Test: Introduction to Friction - Civil Engineering (CE) MCQ

Friction force:

• Friction is the force that resists motion when the surface of one object comes into contact with the surface of another.
• It is a force that opposes the relative motion of two bodies in contact.
• The frictional force acts in a direction opposite to the direction of motion of the bodies in contact.
• Frictional force prevents objects from gliding and ensures contact.
• Friction forces also provide a significant amount of resistance to motion.

Lubricant:

• It is a substance used for lubricating an engine or component, such as oil or grease.
• ​There are 4 types of lubricants i.e., Oil, Grease, Penetrating Lubricants, and Dry Lubricants.
• It aims at reducing friction between two moving objects.

Explanation:

• Friction force acts according to Newton's third law of motion.
• To perform a motion, the force used to initiate it must be greater than the frictional force.
• Friction is caused due to the roughness of the contact surface and the moving body. 
• When the rubber ball moves on the wooden table, a frictional force will be active due to friction on the ball.
• This frictional force prevents the ball from moving.
• To make the ball travel a longer distance, friction between the floor and the ball must be reduced. 
• The friction on the wooden floor can be reduced by sprinkle talcum powder on the wooden surface.
• Talcum powder is an anti-friction material that can be used to reduce friction.

Thus, a boy rolls a rubber ball on a wooden surface. The ball travels a short distance before coming to rest. To make the same ball travel longer distance before coming to rest, he may sprinkle talcum powder on the wooden surface.

Friction force:

• Friction is the force that resists motion when the surface of one object comes into contact with the surface of another.
• It is a force that opposes the relative motion of two bodies in contact.
• The frictional force acts in a direction opposite to the direction of motion of the bodies in contact.
• Frictional force prevents objects from gliding and ensures contact.
• Friction forces also provide a significant amount of resistance to motion.

Explanation:

• She should walk on the gravel path which provides sufficient friction force to walk. 
• As we sink into soft and wet sand, we exert significantly more effort. 
• Gravel on top of mud prevents us from sinking deeper. As a result, walking becomes easier.
• The surface of gravel is rough.
• When a surface is rough, friction increases.
• As a result, there will be enough force of friction for her to walk easily without slipping.

Thus, in a large commercial complex there are four ways to reach the main road. One of the path has loose soil, the second is laid with polished marble, the third is laid with bricks and the fourth has gravel surface. It is raining heavily and Paheli wishes to reach the main road. The path on which she is least likely to slip is gravel. 

Friction force:

• Friction is the force that resists motion when the surface of one object comes into contact with the surface of another.
• It is a force that opposes the relative motion of two bodies in contact.
• The frictional force acts in a direction opposite to the direction of motion of the bodies in contact.
• Frictional force prevents objects from gliding and ensures contact.
• Friction forces also provide a significant amount of resistance to motion.

Explanation:

• To sharpen the blade of a knife on high-frictional surfaces is preferable.
• The friction between the surface and the blade is greater on harder surfaces.
• A stone will be more suitable to sharpen the blade of a knife by rubbing it against a surface.
• It will provide a greater reaction and thus a higher friction force, making it easier to sharpen the blade of a knife by rubbing.

Thus, to sharpen the blade of a knife by rubbing it against a surface, stone will be most suitable.
Additional Information
Wooden blocks: 

• Wooden blocks may be considered hard in comparison to plastic wood, but in general, or in comparison to stone wood is not a very hard material.

Plastic blocks: 

• Plastic blocks are easily deformed and thus cannot provide the necessary force to easily remove the dull corroded particles from the surface.  
• Aside from the blades, they could be cut rather than sharpened.

Glass block:

• Although glass block is somewhat hard by scientific definitions, but it is less hard than stone. 

Friction force:

• Friction is the force that resists motion when the surface of one object comes into contact with the surface of another.
• It is a force that opposes the relative motion of two bodies in contact.
• The frictional force acts in a direction opposite to the direction of motion of the bodies in contact.
• Frictional force prevents objects from gliding and ensures contact.
• Friction forces also provide a significant amount of resistance to motion.

Lubricant:

• It is a substance used for lubricating an engine or component, such as oil or grease.
• ​There are 4 types of lubricants i.e., Oil, Grease, Penetrating Lubricants, and Dry Lubricants.
• It aims at reducing friction between two moving objects. 

Explanation:

• Friction force acts according to Newton's third law of motion.
• To perform a motion, the force used to initiate it must be greater than the frictional force.
• If we apply oil to the door hinges, the friction will decrease because lubrication always decreases friction between two surfaces.
• Oil can be any type of lubricant.
• For example, molybdenum sulphide and graphite.

Thus, if we apply oil on door hinges, the friction will decrease.

Important Points

• Friction is a force between two surfaces that are sliding, or trying to slide, across each other.
  • For example, when you try to push or pull luggage along the floor, friction makes this difficult.
• Friction always works in the opposite of the direction in which the object is moving or trying to move.
• There are four types of friction:
  • Static Friction: Static friction acts on objects when they are resting on a surface.
    • For example, hiking in the woods, there is static friction between shoes and the trail each time put down the foot.
    • Without this static friction, feet would slip out and making it difficult to walk.
  • Sliding Friction: Sliding friction is friction that acts on objects when they are sliding over a surface.
    • Sliding friction is weaker than static friction.
  • Rolling Friction: Rolling friction is friction that acts on objects when they are rolling over a surface.
    • Rolling friction is much weaker than sliding friction or static friction.
    • For example, ground transportation use wheels, including bicycles, cars, 4-wheelers, roller skates, scooters, skateboards, Ball bearings.
  • Fluid Friction: Fluid friction is friction that acts on objects that are moving through a fluid.
    • A fluid is a substance that can flow and take the shape of its container. Fluids include liquids and gases.

Force

• A push or a pull on an object is called a force. 
• The SI unit of force is the newton which is represented by N.
• Thus, we can say that the motion imparted to objects was due to the action of a force.
• A force arises due to the interaction between two objects.
• Force has magnitude as well as direction.
• If the two forces act in opposite directions on an object, the net force acting on it is the difference between the two forces.

Explanation:
Force of friction:

• Friction is the force that resists motion when the surface of one object comes into contact with the surface of another.
• It is a force that opposes the relative motion of two bodies in contact.
• The frictional force acts in a direction opposite to the direction of motion of the bodies in contact.
• Frictional force prevents objects from gliding and ensures contact.
• During motion, kinetic energy is converted to thermal energy by friction.
• Without frictional force, one cannot imagine two surfaces, whether solid, liquid, or gas, cannot remain in contact with the earth's surface.
• In solids, liquids, and gases, friction force acts to oppose motion with respect to each other.

Thus, whenever the surfaces in contact tend to move or move with respect to each other, the force of friction comes into play irrespective of whether the objects are solid, liquid or gaseous.

• Friction: The property of a surface that opposes the relative motion between two surfaces is called friction.

Friction (f) = μ N
Where μ is the coefficient of friction and N is the normal force

Explanation:

• Tension: It is the force which is transmitted through a rope or wire when pulled by forces acting from opposite sides. In other words, it is simply the force exerted by a string when it is subjected to pull.
• Momentum: It is the strength contained in the object due to its motion. It is the product of the mass of an object and the velocity with which it is moving.
• Impulse: It is the product of force and time. One can also say that it is the change in momentum.
• If no force is applied to a moving object, then it will stop due to Friction. So option 4 is correct.
  • As per the first law of motion-" an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force".
  • So, if no force is applied to a moving object, it should remain in motion as per the first law of motion. But, it is stopped due to friction.

Friction Force:
It is a kind of resisting force that acts between two contact bodies to avoid relative motion between them.

• It is formed due to the close interaction of charged particles at the surface of the bodies due to electromagnetic forces.
• Mathematically it is represented as Friction force = μ N.
• Where N = normal force = m × g
• Normal force: It is a force that is exerted by the surface perpendicular to the body. It is a component of the contact force.

Frictional forces are of two kinds:

• Static friction (f_s): When two bodies do not slip on each other, the force of friction is static friction. It can change its value until its max. F_max ≥ f_s
• Kinetic Friction (f_k): When two bodies slip over each other the force of friction is called kinetic friction.

Coefficient of Kinetic friction (μ_k) < Coefficient Static friction (μ_s).

• Here, Applied force = friction force
• Applied force (F) = μ N = μ m g

Explanation:

• Since the block does not move so frictional force (f_s) ≥ Applied force
• This means frictional force (f_s) ≥ 200 N

• Friction is the resistance force that opposes relative motion between two objects. 
  • It is not a fundamental force, like gravity or electromagnetism.
  • The direction of friction is always in such a way that it opposes the relative motion
• The friction force between two surfaces is due to the roughness of the contact surface.

Explanation:

• The friction force between two surfaces is due to the roughness of the contact surface.
• When the surfaces are polished, their roughness decreases.
• They become smooth.
• Hence the friction between them decreases.
• So the correct answer is option 2.

The correct answer is Only I

• The friction force is a contact force.
• Contact force is a force that is applied by objects in contact with each other.
• The contact force is governed by Newton’s Laws.
• Friction is the resistance to motion of one object moving relative to another.
• Magnetic force is the non-contact force.
• Gravitation force is also a non-contact force.

Types of contact forces:

• Frictional Force
• Applied Force
• Normal Force