Unit 1: waves and optics
Short Question answer
Q.1 Differentiate between free and damped oscillation.
Ans: The differences between free and damped oscillation are given below.
|Free oscillation||Damped oscillation|
|1. The oscillation in the absence of external forces is called free oscillation||2. The oscillation in which there is gradual decrement of amplitude due to the resistive forces acting on the oscillating body or system is called damped oscillation.|
|3. The period and amplitude don’t change in time. EG. Oscillation a pendulum, a spring-mass system in a vacuum etc.||4. The period and amplitude change with time. EG. Oscillation of a pendulum in liquid, tuning forks etc.
Q.2 Differentiating between free vibration and forced vibration.
Ans. The difference between free and forced vibration are given below
|Free vibration||Forced vibration|
|1. In free vibration, the body vibrates with its natural frequency.||2. In forced vibration, the body vibrates with the frequency of the source.|
|3. A vibrating body vibrates with the help of internal elastic forces.||4. A vibrating body vibrates with the help of external periodic force.|
|5. It is self-sustained oscillation||6. It is externally sustained oscillation|
Q.3 Explain why soldiers are ordered to break steps while crossing a bridge.
Ans. Soldiers are ordered to break steps while crossing the bridge. If the machine soldiers do not break the steps, the frequency of marching may exactly coincide with the natural frequency of the bridge. As a result, the resonance occurs in the bridge and it vibrates with maximum amplitude. If the displacement of the bridge exceeds the elastic limit of the bridge it may collapse. So soldiers are ordered to break steps while crossing the bridge.
Qn4. What is resonance and cause of resonance?
Ans. When a system is made to execute forced vibration by applying a periodic force of the same frequency as the natural frequency of the system, it is said to execute resonance. If the external periodic force is always in phase with the natural frequency of the system, amplitude progressively builds up and it soon becomes very large. When the frequency of the source is equal to the natural frequency of the body, the body begins to vibrate with maximum amplitude and this kind of vibration of the body known as resonant vibration.
Q5. Ocean waves hitting a beach are always found to be nearly normal to the shore, why?
Ans. The waves produced on the surface of the water are transverse waves. When such waves are produced in water, they spread out and when they reach the breach-shore, they acquire. Such a large radius of curvature that they may be assumed as plane waves, Hence, ocean waves hit the beach nearly normal to the shore.
Q6. How are stationary waves formed?
Ans. When two progressive waves of the same wavelength and amplitude travel with the same speed through a medium in opposite directions, they superimpose upon each other and thus a wave is formed, points of maximum and minimum, ( Zero) amplitudes result alternatively in space, Which is shown in the figure.
Qn7. An explosion occurs inside a lake. What type of waves are produced in a water?
Ans. When an explosion occurs inside a lake, longitudinal waves are produced inside the water. it is because a liquid can have hulk modulus of elasticity but it doesn’t have modulus of rigidity which is responsible for the propagation of transverse waves through the medium.
Qn8. Bells are made of metal and not of wood, Why?
Ans. Wood has much higher damping than a metal i.e the amplitude of vibration goes on decreasing in damped oscillation and it is much higher in the case of wood than metal. The vibrating material for the source of sound must have low damping. Because of this, bells are made of materials and not of wood.
Qn9. Do sound waves need a medium to travel from one point to other point in space? What properties of the medium are relevant?
Ans. Yes, sound wave needs a medium to travel from one point to other point in space. this is because it is longitudinal wave as well as a mechanical wave. the Relevant properties of the medium are as follows:
- The medium should possess the property of elasticity.
- The medium should possess the property of inertia.
- The medium should have minimum friction.
Qn10. Can liquid and gas propagate of the transverse waves through a medium, modulus of rigidity is responsible because the tangential force stress acts on the particle of the medium when a transverse wave travels through it. since the liquid and gas possess very poor modulus of rigidity( modulus of rigidity is practically zero), transverse waves can not be propagated through them.
Qn11. Is velocity of sound more in damp air or in dry air ? Explain
Ans. Velocity of sound is more in damp air. We know that the density of damp air is less than dry air due to the presence of humidity. Also we have the relation than dry air due to the presence of humidity. Also we have the relation
Qn12 Sound can be heard over longer distance on a rainy day. Why?
Ans. On a rainy day, the air contains a large amount of water Vapour . The Density of moist air is less than that of air. Also,we have the relation v is inversely propotional to density. so it is clear that sound travels faster in moist air and hence it can be heard over longer distance on a rainy day.
Qn13. Do sound waves undergo reflection,refraction and polarization phenomena? Explain
Ans. Being a wave, the sound wave undergo reflection and refraction. The phenomena are exhibited by both transverse wave and longitudinal wave, but, polarization phenomenon is exhibited by transverse wave only. Since solids possess both bulk modulus and shear modulus of elasticity, both transverse and longitudinal waves can be produced in solids but only longitudinal waves in gases.
Qn14. Both transverse and longitidinal waves can be produced in solids but only longitidinal waves can be produced in solids but only longitudinal waves in gases.
Qn15. Why is end correction necessary for an organ pipe?
Ans.When an air in an organ pipe vibrates, the reflection of the sound waves takes place a little above the open end of the pipe. so the length of the air column which has been set into oscillation is not exactly the distance between the two ends of the tube but a little bit more than this. This factor is to considered in order to get the accurate value for frequency of vibration, so the end correction is necessary for an organ pipe.
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