71. AV= constant, where A= area of cross-section and V= velocity of fluid. This equation is called _____.

1. Equation of discontinuity
2. Equation of continuity
3. Equation of sustenance
4. Equation of parallelism

Option “2” is correct.
According to the equation of continuity Av = constant. Where A =cross-sectional area and v=velocity with which the fluid flows. It means that if any liquid is flowing in streamline flow in a pipe of non-uniform cross-section area, then rate of flow of liquid across any cross-section remains constant.
72. A body in equilibrium _______.

1. Can move with constant acceleration
2. Is always at rest
3. Can move with constant velocity
4. Can move with variable acceleration

Option “3” is correct.
A body which is in equilibrium is either moving at constant velocity in a straight line, or it is not moving.
73. What is the viscosity of an ideal fluid?

1. Equal to its mass
2. Equal to its weight
3. Zero
4. One

Option “3” is correct.
The viscosity of an ideal fluid is Zero.
74. A parachute descends slowly whereas a stone dropped from the same height falls rapidly, because –

1. Stone is heavier than parachute
2. Special mechanisms are present in parachute
3. A parachute has a larger surface area and air resistance is more
4. None of the above

Option “3” is correct.
A parachute descends slowly because it has a larger surfaace area and air resistance is more.
75. Shaving mirror is –

1. Convex
2. Concave
3. Plane
4. Parabolic

Option “2” is correct.
Concave mirror is used as Shaving mirror.
76. Moving Electric Charge produces –

1. Magnetic field
2. Sound waves
3. Light rays
4. Heat waves

Option “1” is correct.
A charged particle moving without acceleration produces an electric as well as a magnetic field.
77. Kerosene rises in the wick of the stove is due to –

1. Osmosis
2. Diffusion
3. Surface tension
4. Biogen contraction

Option “3” is correct.
Kerosene oil rising in the wick of the stove is due to the surface tension of oil. The wick of the lamp has many holes which act as capillaries. So kerosene keeps on rising in the capillaries.
78. The reason of mirage is-

1. Interference of light
2. Diffraction of light
3. Polarization of light
4. Total internal reflection of light

Option “4” is correct.
The mirage is caused by the Total internal reflection of light at layers of air of different densities.
79. Red light is used in traffic signals because-

1. It has the longest wavelength
2. It is beautiful
3. It is visible to people even with bad eyesight
4. None of the above reasons

Option “1” is correct.
The wavelength of red colour is highest while of violet colour is lowest. Dispersal is inverse to wavelength so it is seen more clear. That’s why the red colour is used in traffic.
80. If Va, Vw and Vs respectively are the speed of sound in air, water and steel, then-

1. Va < Vw < Vs
2. Vs < Vw < Va
3. Vw < Vs < Va
4. Vs < Va < Vw

Option “1” is correct.
As we know that the speed of sound is different for different types of medium. In general, sound travels faster in liquids than gases and faster in solids than in liquids.
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