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Q   1. An airplane produces a sound wave with frequency of 5 KHz and wavelength 30 m. In how much time would the sound wave cover the distance of 4 Km? Solution Frequency, ν = 5 KHz = 5000 Hz Wavelength, λ = 30 cm = 0.3 m Speed, v = ν λ = 0 .3 m x 5000 Hz = 1500 m/s Time = distance/ speed = 4000/1500 = 2.6 seconds Q   2. How is sound produced in the following things? The buzzing of mosquito The sound of drums  The sound of guitar  The sound of trumpet Solution Vibration of mosquito wings produces buzzing sound. The sound of drum is caused by the vibration of its membrane when struck by drumsticks. The vibration of strings produces various sounds in guitar. The vibration of air inside tube of the trumpet produces sound. Q   3. The frequency of a sound source is 600 Hz. The speed of sound in a medium is 330 m/s. What will be the linear distance between an adjacent crest and trough. Solution v = ν λ 330 = 600 x λ Hence, λ = 330/600 = 11/20 m The distance between
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Q   1. What kind of energy is possessed by the following? (a) A stone kept on the roof top. (b) Water stored in the reservoir of a dam. (c) A stretched rubber band. Solution (a) Potential energy (b) Potential energy (c) Potential energy Q   2. Explain how wind mills are used to generate electricity? Solution In a wind mill, kinetic energy of wind is used to move the blades of fan which in turn move the coil of generator thereby converting kinetic energy to electrical energy. Thus, in the above example there is only transformation of energy from one form to another such that the total energy of the system remains conserved. Q   3. Explain the principle behind the wind up toys. Solution Wind up toys have a spring inside it which gets wound up with the help of winding keys. So when we wind-up the spring of a toy car by using a winding key, then some work is done by us. As a result, the spring gets coiled up more tightly. The work done in winding the spring g
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Q     1. What is meant by free fall? Two bodies, one of mass 1g and other of mass 1 kg are dropped from the same height in vacuum. Compare the two time intervals in which the two bodies will hit the ground. Solution Whenever objects fall towards the earth under the influence of gravitational force alone, it is called free fall. Both bodies will hit the ground at the same time as acceleration due to gravity is independent of mass of the falling object. Q     2. What is the force between the Earth and a body called? Solution The force between the Earth and the body is known as the force of gravity or gravitational force. It is also called the weight of the body. Q     3. State Archimedes' principle. Give two applications based on the principle. Solution Archimedes' principle: When an object is wholly (or partially) immersed in a liquid, it experiences a buoyant force (or upthrust) which is equal to the weight of liquid displaced by the object. Buoyant for
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