Friction

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. A sprinkling of powder on the carrom board.…………….friction.
5. Sliding friction is……….than the static friction.

Answer.

1. relative motion
2. smoothness (or irregularities or nature)
3. heat
4. reduces
5. less.

Question 2.
Four children were asked to arrange forces due to rolling, static, and sliding frictions in decreasing order. Their arrangements are given below. Choose the correct arrangement.
 

• (a) rolling, static, sliding
• (b) rolling, sliding, static
• (c) static, sliding, rolling
• (d) sliding, static, rolling
   

Answer.
(c) static, sliding, rolling.

Question 3.
Alida runs her toy car on the 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
 

• (a) wet marble floor, dry marble floor, newspaper, and towel,
• (b) newspaper, towel, dry marble floor, wet marble floor,
• (c) towel, newspaper, dry marble floor, wet marble floor,
• (d) 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.
The frictional force will act parallel to the inclined surface, opposite to the direction of the sliding of the book.

Question 5.
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?
 

Answer.
It is difficult to walk on a soapy floor because a soapy floor applies very little friction and hence we can slip on such floors.

Question 6.
Explain why sportsmen use shoes with spikes.
 

Answer.
Sportsmen use shoes with spikes because spikes produce the desired frictional force and thus help in holding the ground firmly.

Question 7.
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?
 

Answer.
We know that the force of friction is increased if the two surfaces are pressed harder. A heavier box will apply more pressure on the floor and hence Seema will experience more frictional force.

Question 8.
Explain why sliding friction is less than static friction.
 

Answer.
Friction is caused by the interlocking of irregularities in the two surfaces. When the object starts sliding, the contact points on its surface, do not get enough time to lock into the contact points of the other surface. So, the sliding friction is slightly smaller than the static friction.

Question 9.
Give examples to show that friction is both a friend and a foe.
 

Answer.

Friction is both friend and foe, that is why it is said that friction is a necessary evil.

1. Friction as a friend:

• To hold a glass, we have ridges on our palm, which increase the friction between palm and glass.
• We are able to write anything with a pen or pencil because there is friction between the surfaces of paper and the point of a pen (nib) or pencil.
• A teacher can write on the blackboard with chalk because of the friction between the black-board surface and the chalk.
• If there is no friction, then a moving body would never stop.
• A nail could not be fixed in the wall, or a knot could not Be tied, had there been any friction.
• No building could be constructed without friction.

2. Friction as a foe:

• Friction wears out materials, whether they are screws, ball-bearings, or soles of shoes.
• Friction can also produce heat, which increases the wear and tear of machine parts. It also causes much wastage of energy, because this heat is not utilized.

Question 10.
Explain why objects moving in fluids must have special shapes.
 

Answer.
Birds and the fishes have to move about in fluids all the time. Thus, their “bodies have evolved to shapes which make them not to lose much energy in overcoming fluid friction. Such shapes are called streamlined. Cars are also designed so that their shapes resemble the streamlined shapes of birds, fishes, etc.