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Q1. What are the factors on which the magnitude of drag exerted by a fluid on an object moving through it depends?

• 1) All of the above
• 2) Thickness of the fluid
• 3) Speed of the object
• 4) Shape of the object

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.

Q2. 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. 

• 1) Equal to
• 2) Less than
• 3) More than
• 4) Twice

Solution

As friction only depends on the weight and not on the area of contact, friction experienced by both buckets is the same. 

Q3. If a surface is smoothened, how will it affect friction?

• 1) Cannot be computed
• 2) Friction will remain same
• 3) Friction will increase
• 4) Friction will decrease

Solution

Smoothening a surface decreases its friction.

Q4. The S.I. unit of friction is

• 1) newton
• 2) pascal
• 3) Kilogram
• 4) Dyne

Solution

As friction is a force and the S.I. unit of force is Newton, the S.I. unit of friction is also Newton.

Q5. 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.

Q6. Select the correct order

• 1) Sliding friction > static friction > Rolling friction
• 2) Rolling friction > static friction > sliding friction
• 3) Rolling friction > sliding friction > static friction
• 4) Static friction > sliding friction > rolling friction

Solution

Static friction >sliding friction >rolling friction

Q7. 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.

Q8. A force that always tends to slow down the motion of an object on a surface is

• 1) Weight
• 2) Mass
• 3) Inertia
• 4) Friction

Solution

A force of friction always tends to slow down the motion of an object on a surface.

Q9. In which direction does the force of friction act?

• 1) In all directions
• 2) In the direction of applied force
• 3) Perpendicular to the direction of applied force
• 4) Opposite to the direction of applied force

Solution

Force of friction always acts opposite to the direction of applied force.

Q10. 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.

Q11.  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.

Q12. Which kind of friction mainly occurs between the surface of a wheel or other round objects and another surface?

• 1) Fluid friction
• 2) Static friction
• 3) Rolling friction
• 4) Sliding friction

Solution

Rolling friction mainly occurs between the surface of a wheel or other round objects and another surface.

Q13. 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?

• 1) Static friction
• 2) Fluid friction
• 3) Rolling friction
• 4) Sliding friction

Solution

Static friction comes into play when we try to move an object at rest.

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. A block is being pulled by a force. Which of the following actions will decrease the frictional force on the block?

• 1) Pressing the block harder on the surface
• 2) All of the above
• 3) Increasing the contact surface area
• 4) Moving the block on a smooth surface

Solution

Moving the block on a smooth surface will decrease the frictional force on the block.

Q16. 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.

Q17. Which of the following is NOT used to reduce friction?

• 1) Boric powder
• 2) Lubricants
• 3) Wheels
• 4) Cleats

Solution

Cleats are used to increase friction so as to have a better grip on the ground.

Q18. 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.

Q19. What is an advantage of using rollers instead of sliding an object?

• 1) Rolling friction has the advantage of being greater than sliding friction
• 2) Rollers are less expensive
• 3) Rollers decrease the weight of the object to be moved
• 4) Rolling friction is much less than sliding friction

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.

Q20. 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.

Q21. Is friction a self adjusting force?

Solution

No, all types of friction are not self adjusting. Only static friction is self adjusting.

Q22. Why do gymnasts apply a coarse substance to their hands?

Solution

Gymnasts apply a coarse substance to their hands for a better grip.

Q23.  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. 

Q24. Name two common lubricants.

Solution

 Grease and oil are two commonly used lubricants.

Q25. 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.

Q26. 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.