Answer: Option B. -> 5 N
:
B
The force required to just move the block is 10 N. This means that when we apply a force of 5 N on it, it will not move. Static frictionwill act opposite to the applied force to balance it. Hence, frictional force acting on the block will be 5 N in the direction opposite to the applied force.

Answer: Option B. -> Rolling friction is less than sliding friction.
:
B
Wheels are added to trolleys to reducefriction. As rolling friction is lesser than sliding friction, adding wheels to trolleys reduces opposition to motion.

Answer: Option B. -> False
:
B
Any object moving through a fluid (liquid or gas) experiences a frictional or drag force which opposes its motion. Hence, motion of the aeroplane will be retarded by air. Thus, the given statement is false.

Answer: Option C. -> Both the statements are correct.
:
C
Friction is caused by interlocking of irregularities present on the surfaces in contact. Even surfaces which appear smooth have a large number of minute irregularities present on them. These irregularities lock into one another. When we attempt to slideany surface over another, we have to apply a force to overcome the interlocking between them.
Rough surfaces have more interlocking as they have more number of irregularities present on the them. Hence,friction is generally larger between them.​Thus, both the statements are correct.

Answer: Option A. -> True
:
A
Applying lubricants to moving parts of machines reduces friction betweenthem. This means that energy that is lost as heat due to friction is reduced. Hence, efficiency of the machine is increased.

Answer: Option A. -> True
:
A
All fluids exert frictional or drag force on objectsmoving throughthem whichopposes motion of the object. Greater the speed of the vehicle, larger is the drag experienced by them. To minimize this drag force,vehicles are designed to have streamlined shapes.This helps in reducing drag force by allowing air to easily move past them.

Answer: Option B. -> Polished steel < Cardboard < Sandpaper
:
B
Polished steel surface is the smoothest amongthe given surfaces because it has the least irregular surface followed by cardboard and sandpaper respectively. If we try to slide a block with same speed on all these surfaces, the block will move farthest on the polished steel surface because it offers the least friction.

Answer: Option C. -> Ice
:
C

When a body is slid along a horizontal surface, frictional force opposes its motion. This means that the body will travel to the maximum distance on the surface having least friction. Among the given surfaces, ice has the least friction and the ball will travel the farthest on ice.

Answer: Option D. -> Grease
:
D

The substances which when applied between two surfaces reduce friction between them are known as lubricants. They help in reducing heat generation, and wear and tear in machines. Grease is a commonly used lubricant which is used in machines to reduce friction.

Answer: Option D. -> Fixing a nail to wall
:
B, C, and D

Friction is often called a necessary evil as it has both positive and negative effects. Some of the positive effects of friction are:

• It provides the necessary force for walking. It is very difficult to walk on a slippery surface.


• It helps us write on blackboard with a chalk. If friction was absent, chalk wouldn't remain stuck on the blackboard.


• It is because of friction that a nail remains fixed to the wall.

There are many other positive effects of friction as well. However, friction causing wear and tear of shoes is not desirable.