KPK 11th Class Physics Chapter 8 Waves Short Questions Answers

Progressive waves:

The waves which are produced in a medium and they travels from one point to the other are known as progressive waves or travelling waves.

For example, when a pebble is dropped onto a pond of water, then waves are produced due to disturbance. These waves travels from the region of disturbance towards the edge of the pond. These waves transfer energy from one place to another and are called progressive waves.

(ii) stationary (or) standing waves:

These waves are formed when two exactly similar waves are travelling in opposite direction and superpose each other. These waves consists of nodes and antinodes which are produced one after the other in a certain order. At nodes, the amplitude of vibration is 3ero and thus the stationary waves cannot transfer energy from one place to another.

There is no transfer of energy through a medium when a stationary wave is produced in it.

We know that a stationary wave consists of nodes at which the amplitude of vibration is zero i.e. theses points are at rest and their displacements from mean position are zero. Thus the energy can not transfer through a medium when a stationary wave is produced in it. Only inter conversation of K.E and P.E occure at anti nodes.

When two similar wave pulses are traveling in opposite direction and they superpose each other, then standing waves are produced. The standing wave consists of nodes and anti node.  At node the amplitude of vibration is zero. i.e. the particles of the medium remains at rest at the nodes. So there is no transfer of energy through the medium due to nodes. However only the inter conversion of K.E and P.E takes place. At anti node the energy is potential while at means position, the energy is kinetic.

Condition for constructive interference:

The constructive interference takes place at those points where the crest of one wave superpose the crest of the other wave. (OR) when the trough of one wave superpose the trough of the other wave. This is possible only, then the path difference b/w the superposing waves at a point must be equal to an integral multiple of their wave length. i.e.

Path difference = 0,λ,2λ,3λ, ……………..

Or 

P.d = mλ 

Where m=0, 1, 2, 3, ……………… and λ = wavelength of the waves. 

2) Condition for Destructive Interference:

 The destructive interference takes place at those points where the crest of one wave falls upon the trough of another wave. This is possible only, when the path difference b/w the superposing waves at a point is equal to an odd integrat multiple of ‘λ/2’. i.e. path difference = λ/2, 32 , 52 , 72 …………………………

Or P.d= (m+12) λ

Where m = 0, 1, 2, 3, …………….. and λ = wavelength of the waves.

We can locate the position of nodes in a vibrating string easily. It is because, at nodes point, the tension is maximum and the amplitude of vibration is zero. The nodes are at rest and the string appears to us in the form of loops with nodes which are located at the ends of each loop, as shown in the figure.

Similarly, we can located the position of anti-node in a vibrating string easily. It is because, at anti nodes point, the tension is minimum and amplitude of vibration is maximum. The string appears to us in the form of loops. At mid point of each loop, the amplitude is maximum and thus the anti nodes are located at the mid point of each loop above or below the mean position, as shown by ‘A1’ and ‘A2’  in the given figure.

Yes it is possible for an object which is vibrating transversely to produce sound wave. All the stringed and percussion instrument, when played, produces transverse wave. And the energy from these vibrating instruments travels through air in the form of sound waves which are longitudinal waves in nature.

We know that the speed of sound waves in a medium of elasticity ‘E” and density ‘r’ is given by,

V = √E/r ……………….(1)

In case of solids ‘E’ is taken as its young’s modulus while in case of gases, ‘E’ is taken as its bulk modulus. 

It is true that density ‘r’ of solids is greater than that of a gas, but at the same time, the young modulus of a solid is far greater than the bulk modulus of gas. So the term, ‘E/r’ becomes greater for solids than in gases i.e. sound will travel faster in solids than gases.

We know that the speed of sound in a gas is given by,

 V = √E/r …………………………(1)

Eq (1) shows that the speed of sound is inversely proportional to the square root of the density. i.e.  V∝√1/r ……………….(2)

Now we know that when temperature increases, the volume of the gas increase and its density decreases. So according to relation (2), the speed of sound will increases. And when the temperature of gas decreases, its volume decrease and density increases. So according to relation (2), the speed of sound decreases. That is why the speed of a sound wave in a gas changes with temperature.

It is not possible for two astronauts to talk directly to one another even if they remove their helmets. It is because, the sound are longitudinal mechanical wave which need a material medium for their propagation. As there is no material medium b/w the two astronauts, so they can not talk directly to one another.

length of tube = L = 15 cm = 0.15m let speed of sound = v = 340 m/sec, here the test tube behaves like a closed pipe for which the fundamental frequency is given by, 

 ƒ₁=V/2L 

• F1=340/(4×0.15)        = 567 Hz
• F1=567 Hz

 Now the 2nd harmonic is given by, 

ƒ₂=3ƒ₁

• ƒ₂=3×567  =>   ƒ₂=1701 Hz

Similarly 

  ƒ₃=5ƒ₁=5 ×567  => ƒ₃=2835 Hz

ƒ₄= 7ƒ1=7×567   =>   ƒ₄=3969 Hz

And ƒn=2n-1ƒ1=2n-1567 Hz