Chapter 5 : Science - Class 8
Topic: Friction
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A. Fill in the Blanks
1. Friction always opposes any relative motion between the two surfaces that are in contact with each other.
2. Static friction is always greater than sliding friction.
3. Sportsmen use shoes, fitted with spikes, to increase friction between their shoes and the ground.
4. Sprinkling of talcum powder, on the carrom-board, helps to decrease friction.
5. Frictional force, on an object moving in a fluid, depends on its speed.
6. 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
1. When a body slides over smooth and wet surfaces, the amount of interlocking of irregularities, of the two surfaces in contact, increases. — False
2. If a car moves eastwards, the force of friction acts southwards. — False
3. Friction depends on how hard the two surfaces press against each other. — True
4. The sole of shoes and the tyres of the vehicles are ‘treaded’ to decrease the effects of friction. — False
5. Lubricants can be liquids, semi-liquids or solids. — True
6. Friction can be increased by using ball bearings between the moving parts of machines. — False
7. Fluid friction can be minimised by giving suitable shapes to the objects moving through the fluid. — True
C. Tick the correct option.
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 –
i. top-left figure
ii. bottom-left figure
iii. top-right figure
iv. bottom-right figure ✅
2. Two boys are applying oppositely directed, and equal in magnitude, forces on a box. The force of friction, that would exist between the lower end of the box and the ground, would be called—
i. sliding friction
ii. static friction ✅
iii. rolling friction
iv. fluid friction
3. Four students were asked to arrange the (frictional) forces, due to rolling, static and sliding friction, in an increasing order. The correct arrangement is—
i. sliding, rolling, static
ii. static, sliding, rolling
iii. rolling, sliding, static ✅
iv. sliding, static, rolling
4. The energy, ‘used up’ in overcoming friction, gets converted mainly into—
i. sound energy
ii. heat energy ✅
iii. magnetic energy
iv. kinetic energy
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 longer distance, before coming to rest, Rahul should—
i. use a heavier ball
ii. use a metal surface
iii. sprinkle talcum powder on the wooden surface ✅
iv. push the ball harder each time
6. A block is sliding on a horizontal surface. The force of friction between the two can be increased by—
i. polishing the surface
ii. putting a second identical block on top of the given block ✅
iii. lubricating the surface
iv. reducing the contact area
7. Rohini is pushing a box towards the west direction with a force of magnitude F. To help her move the box faster in the same direction, her friend should—
i. push the box in the east direction with a force of magnitude F
ii. pull the box in the east direction with a force of magnitude F
iii. push the box upward with a force of magnitude F
iv. pull the box in the west direction with a force of magnitude F ✅
D. Answer the following questions in brief.
1. Define the terms (a) static friction (b) sliding friction.
Answer:
(a) Static friction – It is the force that prevents a stationary object from moving. This force acts between the object and the surface it is kept on.(b) Sliding friction – It is the force that acts when one object slides over the surface of another object.
Answer:
(a) Rolling friction – It is the frictional force that acts when an object rolls over the surface of another object.
(b) Fluid friction – It is the frictional force that acts on an object when it moves through a fluid like air or water.Answer: Friction is mainly caused by:
The roughness or irregularities present on the surfaces in contact.
The force with which the two surfaces are pressed against each other.4. A box is resting on the floor. To move it, a (variable) force is applied as shown in the diagram below.
(a) Give the (likely) value of the force of limiting friction.Answer: In both these cases, the force applied is less than the limiting friction. That’s why the box does not move.
(c) Mark the direction of frictional force, and mention the kind of frictional force that comes into play, in each case.
Answer:The direction of friction is opposite to the direction in which the box is being pushed.
In case (a) – Static friction acts
In case (b) – Static friction acts
In case (c) – Sliding friction acts
5. Name the kind of frictional force that comes into play when a book, kept on a collection of cylindrical pencils, is moved by pushing it.
Answer: Rolling friction
6. Why are objects given special shapes when they are moving through fluids? What is the name given to these shapes?
Answer: Objects moving in fluids face resistance called fluid friction. To reduce this friction, they are given special shapes.
These shapes are called streamlined shapes.E. Answer the following questions.
1. Define the force of friction. List the factors affecting the force of friction. Explain with examples.
Answer: The opposing force that comes into play whenever an object moves or tends to move over the surface of another object, and acts parallel to the surface of contact, is called friction or force of friction.
Factors affecting the force of friction:
Nature of surfaces in contact – Rough surfaces produce more friction due to more interlocking of irregularities.Force pressing the surfaces together – Greater the normal force, greater the friction.
Example:
A box placed on a smooth surface with less mass is easier to move due to less friction.
A box placed on a rough surface with greater mass will experience more friction, making it harder to move.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 figure. In which case is the ball likely to move the longest distance? Give reason.
Answer: The ball is likely to move the longest distance in Case ‘C’ (glass sheet).
Reason:
The glass surface is very smooth.
There is less interlocking of irregularities.
Therefore, less friction is offered to the rolling ball, allowing it to travel farther.3. Explain why sliding friction is slightly less than static friction.
Answer: Friction occurs due to the interlocking of irregularities on surfaces in contact.
In static friction, the interlocking is stronger because the objects are at rest relative to each other.In sliding friction, the surfaces move past each other, giving less time for the irregularities to lock in.
Hence, sliding friction is slightly less than static friction.
4. Give reasons for the following:
(a) We tend to slip when we step on a banana peel.
Answer: The banana peel reduces friction drastically due to its smooth surface, so our feet cannot grip the ground properly, leading to slipping.
(b) It is easier to push a lighter box than a similar heavier box on the same floor.
Answer: The heavier box applies more force on the surface, increasing the normal force and thus the friction. The lighter box exerts less force and hence faces less friction.
(c) The force needed to start a cart is greater than the force needed to keep it moving with uniform speed.
Answer: Starting the cart requires overcoming static friction, which is higher. Once moving, only sliding or rolling friction (which is lower) needs to be overcome.
(d) Sportsmen use shoes fitted with spikes.
Answer: Spikes increase friction between shoes and the ground, giving a better grip and preventing slipping during movement.
(e) Ball bearings are used in machines.
Answer: Ball bearings reduce friction between moving parts, allowing smoother motion and increasing efficiency.
(f) Tyres of vehicles need to be changed regularly.
Answer: Due to continuous use, tyres wear out and lose grip. Changing them restores the required friction between tyre and road for safety.
(g) Machine parts are frequently oiled or greased.
Answer: Oiling or greasing reduces friction, which prevents wear and tear, and improves efficiency of the machine.
(h) Sports cars, aeroplanes and boats are designed to have a streamlined shape.
Answer: Streamlined shapes reduce fluid friction (air or water resistance), helping them move faster and more efficiently.
(i) Metal chains are wrapped on tyres when they run on icy roads.
Answer: Chains increase grip by increasing friction between the tyre and the icy surface, reducing chances of slipping.
(j) Rollers are used for transporting luggage.
Answer: Rolling friction is much less than sliding friction, so it's easier to move luggage fitted with rollers.
5. State and explain, with examples, how friction can help the cause of motion in certain situations.
Answer: Although friction opposes motion, it also helps in initiating motion in many situations:
Examples:
Walking: When we walk, we push the ground backward. Friction provides the necessary forward force to move us ahead.
Vehicles: Friction between tyres and the road allows vehicles to move without skidding.
Writing: The pen or pencil moves on paper due to friction, helping us to write properly.