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.