Science STD 8 Chapter 9: Friction - Exercises

Science STD 8 Chapter 9: Friction - Exercises

1. Fill in the blanks.

  1. (a) Friction opposes the motion between the surfaces in contact with each other.
  2. (b) Friction depends on the nature of surfaces.
  3. (c) Friction produces heat.
  4. (d) Sprinkling of powder on the carrom board reduces friction.
  5. (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.

  1. (a) rolling, static, sliding
  2. (b) rolling, sliding, static
  3. (c) static, sliding, rolling
  4. (d) sliding, static, rolling

(c) 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

  1. (a) wet marble floor, dry marble floor, newspaper and towel.
  2. (b) newspaper, towel, dry marble floor, wet marble floor.
  3. (c) towel, newspaper, dry marble floor, wet marble floor.
  4. (d) wet marble floor, dry marble floor, towel, newspaper

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.

The frictional force acts in the direction opposite to the motion of the book. Since the book is sliding down the tilted desk, the force of friction will act in the upward direction, along the slope of the desk.

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?

It would be **more difficult** to walk on the floor. Soapy water acts as a lubricant, forming a thin layer between your feet and the floor. This layer reduces the friction between the two surfaces. Without sufficient friction, you would not be able to get a proper grip on the floor, making it difficult to walk and increasing the chances of slipping.

6. Explain why sportsmen use shoes with spikes.

Sportsmen use shoes with spikes to increase friction with the ground. The spikes create a better grip, which is essential for running, jumping, and changing direction quickly without slipping. This increased friction allows them to move safely and efficiently, especially on soft or uneven surfaces like a running track or a field.

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. This is because the force of friction increases when the two surfaces are pressed harder together. Since Seema's box is heavier, it presses down on the floor more, resulting in a greater force of friction that needs to be overcome to move the box. Therefore, she will have to apply a larger force than Iqbal.

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

Sliding friction is less than static friction because when an object starts moving (sliding), the contact points on its surface do not get enough time to interlock with the irregularities of the other surface. In contrast, for an object at rest, the contact points of the two surfaces are firmly locked into each other. Therefore, a larger force is needed to overcome this interlocking and start the motion, which is why static friction is greater than sliding friction.

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

Friction as a friend:

  • It allows us to walk without slipping on the ground.
  • It enables us to write with a pen or pencil on paper.
  • It helps in stopping vehicles when brakes are applied.

Friction as a foe:

  • It causes wear and tear of machine parts and shoe soles.
  • It produces heat, which can damage machine parts and waste energy.
  • It makes it difficult to move objects, requiring more effort.

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

Objects moving in fluids (gases or liquids) must have special shapes to minimise fluid friction, which is also called drag. The frictional force exerted by fluids depends on the shape of the object. By having a streamlined shape, like a fish or an aeroplane, the objects lose less energy in overcoming friction, allowing them to move more efficiently through the fluid. Nature provides many examples of such streamlined bodies, like birds and fish, that have evolved to reduce fluid friction.

Suggested Activities and Projects

1. What role does friction play in the sport of your choice? Collect some pictures of that sport in action where friction is either supporting it or opposing it. Display these pictures with proper captions on the bulletin board of your classroom.

Outline: This activity involves analysing a sport to understand the role of friction. For example, in football, friction is a friend when players' shoes with cleats grip the ground, allowing them to run and stop. It is also a foe as air resistance (a type of fluid friction) opposes the motion of the ball. The project would involve collecting images and writing captions to explain these concepts visually.

2. Imagine that friction suddenly vanishes. How would life be affected. List ten such situations.

Outline: This is a thought experiment to highlight the importance of friction. If friction vanished, life would be impossible. Some situations would be: we couldn't walk or hold anything, vehicles couldn't move or stop, knots would untie themselves, and buildings would crumble as nails and screws wouldn't hold. Writing on paper or a blackboard would also be impossible without friction.

3. Visit a shop which sells sports shoes. Observe the soles of shoes meant for various sports. Describe your observations.

Outline: This activity involves observing and comparing the soles of different sports shoes. You would notice that running shoes have a different tread pattern for grip on hard surfaces, while football or cricket shoes have spikes or cleats to increase friction on grass. This observation reinforces the idea that friction is deliberately increased for better performance and safety in sports.

4. A toy to play with: Take an empty match box. Take out its tray. Cut a used refill of a ball pen of the same width as the tray as shown in the figure below. Fix the refill with two pins on the top of the tray as shown in Fig. 9.18. Make two holes on the opposite sides of the tray. Make sure that the holes are large enough to allow a thread to pass through them easily. Take a thread about a metre long and pass it through the holes as shown. Fix beads at the two ends of the thread so that it does not come out. Insert the tray in the outer cover of the matchbox. Suspend the match box by the thread. Leave the thread loose. The match box will start falling down due to gravity. Tighten the thread now and observe what happens. Explain your observation. Can you relate it to friction?

Outline: This toy demonstrates the effect of friction. When the thread is loose, the matchbox falls due to gravity. When the thread is tightened, it creates friction with the refill and the holes, which opposes the motion of the matchbox. This frictional force prevents the box from falling, showing that friction can be used to stop motion.