CBSE ADDA

Q.1. Can any object have mechanical energy even if its momentum is zero? Explain. Yes, mechanical energy comprises of both potential energy and kinetic energy. Zero momentum means that velocity is zero. Hence, there it no kinetic energy but the object may possess potential energy. Q.2. The potential energy of a body is 39600J. How high is the body if its mass is 20kg? The potential energy of a body = mgh h = PE/mg = 39600j/20kg x9.8m/s 2 =198m Q.3. A force of 20 N displaces a body through a distance of 1 m at an angle of 60° from its own direction. Calculate the amount of work done. Here, force F = 20 N, displacement, s = 1 m. Angle between force and displacement 60°. Work done,W =Fscos0 =20 X 1 X cos60°=20X 1 X 1/2 = 10J. A man of 50 kg jumps up to a height of 1.2 m. What is his potential energy at the highest point? The potential energy of man = mgh = = 50 + 10 X 1.2 J = 600 J Q.4. How much work is done by a force of 10 N in moving an object through a distance o

   CBSE Class 9 - Science - Chapter 10: FLOTATION   Thrust and Pressure, Pressure in Fluids, Buoyancy Fluid: Fluid is a substance that flows.  For example, liquids and gases. They take the shape of container in which they are stored. The study of fluids at rest is known as hydrostatic or fluid static’s.   The study of fluids in motion is termed as hydrodynamics. THRUST:   The normal force acting on a unit surface area of the fluid is called thrust Density: The mass per unit volume of a substance is called density. D = mass /volume  Þ Mass of substance = d x v ------------------------ (i)  SI unit is  kg/m 3 The density of water (1 g cm −3 ) PRESSURE IN A FLUID The pressure (P) is defined at the magnitude of the normal force acting on a unit surface area of the fluid. P = F/A .  Hence pressure is in directly proportional to Area of contact. For example a nail or a knife have pointed and sharp edge to reduce area of contact to ap

Science CBSE Physics Flotation Term-II Class IX Force :  Pressure :Thrust :   Atmospheric pressure: Buoyant force .                                 Atmospheric pressure                                                          The pressure at any place due to the atmosphere is called atmospheric pressure. Its value varies from place to place and also with the time.  Atmospheric pressure at the earth’s surface near the sea level is around 1.01x105 Pa. This value is known as 1atmosphere of pressure (1atmosphere = 760mm of Hg). Pressure in fluids –  All liquids and gases are fluids. A solid exerts pressure on a surface due to its weight Similarly, fluids have weight, and they also exert pressure on the base and walls of the container in which they are enclosed.  Pressure exerted in any confined mass of fluid is transmitted undiminished in all directions. The pressure in a liquid is the same at all points at the same horizontal level. As we go deeper in the liq

Q. a cube of mass 1kg with each side of 1cm is lying on the table. find the pressure exerted by the block on the table. take g=10 m/s 2 Ans: Pressure is given as force/area  so, Force, F = mg = 1000 X 1000 gm.cm/s 2 and   area, A = 1x1 cm 2  = 1 cm 2 T hus, the pressure exerted would be P = (1000 X 1000) / 1 or P = 1 x 10  6  pa Q. The mass of a solid iron cube of side 3cm is to be determined usig a spring balance. If the of iron is approximately 8.5 g/cm 3 , the best suited spring balance for determining weight of the solid would be of  1. range 0-250gwt ; least count 1gwt              2. range 0-250gwt ; least count 5gwt 3. range 0-1000gwt ; least count 5gwt            4.  range 0-1000gwt ; least count 10gwt Ans: Edge=3 cm ,  Density=8.5 g/cm 3 Mass=  density x volume = 8.5 x(3x3)=229.5gwt Therefore second spring balance of range 0-250 gwt with least count 5gwt will be suitable. Q. The density of turpentine oil is 840 Kg/ m3. What will be its relati

Question:  A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 ms–1 .  Ans. (I) Wavelength of sound waves corresponding to 20 Hz, u = u = 344 ms–1, v = 20 Hz λ 1  =  v/n = 334/20  = 17.2 m  λ 2 =v/n = 334/20,000  = 0.0172 m Question:   The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute? Ans. Frequency of sound = 100 Hz = 100s –1   Number of vibrations in 1 s = 100 Number of vibrations in 1 minute, i.e. 60 seconds =100x60 =6000vibrations Question:   Explain how bats use ultrasound to reach a prey. Ans. Bats produce high pitched ultrasonic waves that are not heard by humans. These ultrasonic waves strike the prey like an insect and it is reflected back in the form of an echo, which is heard by the bat. This enables the bat to hover on the insect and catch it. Question:   A sonar device on a subma

CBSE PHYSICS: IX Physics Sound NCERT Solutions Explain how sound is produced by your school bell. When the bell continues to move forward and backward, it creates a series of compressions and rarefactions. In this  way a bell  produce the sound . Why sound waves are called mechanical waves? Sound waves need the material medium to vibrate and propagate. Hence, these waves are known as mechanical waves. Sound waves propagate through a medium because of the interaction of the particles present in that medium. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend? Sound needs a medium to propagate. Since there is no material medium on the moon due to absence of atmosphere, you cannot hear any sound on the moon. Page 166 : Which wave property determines (a) loudness, (b) pitch? (a) Amplitude of sound wave determines the loudness of a sound. The amplitude of a sound directly proportional to amplitude of wave. If the amplitude of a sou