Mechanics

81.A stone tied to a string is rapidly rotated in a circle. When the string suddenly breaks while twisting, then –
 
A. The stone flies off tangential to the circle of rotation
 
B. The stone moves radially inward
 
C. The stone moves radially outward
 
D. The motion of the stone depends upon its velocity
 
Option”A” is correct.
When a stone is going around a circular path, the instantaneous velocity of stone is acting as tangent to the circle. When the string breaks, the centripetal force stops to act. Due to inertia, the stone continues to move along the tangent to circular path. So, the stone flies off tangentially to the circular path
82.Egg sinks in soft water but floats in concentrated solution of salt because
 
A. Egg absorbs salt from solution and expands
 
B. Albumin dissolves in salt solution and becomes lighter
 
C. The density of the salt solution exceeds the density of the egg
 
D. Water has a higher surface tension
 
Option”C” is correct.
As the density of strong salt solution is denser than fresh water, it exerts more upthrust on the egg that balance the weight of the egg hence the egg sinks in fresh water but floats on a strong salt solution.
83.What should a person on a freely rotating turn table do to decrease his (angular) speed ?
 
A. Put your hands together
 
B. Get your hands up
 
C. Spread his hands outwards
 
D. Sit down with raised hands
 
Option”C” is correct.
When a person on a turn table will raise his hands, there will be increase in the momentum of inertia. Hence, the angular velocity of the turntable will decrease.
84.Why does soap help in better cleaning of clothes?
 
A. It lowers the surface tension of the solution.
 
B. It absorbs dirt
 
C. It gives the strength of the solution
 
D. Soap acts as a catalyst
 
Option”A” is correct.
Soaps decreases the surface tension of water so as to increase wetting ability of water which helps in cleaning clothes.
85.Newton meter is the unit
 
A. Of acceleration
 
B. Of force
 
C. Of power
 
D. Of torque
 
Option”D” is correct.

  1. Torque is also called the moment of force
  2. Moment of force is defined as the product of the force applied and the distance between the pivot and the point of application of the force

86.Dynamo converts it to –
 
A. Mechanical energy into magnetic energy
 
B. Mechanical energy into electrical energy
 
C. Electrical energy into mechanical energy
 
D. None of these
 
Option”B” is correct
87.On extracting butter from milk-
 
A. Milk density increases
 
B. Density of milk decreases
 
C. Density of milk remains unchanged
 
D. None of the above
 
Option”A” is correct
88.Which one of the following statements is not correct?
 
A. The weight of any body varies on different planets
 
B. The mass of any body is the same on the earth, on the moon and in empty space
 
C. The state of weightlessness of any body occurs when it is balanced against the gravitational force exerted on it.
 
D. The weight and mass of any body at sea level on the surface of the earth are the same.
 
Option”D” is correct
89.The amount of matter in a ball of steel is its :
 
A. Weight
 
B. Mass
 
C. Density
 
D. Volume
 
Option”B” is correct
90.There is no atmosphere on the moon because
 
A. It is near the earth
 
B. It orbits the sun
 
C. It receives light from the sun
 
D. Here the escape velocity of the atoms is less than their root mean square velocity.
 
Option”D” is correct.
The escape velocity on the surface of moon is lesser than the RMS velocity of the molecules of gas at the surface temperature of moon. Therefore, the gas molecules escape and hence moon cannot hold an atmosphere.

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