A. Fill in the blanks
Question 1
Friction always __________ any relative motion between the two surfaces that are in contact with each other.
Answer:
Friction always opposes any relative motion between the two surfaces that are in contact with each other.
Question 2
Static friction is always __________ than sliding friction.
Answer:
Static friction is always greater than sliding friction.
Question 3
Sportsmen use shoes, fitted with spikes, to __________ friction between their shoes and the ground.
Answer:
Sportsmen use shoes fitted with spikes to increase friction between their shoes and the ground.
Question 4
Sprinkling of talcum powder, on the carrom-board, helps to __________ friction.
Answer:
Sprinkling of talcum powder on the carrom-board helps to reduce friction.
Question 5
Frictional force, on an object moving in a fluid, depends on its __________.
Answer:
Frictional force on an object moving in a fluid depends on its speed.
Question 6
The shape of an aeroplane is __________ to reduce the effects of friction, due to air.
Answer:
The shape of an aeroplane is streamlined to reduce the effects of friction due to air.
B. State True or False for the following statements
Question 1
When a body slides over smooth and wet surfaces, the amount of interlocking of irregularities of the two surfaces in contact increases.
Answer: False
Reason: Smooth and wet surfaces reduce the interlocking of irregularities, so friction decreases.
Question 2
If a car moves eastwards, the force of friction acts southwards.
Answer: False
Reason: Friction always acts opposite to the direction of motion. Therefore, it acts westwards.
Question 3
Friction depends on how hard the two surfaces press against each other.
Answer: True
Reason: Greater pressing force (normal force) increases friction.
Question 4
The sole of shoes and the tyres of the vehicles are ‘treaded’ to decrease the effects of friction.
Answer: False
Reason: Treads are provided to increase friction and improve grip.
Question 5
Lubricants can be liquids, semi-liquids or solids.
Answer: True
Reason: Lubricants are available in different forms such as oil, grease and graphite.
Question 6
Friction can be increased by using ball bearings between the moving parts of machines.
Answer: False
Reason: Ball bearings reduce friction by changing sliding friction into rolling friction.
Question 7
Fluid friction can be minimised by giving suitable shapes to the objects moving through the fluid.
Answer: True
Reason: Streamlined shapes reduce fluid friction.
C. Tick (✓) the correct option
Question 1
Suppose your writing desk is tilted to a position where a book kept on it just starts sliding down. The figure showing the correct direction of frictional force acting on it is ________.
Answer: ✅ Bottom-left figure
Reason: Since the book tends to slide downward, friction acts upward along the inclined plane.
Question 2
Two boys are applying equal and opposite forces to a box as shown in the figure. In such a case, the force of friction, that would exist between the lower end of the box and the ground, would be called ________.
(a) Sliding friction
(b) Static friction
(c) Fluid friction
(d) Rolling friction
Answer: ✅ (b) Static friction
Question 3
Four students were asked to arrange the (frictional) forces, due to rolling, static and sliding friction, in an increasing order. Their arrangements are listed below. The correct arrangement is ________.
(a) Rolling, Static, Sliding
(b) Rolling, Sliding, Static
(c) Static, Sliding, Rolling
(d) Static, Rolling, Sliding
Answer: ✅ (b) Rolling, Sliding, Static
Question 4
The energy ‘used up’ in overcoming friction gets converted mainly into ________.
(a) Sound energy
(b) Heat energy
(c) Light energy
(d) Chemical energy
Answer: ✅ (b) Heat energy
Question 5
Rahul rolls a ball on a wooden surface. The ball covers a certain distance before coming to rest. To make the same ball cover a (much) longer distance, before coming to rest, Rahul should ________.
(a) Spread a newspaper on the wooden surface.
(b) Spread a towel on the wooden surface.
(c) Sprinkle talcum powder on the wooden surface.
(d) Spread a jute bag on the wooden surface.
Answer: ✅ (c) Sprinkle talcum powder on the wooden surface.
Question 6
A block is sliding on a horizontal surface. The force of friction between the two can be increased by ________.
(a) Decreasing the area of contact of the block with the surface.
(b) Applying a layer of some lubricant on the surface.
(c) Polishing the surface.
(d) Putting a second identical block on top of the given block.
Answer: ✅ (d) Putting a second identical block on top of the given block.
Reason: Adding another block increases the normal force, which increases friction.
D. Answer the following questions in brief
1. Define the terms (a) static friction (b) sliding friction.
Answer
(a) Static Friction:
Static friction is the frictional force that acts between two surfaces which are in contact but are not moving relative to each other. It prevents an object from starting to move. The value of static friction increases with the applied force until it reaches its maximum value, called limiting friction.
(b) Sliding Friction:
Sliding friction is the frictional force that acts when one surface slides over another. It opposes the motion of the moving object and is less than static friction. For example, pushing a book across a table experiences sliding friction.
2. State the meanings of the terms (a) rolling friction (b) fluid friction.
Answer
(a) Rolling Friction:
Rolling friction is the frictional force that acts when a body rolls over a surface, such as a wheel, ball or roller. It is much smaller than sliding friction, which is why wheels and ball bearings are used in vehicles and machines.
(b) Fluid Friction:
Fluid friction is the frictional force exerted by fluids (liquids and gases) on an object moving through them. It is also known as drag. The amount of fluid friction depends on the speed, shape and size of the object.
3. State the likely cause of friction.
Answer
The main cause of friction is the interlocking of the tiny irregularities present on the surfaces in contact. Although surfaces may appear smooth, they have microscopic roughness. These irregularities lock with each other and oppose motion. Greater the roughness of the surfaces, greater is the friction.
4. A box is resting on the floor. To move it, a (variable) force is applied as shown below.
(a) Give the (likely) value of the force of limiting friction.
Answer
The force of limiting friction is 30 N, because the box just begins to move when the applied force reaches 30 N. Limiting friction is the maximum value of static friction acting just before motion starts.
(b) Why does the box stay at rest in the cases corresponding to Fig. (a) and Fig. (b)?
Answer
In Fig. (a) and Fig. (b), the applied force is less than the limiting friction. Therefore, the static friction balances the applied force by acting in the opposite direction. Since the forces are balanced, the box remains at rest.
(c) Mark the direction of frictional force, and mention the kind of frictional force that comes into play, in each case.
Answer
| Figure | Direction of Friction | Kind of Friction |
|---|---|---|
| Fig. (a) | Opposite to the applied force (towards the right) | Static friction |
| Fig. (b) | Opposite to the applied force (towards the right) | Static friction |
| Fig. (c) | Opposite to the direction of motion (towards the right) | Sliding friction |
5. Name the kind of friction that comes into play when a book, kept on a collection of cylindrical pencils, is moved by pushing it.
Answer
The friction acting in this case is rolling friction. As the cylindrical pencils roll under the book, sliding friction is replaced by rolling friction. Since rolling friction is much smaller, the book moves more easily.
6. Why are objects moving through gases shown special shapes? Write the name given to these special shapes.
Answer
Objects moving through gases are given special streamlined shapes to reduce air resistance (fluid friction). These shapes allow air to flow smoothly around the object, making it easier to move and saving energy. Aeroplanes, birds, cars and high-speed trains have streamlined shapes to reduce friction with air.
Remember (Exam Points)
- Static friction acts before motion begins.
- Limiting friction is the maximum value of static friction.
- Sliding friction is less than static friction.
- Rolling friction is the least among static, sliding and rolling friction.
- Fluid friction is also called drag.
- Streamlined shapes help in reducing fluid friction.
These answers are ideal for Class 8 DAV/CBSE students and are written in a neat, easy-to-understand notebook style.
E. Answer the following questions
Chapter – Friction (Class 8)
(Notebook-style answers with a little detail)
Question 1
Define force of friction. List the factors affecting the force of friction. Explain with examples.
Answer
Force of Friction
The force of friction is the force that opposes the relative motion or the tendency of motion between two surfaces in contact. It always acts in the direction opposite to the motion.
For example, when we push a book on a table, the book slows down and finally stops because of friction between the book and the table.
Factors affecting the force of friction
1. Nature of the surfaces in contact
- Rough surfaces produce more friction.
- Smooth or polished surfaces produce less friction.
Example: It is easier to slide a book on a polished table than on a rough wooden surface.
2. How hard the surfaces are pressed together
- Greater the force pressing the surfaces together, greater is the friction.
Example: A loaded trolley is more difficult to push than an empty trolley.
3. Speed of the object (in fluids)
- Fluid friction depends on the speed of the object.
- As speed increases, fluid friction also increases.
Example: A fast-moving car experiences more air resistance than a slow-moving car.
Question 2
A ball is allowed to roll down an inclined wooden plane from a given height. At the foot of the inclined plane, it moves on a horizontal surface, differently covered, one by one, as shown in the figures.
(A) Newspaper
(B) Woollen Cloth
(C) Fine Grass
In which case will the ball likely move the longest distance? Give reason for your answer.
Answer
The ball will move the longest distance in Case ‘A’ (Newspaper).
Reason:
A newspaper provides a smoother surface than woollen cloth and fine grass. Therefore, the friction between the ball and the surface is the least. Since less friction opposes the motion of the ball, it travels a longer distance before coming to rest.
Order of friction:
Newspaper < Woollen Cloth < Fine Grass
Hence, the ball travels the maximum distance on the newspaper.
Question 3
Explain why sliding friction is slightly less than static friction.
Answer
Sliding friction is slightly less than static friction because once an object starts moving, the tiny irregularities on the two surfaces do not remain interlocked for long. They continuously break and reform, reducing the frictional force.
On the other hand, static friction acts when the object is at rest. The irregularities of the two surfaces remain firmly interlocked, so a greater force is needed to start the motion.
Therefore, sliding friction is always less than static friction.
Question 4
Give reasons for the following.
(a) We tend to slip when we step on a banana peel.
Answer
A banana peel acts like a lubricant between our feet and the ground. It reduces the friction between the shoe and the floor, making it difficult to maintain grip. As a result, we tend to slip.
(b) It is easier to push a lighter box than a similar heavy box on the same floor.
Answer
A heavier box presses the floor with greater force, which increases friction. A lighter box exerts less force on the floor, so the friction is less. Therefore, less force is needed to push a lighter box.
(c) The force, needed to start a cart, is greater than the force needed to keep it moving with a uniform speed.
Answer
To start the cart, we have to overcome static friction, which is greater. Once the cart starts moving, sliding (or rolling) friction acts, which is smaller. Therefore, less force is required to keep the cart moving at a uniform speed.
(d) Sportsmen use shoes fitted with spikes.
Answer
Spikes increase the friction between the shoes and the ground. This provides a better grip and prevents slipping while running or jumping.
(e) Ball-bearings are used in machines.
Answer
Ball bearings reduce friction by converting sliding friction into rolling friction. Since rolling friction is much smaller than sliding friction, machines work more smoothly, wear out less, and require less energy.
(f) Tyres of the vehicles need to be changed regularly.
Answer
The treads of tyres wear out after continuous use. Worn-out tyres provide less friction with the road, increasing the chances of skidding, especially on wet roads. Therefore, tyres should be replaced regularly for safety.
(g) Machine parts are frequently oiled or greased.
Answer
Oil and grease act as lubricants. They reduce friction between moving machine parts, preventing wear and tear, reducing heat generation, and increasing the efficiency and life of machines.
(h) Sport cars, aeroplanes and boats are designed to have a streamlined shape.
Answer
A streamlined shape reduces air or water resistance (fluid friction). As a result, these vehicles can move faster, consume less fuel, and perform more efficiently.
(i) Metal chains are wrapped on tyres when they run on icy roads.
Answer
Icy roads are very slippery because friction is very low. Metal chains increase the friction between the tyres and the road, providing better grip and preventing the vehicle from skidding.
(j) Rollers are used for transporting luggage.
Answer
Rollers reduce friction by changing sliding friction into rolling friction. Since rolling friction is much less, heavy luggage can be moved easily with less effort.
Question 5
State and explain, with examples, how friction can help the cause of motion in a number of situations.
Answer
Although friction opposes motion, it is also very useful because it helps us perform many everyday activities.
Some examples are:
- Walking: We can walk because friction between our feet and the ground provides the necessary grip. Without friction, we would slip.
- Writing: Friction between the pen or pencil and paper helps us write.
- Driving vehicles: Friction between the tyres and the road enables vehicles to move, turn, and stop safely.
- Holding objects: Friction between our hands and objects allows us to hold and lift them without slipping.
- Braking: Brakes stop vehicles due to friction between the brake pads and the wheels.
- Lighting a matchstick: Friction between the matchstick and the rough surface of the matchbox produces heat, which ignites the match.
Conclusion:
Friction is a necessary force. Although excessive friction causes wear and energy loss, an appropriate amount of friction is essential for movement and many daily activities.
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