Friction – MCQs with Answers and Explanations
CBSE Class 8 – Science
Chapter 12: Friction
A. Basic Idea and Nature of Friction
Correct Answer: (b)
Friction is defined as a force which opposes motion or the tendency of motion between two surfaces in contact.
Correct Answer: (b)
Friction arises only when two surfaces are in physical contact, hence it is a contact force.
Correct Answer: (c)
Friction always acts in a direction opposite to the motion or attempted motion of a body, thus opposing it.
Correct Answer: (c)
We can walk because friction between our feet and the ground provides the necessary grip to push the ground backward and move forward.
Correct Answer: (c)
Writing with a pencil requires friction between the pencil tip and paper so that graphite can stick and form marks. Without friction, writing would be almost impossible.
Correct Answer: (c)
The ball loses speed because friction from the ground and air resistance oppose its motion and gradually bring it to rest.
Correct Answer: (c)
Friction helps us in walking, writing, gripping, etc., but also causes wear and tear and energy loss as heat. So it is both helpful and harmful, a “necessary evil”.
Correct Answer: (c)
The basic nature of friction is to oppose motion or the tendency of motion between surfaces in contact.
B. Cause of Friction and Factors Affecting It
Correct Answer: (b)
Even smooth-looking surfaces have tiny irregularities. These interlock when surfaces touch, creating friction.
Correct Answer: (c)
Smooth glass on smooth metal has fewer irregularities interlocking, so friction is relatively low compared to rough combinations.
Correct Answer: (c)
More pressing force causes stronger interlocking of surface irregularities, hence friction increases.
Correct Answer: (c)
Under a microscope, every surface shows tiny bumps and hollows; no real surface is perfectly smooth.
Correct Answer: (c)
Polishing makes surfaces smoother and reduces interlocking of irregularities, thereby reducing friction.
Correct Answer: (c)
Friction depends on the nature of surfaces (rough/smooth) and the normal force pressing them together.
Correct Answer: (d)
A heavy box increases normal force and a rough floor increases interlocking, so both together cause maximum friction.
C. Types of Friction – Static, Sliding and Rolling
Correct Answer: (c)
When an object is at rest and we try to move it, the resisting force is static friction, up to a maximum value.
Correct Answer: (b)
The highest value of static friction just before the body begins to move is called limiting friction.
Correct Answer: (c)
Sliding friction occurs when one surface actually slides over another, like a book pushed across a table.
Correct Answer: (c)
Rolling friction is the least because only a small area is in contact and interlocking is minimum when surfaces roll.
Correct Answer: (b)
For the same surfaces: Static friction is greatest, followed by sliding, and rolling friction is the least.
Correct Answer: (c)
A suitcase on wheels uses rolling friction, which is smaller than sliding friction, making movement easier.
Correct Answer: (c)
Rotating tyres roll on the road, so the friction involved is mostly rolling friction.
Correct Answer: (c)
Static friction increases or decreases to equal the applied force until it reaches its maximum value (limiting friction).
Correct Answer: (c)
In rolling, only a small contact region touches at a time, so interlocking and resistance are much less than in sliding.
Correct Answer: (c)
Using wheels makes the body roll instead of slide, changing sliding friction to much smaller rolling friction.
D. Advantages and Uses of Friction
Correct Answer: (c)
Walking requires adequate friction between feet and ground to provide grip; this is a helpful effect.
Correct Answer: (b)
Rough soles increase friction with the floor, giving better grip and preventing slipping.
Correct Answer: (a)
Holding a glass firmly requires friction between the glass surface and our hand so that it does not slip.
Correct Answer: (c)
Brakes work due to friction between brake shoes and wheels, which slows and stops the vehicle safely.
Correct Answer: (c)
Spikes increase friction between shoes and track, giving athletes better grip and control.
Correct Answer: (c)
Friction between rope surfaces prevents slipping, so the knot stays tight and secure.
Correct Answer: (c)
Wear and tear of shoe soles is a harmful effect of friction, not a useful one.
E. Disadvantages of Friction and Heating Effect
Correct Answer: (b)
Continuous friction between tyres and road gradually wears out the tyres, which is a disadvantage.
Correct Answer: (b)
Friction in machines converts useful mechanical energy into heat energy, leading to energy loss.
Correct Answer: (b)
Excessive friction produces heat and wear, causing loss of energy and reduced machine efficiency.
Correct Answer: (b)
When a matchstick is rubbed against the rough matchbox surface, friction produces heat to ignite the match.
Correct Answer: (c)
Large friction leads to wear and tear of moving parts, so machines require maintenance and part replacement.
Correct Answer: (c)
Friction in machines wastes energy as heat and causes damage to parts, both being undesirable.
F. Methods to Reduce and Increase Friction
Correct Answer: (b)
Materials that form a smooth film between surfaces and reduce friction are called lubricants.
Correct Answer: (c)
Lubricants fill up microscopic irregularities and create a thin smooth layer, so contact between rough surfaces is reduced.
Correct Answer: (b)
Ball bearings use small balls to change sliding friction into rolling friction, which is much smaller.
Correct Answer: (c)
Rough surfaces have more irregularities, which increases interlocking and therefore increases friction.
Correct Answer: (b)
Powder reduces friction between coins and board, so coins can slide easily with less force.
Correct Answer: (b)
Tyre treads increase friction with the road, especially on wet roads, giving better grip and safety.
G. Fluid Friction (Drag) and Streamlined Shapes
Correct Answer: (c)
Fluids exert a resistive force called drag or fluid friction on objects moving through them.
Correct Answer: (d)
Fluid friction depends on speed, shape and nature of the fluid, but not on the colour of the object.
Correct Answer: (b)
A streamlined shape (narrow front, wider middle, tapered back) allows fluid to flow smoothly, reducing drag.
Correct Answer: (b)
A fish has a streamlined body which helps it swim easily through water with less fluid friction.
Correct Answer: (b)
Streamlined shape reduces air drag, helping aeroplanes move faster and more efficiently with less fuel.
Correct Answer: (b)
The large surface of a parachute increases air resistance (fluid friction), slowing the skydiver for a safe landing.