Olympiad Test: Friction - Free MCQ with solutions Class 8 Science

• Friction: Friction is a force that opposes the motion of an object when it is in contact with another object.


• Contact Force: Friction is considered a contact force because it only occurs when two objects are in physical contact with each other.


• Types of Forces: Contact forces are forces that require physical contact between two objects, unlike non-contact forces such as gravity or magnetic forces.


• Examples: Examples of contact forces include friction, normal force, tension force, and applied force.


• Explanation: When two objects are in contact, the surface irregularities of the objects interact with each other, creating a force that resists the motion between the two surfaces. This force is known as friction.

• Surface roughness: A smooth surface has fewer irregularities, bumps, and rough patches compared to a rough surface. This reduces the microscopic interactions between the surfaces, resulting in less frictional force.



• Contact area: A smooth surface allows for less contact area between two surfaces in contact. With less surface area in contact, there are fewer points where friction can act, leading to lower frictional force.



• Surface adhesion: Smoother surfaces tend to have lower adhesion forces between them. Adhesion can contribute to frictional force, so reducing adhesion by having a smooth surface helps in reducing friction.



• Surface lubrication: Smooth surfaces can be easily lubricated, which further reduces friction between the surfaces. The presence of a lubricant layer helps in reducing the resistance to motion and lowers the frictional force.

A lubricant is a substance, usually organic, introduced to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces.

Force of friction depends on smoothness, roughness of surface, inclination of surface. 
When the two surfaces in contact are very rough, then the friction between them will be very large because the interlocking of very rough surface is too much. 

As the angle of incline is increased, the normal force is decreased, which increases the frictional force.

Friction is both friend as well as foe. 
Friend:-  as friction helps us to write, walk to control our speed .
Foe:-  as it cause wear and tear in object. 
So, friction is both Friend and foe.

If a body moves on a sandy surface, its motion will generally decrease (option b).

This is because the sandy surface creates more friction and resistance compared to smoother surfaces, making it harder for the body to move. The increased friction slows down the body's motion.

The correct answer is The friction between the feet and the path is reduced.

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.

Note: 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.

Airplanes have wings, just like birds. They also have a light Skelton to decrease their weight, and they have a streamlined shape to decrease drag. Airplanes wings are designed to create lift by changing the pressure, rather than by flapping them up and down.

• True: Friction is a force that opposes the motion of an object when it comes into contact with another object or surface.


• Frictional Force: The direction of frictional force is always opposite to the direction of motion of the object.


• Causes of Friction: Friction is caused by the irregularities on the surface of objects coming into contact, which create resistance to motion.


• Types of Friction: There are different types of friction, such as static friction, kinetic friction, and rolling friction, all of which act in the opposite direction of motion.


• Examples: When you push a box on the floor, the frictional force acts in the opposite direction to your push, slowing down the box's motion.


• Importance of Friction: Friction is essential for various everyday activities, such as walking, driving, and preventing objects from sliding off surfaces.

Friction is indeed caused by the interlocking of irregularities on the surfaces of objects in contact. When two surfaces try to slide past each other:

• Their microscopic projections and valleys catch.
• This catching resists movement.
• This resistance creates a frictional force.

Thus, the statement is true.

• Friction: Friction is the force that resists the relative motion or tendency of such motion of two surfaces in contact.


• Smoothness of the surface: The smoothness of a surface refers to how rough or uneven it is.


• Relationship between friction and smoothness:
  
  
  
  • Friction is inversely proportional to the smoothness of the surface.
  
  
  • When the surface is smoother, there are fewer irregularities for the surfaces to interact, resulting in lower friction.
  
  
  • Conversely, rough surfaces have more irregularities that interlock with each other, leading to higher friction.
  
  
  
  


• Conclusion: Therefore, friction decreases with an increase in the smoothness of the surface, making the statement false.

Soapy floor is slippery due to decreased friction. The soap covers the roughness of floor and makes it smooth, which restricts our walk on the floor.
So, the given statement is false.

• Friction is essential for various reasons:




• Friction allows us to walk on the ground without slipping.


• It helps in stopping vehicles by providing necessary traction.


• Friction is crucial in various machines and mechanisms for their proper functioning.




• Friction can be considered a necessary evil due to the following reasons:




• Friction causes wear and tear in machines and surfaces, leading to the need for maintenance and replacement.


• It opposes the motion of objects, requiring additional energy input to overcome it.


• Friction can sometimes cause heat generation, leading to inefficiencies in systems.




• While friction has its downsides, it is indispensable in many aspects of our daily lives and industrial processes.

A spring scale or spring balance or newton meter is a type of mechanical force gauge or weighing scale. It consists of a spring fixed at one end with a hook to attach an object at the other. It works by Hooke's Law, which states that the force needed to extend a spring is proportional to the distance that spring is extended from its rest position. Therefore, the scale markings on the spring balance are equally spaced. A spring scale cannot measure mass, only weight.

• Frictional Force: Frictional force is the force that resists the relative motion or tendency of such motion of two surfaces in contact.


• Smooth Surfaces: Smooth surfaces have less roughness and irregularities compared to rough surfaces.


• Friction on Smooth Surfaces: Smooth surfaces actually have less frictional force compared to rough surfaces.


• Reasoning: The irregularities and roughness of surfaces create more contact points which result in higher frictional force. Smooth surfaces have fewer contact points, hence less friction.

• True: Therefore, it is true that smooth surfaces have less frictional force compared to rough surfaces.

A fluid is a substance that continually flows under an applied  shear stress, or external force. Fluids are a phase of matter and include liquids, gases, and plasmas. 
So, the given statement is false.

When the road is wet or muddy, water covers the surface. The coefficient of friction of water is much lesser than that of the road's surface. These will cause the friction to be reduced between the road and the tyres and will lead to the car to skid.