UPSC Prelims 2013 Q88: Science & Technology – Physics
Ball bearings are used in bicycles, cars, etc., because
- (a) the actual area of contact between the wheel and axle is increased
- (b) the effective area of contact between the wheel and axle is increased
- (c) the effective area of contact between the wheel and axle is reduced
- (d) None of the above statements is correct
Answer: (c) the effective area of contact between the wheel and axle is reduced
Explanation
Why ball bearings are used
Rolling friction is far smaller than sliding friction.
Ball bearings put small steel balls between the wheel and the axle.
The parts then roll over each other, touching at points only.
| Option | Verdict | Reason |
|---|---|---|
| (a) The actual area of contact is increased | Incorrect | Contact is reduced |
| (b) The effective area of contact is increased | Incorrect | Contact is reduced |
| (c) The effective area of contact is reduced | Correct | Balls touch at points; sliding becomes rolling |
| (d) None of the above | Incorrect | Option (c) is right |
Types of friction, largest to smallest:
| Type | Example |
|---|---|
| Static | A box that will not start moving |
| Sliding | A box dragged on the floor |
| Rolling | A wheel or ball |
Other ways to reduce friction: lubricants (oil, grease), polishing, streamlining.
Answer: (c) the effective area of contact between the wheel and axle is reduced.
Why each option is right or wrong
- (a) Incorrect. Ball bearings do not increase the area of contact.
- (b) Incorrect. The effective area of contact is reduced, not increased.
- (c) CORRECT. Ball bearings reduce the effective area of contact and replace sliding by rolling.
- (d) Incorrect. Option (c) gives the right reason.
Key point
Ball bearings convert sliding friction into much smaller rolling friction by reducing the effective contact between wheel and axle.
Remember
Ball bearings = rolling friction, less contact.
Common mistakes
- Thinking more contact area makes motion smoother.
- Forgetting the order: static > sliding > rolling friction.
- Confusing ball bearings with lubrication, a separate method.
Exam tip
Rolling beats sliding. That is why wheels and ball bearings exist.
Sources
- NCERT Class VIII — Science, Ch 12 (Friction)
- NCERT Class XI — Physics Part I (Laws of Motion: friction)