Lakshya Education MCQs

Question: Which of the following forces helps us walk on ground?
A.Buoyant force
C.Frictional force
Answer: Option C
: C

While walking, we apply a force on ground in backward direction. The ground, in turn, applies a force on us in forward direction. This force is basically the frictional force which opposesslipping between our foot and ground.

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Question 1. For the same pair of bodies, the correct relation between different types of friction is:
  1.    Static friction > sliding friction > rolling friction.
  2.    Sliding friction > rolling friction > static friction.
  3.    Static friction > rolling friction > sliding friction.
  4.    Sliding friction > static friction > rolling friction.
Answer: Option A
: A

When a body starts sliding, the contact points on its surfacedo not get enough time to lock into the other surface. So, sliding friction is slightly smaller than maximum static friction. It is always easier to roll than to slide a body over another.So, rolling friction is further smaller than sliding friction. Hencethe correct order is: static friction > sliding friction > rolling friction.
Question 2. A block, kept on ground, requires 10 N of horizontal force to just start moving. If we apply a force of 5 N on the block in horizontal direction, what will be the magnitude of frictional force acting on it?
  1.    10 N
  2.    5 N
  3.    0 N
  4.    7.5 N
Answer: Option B
: 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.
Question 3. If a book placed on a horizontal desk is pushed towards right, what will be the direction of frictional force acting on it?
  1.    Towards right
  2.    Vertically upwards
  3.    Vertically downwards
  4.    Towards left
Answer: Option D
: D

Frictional force always opposes relative motion between two surfaces in contact. As the book is being pushed towards right, friction will act towards left to oppose this impending motion.
Question 4. Since rolling friction is less than sliding friction, sliding friction is replaced with rolling friction in most machines using ball bearings.
  1.    True
  2.    False
  3.    Vertically downwards
  4.    Towards left
Answer: Option A
: A

The given statement is true. Ball bearings are used in many machines to utilize rolling friction instead of sliding friction between moving parts. Rolling friction is preferred as it is less in magnitude than sliding friction and henceimproves efficiency of the machine.
Question 5. What is the frictional force exerted by fluids on objects moving through them known as?
  1.    Lift
  2.    Thrust
  3.    Weight
  4.    Drag
Answer: Option D
: D

Frictional force exerted byfluids, i.e., liquids and gases on objects moving through themis known as "drag". Itis the force which opposesmotion of any object moving with respect to the surrounding fluid.

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