Mechanics

71.Adding kerosene to stagnant water reduces mosquitoes because it-
 
A. Interferes with reproduction
 
B. Is highly toxic to mosquitoes
 
C. Hinders the breathing of larvae
 
D. Repels mosquitoes
 
Option”C” is correct.
  1. Mosquitoes lay their eggs in stagnant water.​
  2. Kerosene floats on water, so adding kerosene to water stops the supply of air to larvae preventing them from breathing. ​
  3. Thus, it prevents malaria from spreading. ​
72.The hairs of the shaving brush stick together when removed from the water, the reason for this is?
 
A. Surface tension
 
B. Viscosity
 
C. Elasticity
 
D. Friction
 
Option”A” is correct.
When the brush is taken out of water, the hairs cling together due to surface tension. This happens because the free surface of water tries to acquire a minimum surface area.
73.A boy sitting in an open car going with a constant speed throws a ball straight up in the air, the ball falls
 
A. Behind him
 
B. In front of him
 
C. In his hand
 
D. Beside her
 
Option”C” is correct.
Since the car is moving with a constant velocity, so no pseudo force acts on the ball while it is in air. Thus the ball goes vertically upwards and the falls downwards (w.r.t. car frame) and falls exactly in the hands of the person.
74.A person is at rest in the middle of a horizontal plane of perfectly smooth ice, using which Newton’s law can he bring himself to the shore?
 
A. First motion law
 
B. Second law of motion
 
C. Third motion law
 
D. First second third law of motion
 
Option”C” is correct.
When the person pushes forward, the reaction force offered by the ice is very little. But this reaction force will help him to move forward. So, by Newton’s third law of motion, he can reach the shore.
75.Why do small pieces of camphor dance on the surface of water
 
A. Due to surface tension
 
B. Due to viscosity
 
C. Camphor has this quality
 
D. Due to the density of water
 
Option”A” is correct.
The camphor slowly dissolves in the water lowering the water’s surface tension in the immediate neighbourhood. The strange pull exerted by the uncontaminated portion of water brings about a movement of the surface and the camphor particles are carried along with it.
76.When detergent is added to pure water, the surface tension
 
A. Increases
 
B. Decreases
 
C. Remains unchanged
 
D. Becomes infinite
 
Option”B” is correct.
When detergent is added to water, the detergent molecules break down the cohesive attractive forces of the water molecules at the surface. Thus, the surface tension is broken and viscosity increases due to attractive forces between water molecules and detergents.
77.surface tension when soap is dissolved in water
 
A. Decreases
 
B. Increases
 
C. Remains unchanged
 
D. First decreases then increases
 
Option”A” is correct.
Soap decreases the surface tension of water. The attractive forces between the water molecules decreases. Hence it is easier to wash our clothes with soap as the water molecules break apart and trap the dirt in between them.
78.Ventilators are placed near the ceiling of the room because-
 
A. Exhaled hot air rises up and goes out
 
B. They provide cross ventilation in the room
 
C. They let some light into the room
 
D. She doesn’t suit the lower part
 
Option”A” is correct.
Ventilators are provided near ceilings in the rooms of our houses because the hot air is lighter and rises out of the room through the ventilators. Cool fresh air rushes into the room through the doors and windows to to occupy its place.
79.Assertion (A) : Detergents successfully remove oil and dirt from dirty clothes Reason (R) : Detergents increase the surface tension of water.
 
A. Both A and R are true and R is the correct explanation of A.
 
B. Both A and R are correct, but R is not the correct explanation of A.
 
C. A is correct but R is incorrect
 
D. A is wrong but R is correct
 
Option”C” is correct
80.Astronauts cannot stand upright in space because-
 
A. No gravity
 
B. Viscous force is very strong in the atmosphere
 
C. Solar wind exerts upward force
 
D. Atmospheric pressure is very low
 
Option”A” is correct

Pages: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22