Chapter 12: Friction

Extra Internal Question and Answer:

Question: What is friction? Why does it occur between two surfaces?

Answer: Friction is the force that opposes the relative motion between two surfaces in contact. It occurs because the surfaces are not perfectly smooth. When they touch, their irregularities interlock with each other, which resists motion and produces friction.

Question: Name the factors on which friction depends.

Answer: Friction depends mainly on the nature of the surfaces in contact (whether they are rough or smooth) and the force pressing the two surfaces together (normal force).

Question: Why is friction greater on rough surfaces than smooth surfaces?

Answer: Friction is greater on rough surfaces because rough surfaces have more irregularities. These irregularities interlock more strongly with each other when the surfaces are in contact. This increases resistance to motion, so friction becomes higher compared to smooth surfaces.

Question: How does polishing a surface affect friction?

Answer: Polishing a surface makes it smoother by filling the irregularities present on it. Due to this, the interlocking between the surfaces reduces. So, the force of friction decreases, and objects can move more easily over the polished surface.

Question: What are lubricants? Give two examples

Answer: Lubricants are substances which are used to reduce friction between two moving surfaces by forming a thin layer between them. They make motion smooth and reduce wear and tear. Examples of lubricants are oil and grease.

Question: What is rolling friction? Give two examples .

Answer: Rolling friction is the force of friction which acts when a body rolls over a surface. It is much smaller than sliding friction. Two examples are: a ball rolling on the ground and a wheel of a bicycle moving on the road.

Question: What is sliding friction? Give two examples.

Answer: Sliding friction is the force of friction that acts when one surface slides over another surface. It always opposes the motion of the object.

Examples: (i) A book sliding on a table, (ii) A box moving on the floor.

Question: Why is rolling friction less than sliding friction?

Answer: Rolling friction is less than sliding friction because in rolling, the surface irregularities do not get much time to interlock. The contact area is also smaller and deformation is less. So, less force is needed to move an object by rolling than by sliding.

Question: What is fluid friction? Give two examples.

Answer: Fluid friction is the force that opposes the motion of objects moving through a fluid, such as air or water.

Two examples are: (i) Motion of a boat in water  (ii) Motion of an aeroplane in air

Question: Why is fluid friction also called drag?

Answer: Fluid friction is also called drag because it is the force exerted by fluids like air or water on objects moving through them. This force acts opposite to the motion of the object and slows it down, so it is called drag.

Question: On which factors does fluid friction depend?

Answer: Fluid friction depends on the speed of the object, the nature of the fluid, and the shape of the moving object. A larger speed and less streamlined shape increase fluid friction, while a smooth and streamlined shape reduces it.

Exercises

1. Fill in the blanks.
(a) Friction opposes the _____________ between the surfaces in contact with each other.
(b) Friction depends on the _____________ of surfaces.
(c) Friction produces __________.
(d) Sprinkling of powder on the carrom board ________ friction.
(e) Sliding friction is ___________ than the static friction.

Answer: (a) Friction opposes the relative motion between the surfaces in contact with each other.

(b) Friction depends on the nature (or roughness) of surfaces.

(c) Friction produces heat.

(d) Sprinkling of powder on the carrom board reduces friction.

(e) Sliding friction is less than the static friction.
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.
(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling

Answer:  (c) static, sliding, rolling

Friction is maximum in static friction, then comes sliding friction, and minimum is rolling friction. So, the decreasing order is: static > sliding > rolling
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
(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.

[ Wet marble floor has the least friction because water acts like a thin layer and makes the surface more slippery. Dry marble floor has more friction than wet marble. Newspaper increases friction further due to its roughness. Towel has the highest friction because its surface is very rough, so it resists motion the most. ]     
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 the desk is tilted and the book starts sliding down, the frictional force acts along the surface of the desk in the upward direction, opposite to the motion of the book. It always opposes the relative motion between the surfaces in contact.
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 becomes more difficult to walk on a marble floor with soapy water. This is because soapy water acts like a lubricant and reduces friction between the feet and the floor. Due to less friction, there is less grip, so slipping becomes easier.         
6. Explain why sportsmen use shoes with spikes.

Answer: Sportsmen use shoes with spikes to increase friction between their feet and the ground. The spikes grip the surface better, which helps them run, walk, and stop without slipping, giving better balance and performance during sports.
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: Seema will have to apply a larger force.This is because the heavier box presses more on the floor, which increases the normal reaction and hence increases friction between the box and the floor. So, more force is needed to move the heavier box compared to the lighter one.
8. Explain why sliding friction is less than static friction.

Answer:  Sliding friction is less than static friction because when an object is at rest, the interlocking between the irregularities of the two surfaces is stronger. More force is needed to break this contact. Once the object starts moving, these interlocking reduces, so less force is required to keep it sliding.

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

Answer: Friction is a friend because it helps us to walk, write and hold objects. It also helps in stopping moving vehicles using brakes. But friction is a foe because it causes wear and tear of machine parts and produces heat, which reduces efficiency of machines.
10. Explain why objects moving in fluids must have special shapes.

Answer: Objects moving in fluids have special shapes to reduce fluid friction (drag). A streamlined shape helps the fluid flow smoothly around the object, reducing resistance. This allows the object to move faster and with less energy loss. Examples include fish, birds and aeroplanes which have streamlined bodies for easy movement.