The period of oscillations of a simple pendulumincreases when the

________________________ increases.

A length of the pendulumB mass of the bob of the pendulumC acceleration due to gravityD amplitude of the oscillations of the

pendulum

8 The figure shows P,Q,R and S are fourpendulums of different lengths and masses ofthe bob.

Which pendulum swings the slowest?

A P B QC R D S

9 Pak Abu has a pendulum clock as shown in thediagram below.

Every day it was observed that the clock wasslow about 2 minutes.To correct the clock Pak Abu

A raised the position of the bobB lowest the position of the bobC increased the mass of the bobD decreased the mass of the bob

10 The figure shows a simple pendulum.

Which of the following is true?

A The amplitude of oscillation is 20 cmB The period of oscillation is the taken to

move from A to CC The frequency of oscillation increases

when the length of pendulum decreases

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Which of the following is not true when the bobis displaced and released ?

A At point B ,the bob has the highest velocityB The time taken to move from to A to B

same as from B to C.

C The gravitational potential energy at Asame as at C

D The gravitational potential energy at Asame as the kinetic energy at B

17 The figure shows five pendulums that are hungfrom th wire MN. The pendulum P is displacedand released.

Which of the pendulums will oscillate with thehighest amplitude?

A Pendulum W B Pendulum XC Pendulum Y D Pendulum Z

18 Which of the following physical quantities has amaximum value when resonance occurs?

A frequency B periodC amplitude D acceleration

19 A spring vibrates at a natural frequency 50 Hz.Resonance occurs when the frequency of forced

vibrations on the spring is

A 0 Hz B 25 HzC 50 Hz D 100 Hz

20 The figure shows the arrangement of theapparatus to show the phenomenon ofresonance but the resonance not occurs.

To improve the experiment ,

A the pendulums are oscillated with amaximum speed

B the iron rod is replaced by a string

C increase the mass of the bobs

21 Which of the following phenomenon is notcaused by resonance ?

A The violent vibrations of a washingmachine at some spin speeds

B The tuning circuits in TV and radio setswork at the frequency of the station youselect

C The wind instruments produce sound byforcing the air inside the tube

D The shock absorbers in cars causeoscillations to die down after a car hasgone over a bump in the road

22 A system is oscillating at its natural frequencywithout damping.What happens to the total energy of theoscillating system?

A It remains constantB It decreasesC It increases

23 As the amplitude decreases the______________ decrease.

A period B frequencyC energy D number of

oscillations

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24 The figure shows a mass hanging from a springis immersed in water.

Which of the following graph shows thevibrations the spring ?

25 Thefigure shows a paper cone filled with waterattached to spring. The paper cone has a hole atits end.

Which of the following graph shows the vibrationsthe spring while the water droplet drop from thehole?

26 Diagram shows a simple pendulum whichconsists of a bob of mass 40.0 g at the end of alight inelastic string of length 20.0 cm. Thependulum makes 20 complete oscillations in 5.0s.

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(a) By using the letters A,B and C in thediagram above state the

(i) equilibrium position

...........................................................

(ii) the meaning of period

..

(b) What is the frequency of the oscillations ofthe pendulum ?

(c) What is the frequency of the oscillations ofthe pendulum when a bob of mass 50.0 gis used?

(d) What happens to the frequency of theoscillations of the pendulum when the

length of the pendulum is 80.0 cm ?

.............................................................................

(e) After a while the pendulum stops tooscillate .(i) Explain why?

.............................................................................

.............................................................................

.............................................................................

(f) Plot a graph displacement time to showwhy the pendulum stops to oscillate.

(g) State the form of energy when thependulum(i) at B

............................................................(ii) between B and C

............................................................

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A trolley rests on a horizontal frictionless surfaceand is connected to two walls by two steelsprings. The trolley vibrates with its natural

frequency when it is pulled aside and released.

(a) What is meant by the natural frequency ?

...................................................................(b) What will happen to the nature frequency of

theboth springs when

(i) the steel springs are replaced by thecooper springs.

............................................................(ii) the number of trolley is increased.

............................................................(iii) the experiment is conducted at the

surface of the Moon.

............................................................

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29 Each figure below shows a boxer punches thepunch-ball with two different sizes. The bigpunch-ball vibrates more slowly than the smallpunch-ball.

Observe the vibrations of each of the punch-ball.Based on the observations:

(a) State one suitable inference that can be

made.(b) State one appropriate hypothesis for an

investigation.(c) With the use of apparatus such as spring,

slotted mass and other apparatus ,describe an experimental framework to testyour hypothesis.In your description , state clearly thefollowing:(i) Aim of the experiment(ii) Variables in the experiment(iii) List of apparatus and materials(iv) Arrangement of the apparatus

(v) The procedure of the experimentwhich include the method ofcontrolling the manipulated variableand the method of measuring theresponding variable.

(vi) Way you would tabulate the data

(vii) Way you would analysis the data

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lesson 6.1 physics f5

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