NCERT Class VIII Science Solutions: Chapter 12-Friction Part 1

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Question 1:

Fill in the blanks:

  1. Friction opposes the _____________ between the surfaces in contact with each other.

  2. Friction depends on the _____________ of surfaces.

  3. Friction produces __________.

  4. Sprinkling of powder on the carom board ________ friction.

  5. Sliding friction is ___________ than the static friction.

Answer:

  1. Friction opposes the motion between the surfaces in contact with each other.

  2. Friction depends on the roughness of surfaces.

  3. Friction produces heat.

  4. Sprinkling of powder on the carom board reduce friction.

  5. Sliding friction is lesser than the static friction.

Question 2:

Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their arrangements are given below. Choose the correct arrangement.

  1. Rolling, static, sliding

  2. Rolling, sliding, static

  3. Static, sliding, rolling

  4. Sliding, static, rolling

Answer: C

Friction comes into play when irregularities present in the surfaces of two objects in contact get interlocked with each other. Static friction comes into play when we try to move an object which is at rest.

Sliding friction comes into play when an object slides over the surface of another object. In sliding friction, the time given for interlocking is very small. Hence, interlocking is not strong. Therefore, less force is required to overcome this interlocking. Because of this reason, sliding friction is less than static friction.

This image shows three types of friction

Types of Friction

This image shows three types of friction

Similarly, the area of contact in case of rolling friction is smallest as compared to static or sliding friction. This area of contact changes gradually because of rolling. Hence, rolling friction is lesser than both static and sliding friction.

Thus, the correct sequence is – static, sliding, rolling.

Question 3:

Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be:

  1. Wet marble floor, dry marble floor, newspaper and towel.

  2. Newspaper, towel, dry marble floor, wet marble floor.

  3. Towel, newspaper, dry marble floor, wet marble floor.

  4. Wet marble floor, dry marble floor, towel, newspaper

Answer:

A. Wet marble floor, dry marble floor, newspaper and towel.

Question 4:

Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.

Answer:

When a book slides on the writing desk, a frictional force acts between the book and the surface of the desk. The direction of frictional force on the book is opposite to the direction of its motion and acts in upward direction, as shown in the following figure.

This diagram shows friction force

Friction Force

This diagram shows friction force

Question 5:

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

Answer:

It would be difficult to walk on the floor. Soapy water fills the floor irregularities, thus reduces the friction considerably. Feet do not make necessary grip with the floor surface, thus increasing the chances of falling.