9.9. 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.

This exercise encourages students to recall key facts about friction.

• The first blank emphasizes that friction obstructs movement between surfaces.
• The second blank focuses on understanding that the nature of the surfaces affects friction.
• The third blank requires students to identify the effects of friction, such as heat generation.
• The fourth blank relates to how certain substances can reduce friction.
• The final blank helps reinforce the idea that sliding friction is typically lower than static friction.

Think about sliding down a slide at a playground. The surface of the slide and your speed determine how fast you go. A smooth slide (like one without friction) would let you slide down quickly, while a rough slide (like one with friction) would slow you down because the friction is opposing your motion.

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

This exercise tests students' understanding of the relative strengths of different types of friction. Generally:

• Static friction is the strongest as it prevents motion from starting.
• Sliding friction is weaker because it allows movement but still opposes it.
• Rolling friction is the weakest since it involves wheels or spheres that rotate instead of sliding. Students need to identify that the correct order is (c) static, sliding, rolling.

Consider a heavy sofa. When you try to push it, you first have to overcome static friction — this is hard! Once it starts moving, you face sliding friction, which is easy but still present. If you were to put the sofa on a rolling dolly, moving it becomes much easier because rolling friction is at play.

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

This question helps students understand how surface texture impacts friction. Wet surfaces generally reduce friction compared to dry surfaces. The increasing order of friction would typically follow this logic, leading students to determine that the correct answer is (c) towel, newspaper, dry marble floor, wet marble floor, since the towel is the most absorbent and least frictional.

Picture trying to move a toy car on different carpets—some are plush, which creates more resistance, while others are flat. A wet or polished floor can feel slippery compared to a rough, dry one, similar to how their toy car behaves when running over those surfaces.

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

In this situation, as the book slides down due to gravity, the frictional force will act in the opposite direction of the slide — that is, upwards along the desk surface. This opposing force is crucial as it seeks to slow down the book's descent. Students should illustrate this concept by drawing an arrow pointing upward along the desk surface.

Think about riding a bicycle up a hill. As you pedal harder, gravity tries to pull you down. The friction between your tires and the road pushes you up, opposing gravity's pull - just like the friction for the sliding book.

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?

Soapy water creates a slippery layer on the marble, significantly reducing friction. Consequently, it becomes more difficult to walk as your feet have less grip on the floor. Thus, students should answer that walking becomes challenging due to decreased friction.

Imagine trying to walk on ice versus a rough pavement. Trying to maintain balance on a slippery floor is similar to trying to walk on ice — you would slide instead of grip the surface.

6. Explain why sportsmen use shoes with spikes.

Spikes are designed to dig into the ground, which increases friction between the shoe and the surface. Therefore, when athletes sprint or change direction, they have better control and stability. This innovation greatly helps in various sports by enhancing performance.

Think about a race car’s tires; they are designed for maximum grip on the road. Similarly, spikes on shoes provide that essential grip for sports, preventing slipping and allowing for quick movements.

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?

Seema will have to apply a larger force to push her heavier box due to increased static friction. Static friction depends on the weight of the box, which means the harder pressing force makes it more difficult to start moving. This exercise elaborates on Newton's second law relating force, mass, and acceleration.

Consider lifting a full backpack versus an empty one. The full backpack requires more effort due to its weight—pushing a heavy box is similarly strenuous compared to a lighter one.

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

Static friction is greater because it has to overcome the 'interlocking' or 'locking' of microscopic surface irregularities. Once those surfaces start sliding against each other, the surfaces move past these interlocks, reducing the frictional force required to maintain motion. Thus, sliding friction is generally less than static friction.

Imagine trying to move a heavy piece of furniture. Taking that first step is tough, equivalent to static friction. Once it's in motion, it's easier to slide across the floor — that’s the sliding friction at work!

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

Friction serves as a friend in various scenarios, such as allowing us to walk without slipping and enabling vehicles to stop. However, it can be a foe as well, causing wear and tear and generating unwanted heat in machinery. This contrast illustrates that while friction is essential, it can also lead to inefficiency. Understanding these dual roles fosters a comprehensive view of friction's impact.

Think of friction as a reliable friend who helps you climb a steep hill (like walking) but can also dance a little too close and cause a fall (like excessive wear on a moving part).

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

To minimize fluid friction, objects like boats and airplanes are designed with aerodynamic shapes that reduce drag. For example, a streamlined shape not only helps vehicles move faster through air or water but also enhances fuel efficiency. Understanding this allows students to appreciate the application of physics in engineering.

Consider how a fish swims; its body is streamlined naturally to cut through water smoothly. Engineers mimic nature by designing cars and airplanes to have similar fluid shapes.

--