Friction Class 8 Worksheet Science Chapter 9

Q.1. Why do kabaddi players rub their hands with dry soil? 

Kabaddi players rub their hands with dry soil to enhance their grip on opponents. This technique helps them:

• Increase friction between their hands and the opponent.
• Prevent slipping during tackles.
• Improve overall performance in the game.

Q.2. When does sliding friction come into play? 

Sliding friction occurs when an object is in motion over another surface. Key points include:

• It acts against the direction of movement.
• It is generally less than static friction, which prevents motion.
• Common examples include sliding a book across a table or a sled on snow.

Q.3. What is done to increase friction between the tyres and road? 

The tyres of a vehicle are designed with treads to enhance grip on the road. This is achieved through:

• Tread patterns that create channels for water to escape, preventing hydroplaning.
• Rough surfaces that increase contact area with the road.
• Material composition that provides better traction.

These features collectively help to increase friction between the tyres and the road, improving safety and control while driving.

Q.4. Name the force responsible for wearing out car tyres. 

The frictional force is responsible for the wearing of car tyres.

Q.5. Explain why sportsmen use shoes with spikes. 

Sportsmen use shoes with spikes for several reasons:

• Increased grip: Spikes enhance the friction between the shoe and the ground.
• Improved traction: This allows athletes to run faster and change direction more easily.
• Stability: Spikes provide better stability during performance, reducing the risk of slipping.

Q.6. Can we reduce friction to zero by polishing surfaces or using a large amount of lubricants?

Friction can never be completely eliminated. Key points include:

• No surface is perfectly smooth; there are always some irregularities.
• Even with polishing or lubricants, friction persists.
• Lubricants can reduce friction but cannot remove it entirely.

Q.7. What is rolling friction? 

Rolling friction occurs when one object rolls over the surface of another. It is a type of friction that resists this motion.

• Rolling friction is generally less than sliding friction.
• It makes it easier to move objects, such as luggage with rollers.
• This principle is why the wheel is considered a significant invention.
• In machines, rolling friction is often reduced using ball bearings.

Q.8. Why do tyres of cars wear out gradually? 

The tyres of cars wear out gradually due to:

• Friction between the tyres and the road surface.
• Constant contact as the vehicle moves.
• Heat generated from the friction, which contributes to wear.

This gradual wear affects the tyre's performance and safety.

Q.9. Explain why the sliding friction is less than the static friction. 

Friction occurs due to the interlocking of tiny irregularities on two surfaces. When an object begins to slide, the contact points on its surface do not have enough time to fully engage with the surface beneath it. This results in:

• Sliding friction being less than static friction.
• It is generally easier to keep an object moving than to start moving it from rest.

Thus, the force needed to overcome static friction is greater than that required for sliding friction.

Q.10. When we strike a matchstick against the rough surface, it catches fire. Give reason.

When a matchstick is struck against a rough surface, the following occurs:

• The friction between the matchstick's head and the surface generates heat.
• This heat ignites the chemicals in the matchstick's head.
• As a result, the matchstick catches fire.

Q.11. What is a spring balance? 

A spring balance is a tool for measuring the force acting on an object. It works as follows:

• It contains a coiled spring that stretches when a force is applied.
• The amount of stretch is indicated by a pointer on a graduated scale.
• The reading on this scale shows the magnitude of the force.

Q.12. Why do car wheels often spin on icy roads? 

A car can spin on an icy road because:

• The treads of the tyres lose their grip.
• Ice reduces the friction needed for control.
• This lack of friction causes the car to lose stability and spin.

Q.13. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?

The force of friction occurs due to the interlocking of tiny irregularities on the surfaces in contact. When a heavy object, like Seema's box, is placed on the floor, these irregularities press together more firmly, resulting in a stronger interlocking effect.

• The greater the weight of the object, the stronger the interlocking.
• This means that more force is needed to overcome this interlocking.
• Therefore, Seema must apply a greater force to push her heavier box compared to Iqbal's lighter box.

Q.14. You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why? 

We are able to walk because of the friction present between our feet and the ground. In order to walk, we push the ground in the backward direction with our feet. The force of friction pushes it in the forward direction and allows us to walk. The force of friction between the ground and feet decreases when there is soapy water spilled on the floor. Hence, it becomes difficult to walk on the soapy floor.

Q.15. Why will a pencil write on paper, but not on a glass?

We are able to write on paper because there is friction between the tip of the pencil and the paper. We are not able to write on a glass because the glass surface is very smooth due to which friction between the tip of the pencil and the glass surface is less. This friction is not enough to rub off black graphite particles from the tip of a pencil.