Question 1. Fill in the blanks

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

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

(c) Friction produces heat.

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

(e) Sliding friction is less than static friction.

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

Solution: (c) static, sliding, rolling

• Friction due to static: When an object moves while still at rest

• Sliding friction is the force acting on a moving body

• Compared to static and sliding friction, rolling friction has the smallest contact area

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

Solution: (a) Wet marble floor, dry marble floor, newspaper, and towel.

The friction will be higher if the surface is rough and lower if smooth. As a result, wet marble floor, dry marble floor, newspaper, and towel is the correct sequence when arranged according to the increase in friction acting on the car.

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.

Solution: Friction will always be upward, contrary to the motion of a sliding book. Friction will always be parallel to the surfaces in contact.

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?

Solution: The layer of soap makes the floor smooth, which reduces friction. As a result, the floor becomes slippery, and the foot cannot grip it properly. This makes walking on a soapy floor difficult.

Question 6. Explain why sportsmen use shoes with spikes?

Solution: Sportspeople wear shoes that have a rough surface due to spikes. This increases friction. The more the friction, the better the grip. This makes walking and running easier.

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?

Solution: Friction is directly proportional to mass. The heavier an object, the more the force is to be applied to move it. Due to this, Seema has to exert more force than Iqbal.

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

Solution: During sliding, contact points do not get enough time to interlock properly. Friction forces come into play when irregularities present in the surface of the two objects in contact get interlocked with each other. When an object is in motion, the sliding friction is smaller than the static friction as the interlocking during motion is small. Thus, sliding friction is less than static friction.

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

Solution: Here are some points that illustrate how friction can be both a friend and a foe:

• Friction as a friend: Friction makes life possible for us: It enables us to walk and hold objects. It is helpful in construction activities. It makes matchstick ignition possible.

• Friction can be a foe: It can cause wear and tear of objects. It can damage machine parts, making them more expensive to maintain. It slows down moving objects by reducing their speed. It causes obstructions to the free movement of objects.

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

Solution:  It is necessary for objects moving in fluids to have a special shape to overcome the friction they experience. Objects are designed to have pointed fronts with broad middle portions that taper at the back, known as streamlined shapes to reduce friction.

Question 1:Define Friction.

Solution: Friction is defined as the resistance offered by surfaces in contact when they move past each other. It provides the traction necessary to walk without slipping. Most of the time, friction is helpful. However, it also offers resistance to motion.  

Question 2: How does friction occur? 

Solution: The cause of friction is the irregularities of the two surfaces in contact. Therefore, when objects move over each other, these irregularities on the surface become entangled, which results in friction. Increasing roughness will create more irregularities and result in greater friction.

Question 3: What are different kinds of friction?

Solution: There are four types of friction: static, sliding, rolling, and fluid. Solid surfaces experience static, sliding, and rolling friction. The strongest friction is static friction, followed by sliding friction, and the weakest is rolling friction. When liquids or gases interact, fluid friction occurs.

Question 4: Explain how frictional force in fluids is affected by three factors.

Solution: A fluid's frictional force depends on the nature of the fluid, shape of the object, and speed of the object concerning the liquid.

Question 5. A worker has to push a heavy trunk. How can he reduce his effort in pushing it?

Solution: A worker should place a few cylindrical logs of wood beneath the trunk as a solution. Hence, the area of contact between two surfaces is reduced. There is less friction during rolling than during sliding. Question 6. Why does it take less effort to move a box that's already moving than one that's at rest? Solution: When the box starts sliding, the surfaces of the box do not have enough time to lock into the contact points of the floor, once it has begun sliding. Sliding friction is, therefore, less than static friction.

Question 7.What causes a car to stop when its brakes are applied?

Solution: The friction in brake systems is increased by using brake pads in cars. While the brake pads do not touch the wheels, they stop them from moving when applying the brake lever.

Question 8. In what ways can friction be reduced?

Solution: Friction can be reduced using the following methods:

• The surfaces can be made a bit smoother.
• A surface can also be lubricated to make it smoother.
• Streamline the object.
• Reduce the forces acting between the surfaces in contact.
• Reduce the contact between surfaces so that fewer bonds are formed.   

Question 9. How would you define "streamlined shape"? Where and why is it used? For whom has nature designed streamlined bodies?

Solution: Streamlined shapes are narrow at the front and wide at the back. This reduces friction between the bodies of aeroplanes, missiles, and rockets. To reduce friction with air, ships and boats have streamlined shapes. Animals such as birds and fish have streamlined bodies.

Question 10:How can students benefit from using NCERT Solutions for Class 8 Science Chapter 12?

Solution: In this NCERT Solutions for Class 8 Science Chapter 12, students strengthen their grasping abilities. MSVgo solutions are created by subject experts and are up-to-date according to the latest syllabus and guidelines. With self-study, students can clearly understand the concepts that are important for the exams. Most questions appearing in board exams and competitive exams are taken from NCERT textbooks.