Q1. How does a matchstick light up? Explain.
Solution
When we rub a matchstick against the rough surface of a matchbox, then friction between the head of matchstick and rough side of matchbox produces heat. This heat burns the chemicals present on the head of matchstick due to which the matchstick lights up.
Q2. Is frictional force a contact or non contact force. Explain
Solution
Frictional force is a contact force because it comes into play only when two surfaces are in physical contact with each other.
Q3. Name two common lubricants.
Solution
Grease and oil are two commonly used lubricants.
Q4. Explain with the help of diagrams, how the use of oil reduces friction between two surfaces in contact with each other.
Solution
When oil is applied between the two surfaces in contact, a thin layer of oil is formed between the two surfaces. This layer separates the two surfaces a little bit due to which their interlocking is reduced to a large extent when they rub against each other.
Q5. The S.I. unit of friction is
Solution
As friction is a force and the S.I. unit of force is Newton, the S.I. unit of friction is also Newton.
Q6. A force that always tends to slow down the motion of an object on a surface is
Solution
A force of friction always tends to slow down the motion of an object on a surface.
Q7. What is an advantage of using rollers instead of sliding an object?
Solution
The advantage of using rollers instead of sliding an object is that rolling friction is much less than sliding friction. Thus, rolling reduces friction.
Q8. Why do gymnasts apply a coarse substance to their hands?
Solution
Gymnasts apply a coarse substance to their hands for a better grip.
Q9. Is friction a self adjusting force?
Solution
No, all types of friction are not self adjusting. Only static friction is self adjusting.
Q10. Select the correct order
- Rolling friction > static friction > sliding friction
- Rolling friction > sliding friction > static friction
- Sliding friction > static friction > Rolling friction
- Static friction > sliding friction > rolling friction
Solution
Static friction >sliding friction >rolling friction
Q11. Will it be possible to cut wood without the force of friction? Briefly explain.
Solution
We are able to cut wood because there is friction between the saw blade and log of wood. If there were no friction between the saw blade and log of wood, then cutting of wood would not be possible.
Q12. Two buckets of different surface areas and the same weight are being pushed on the floor. The friction exerted by the floor on the bucket having a larger surface area will be _________ the bucket having smaller surface area.
Solution
As friction only depends on the weight and not on the area of contact, friction experienced by both buckets is the same.
Q13. What are the factors on which the magnitude of drag exerted by a fluid on an object moving through it depends?
Solution
The frictional force (or drag) on an object in a fluid depends on its speed with respect to the fluid. It also depends on the shape and size of the object and also the nature of the fluid.
Q14. Explain why sportsmen use shoes with spikes.
Solution
Sportsmen use shoes with spikes so as to have a better grip while running. This is because the force of friction between the shoes and the ground increases with the help of spikes.
Q15. In which direction does the force of friction act?
Solution
Force of friction always acts opposite to the direction of applied force.
Q16. Which of the following is NOT used to reduce friction?
Solution
Cleats are used to increase friction so as to have a better grip on the ground.
Q17. Which kind of friction mainly occurs between the surface of a wheel or other round objects and another surface?
Solution
Rolling friction mainly occurs between the surface of a wheel or other round objects and another surface.
Q18. A block is being pulled by a force. Which of the following actions will decrease the frictional force on the block?
Solution
Moving the block on a smooth surface will decrease the frictional force on the block.
Q19. Give one example from everyday life where wheels (or rollers) are used to reduce friction.
Solution
Heavy machines can be easily moved from one place to another by placing round logs of wood under them and then pushing with the force of hands. The round logs of wood act as rollers and make it easier to move the heavy machine kept on them.
Q20. Why winter tires usually have deeper treads than regular car tires?
Solution
Winter tires have deeper treads than regular car tires. This makes their surface rougher and help them get a good grip on snow and ice covered roads.
Q21. Our palms become hot when rubbed against each other. Why?
Solution
It is because friction takes place between the surfaces of our palms which produces heat.
Q22. An object is placed at a surface and we try to move it by applying force. The object does not move unless we apply sufficient force. What is responsible for this?
Solution
Static friction comes into play when we try to move an object at rest.
Q23. If a surface is smoothened, how will it affect friction?
Solution
Smoothening a surface decreases its friction.
Q24. What are the factors on which friction depends?
Solution
Friction depends on two factors:(i) Nature of the surfaces in contact.(ii) How hard the surfaces press together.
Q25. How can we say that friction is an evil?
Solution
Friction is an evil because:(i) It produces heat which may damage machines.(ii) It results in wastage of energy and fuel.
Q26. Why aeroplanes and ships are designed with special shaped bodies? What is such a shape called?
Solution The aeroplanes and ships have streamlined shaped bodies to reduce drag. The streamlined shaped body experiences minimum resistance when travelling through water or air. It helps to reduce friction.