Question
List the various ways to reduce friction and explain how each method works.
(NCERT Class 8, Chapter: Friction)
Solution — Step by Step
Friction happens because surfaces have tiny irregularities — microscopic bumps and grooves — that interlock when two surfaces are in contact. To reduce friction, we either separate the surfaces, make them smoother, or change their shape.
A lubricant fills the tiny gaps between surfaces and creates a thin layer that prevents direct contact. Machine engines use oil, door hinges use grease, and locks use dry graphite powder because each material suits the operating condition.
The key idea: the surfaces are no longer rubbing against each other — they’re sliding against the lubricant layer, which has much lower friction.
Ball bearings replace sliding friction with rolling friction. Rolling friction is significantly smaller than sliding friction — this is why wheels were invented.
Inside a ball bearing, small steel balls sit between two rings. Instead of the axle grinding against its housing, it rolls on these balls. Bicycles, motors, and car wheels all use this principle.
Sanding or polishing removes large irregularities from a surface. Smoother surfaces have fewer interlocking bumps, so friction decreases. This is why machine parts are precision-finished to very tight tolerances.
When objects move through air or water, they experience fluid friction (drag). A streamlined shape — tapered at the front, smooth at the back — lets fluid flow around the object without creating turbulent zones.
Fish, aircraft, and high-speed trains are all streamlined. This isn’t about surface contact; it’s about reducing friction with the surrounding fluid.
Why This Works
Friction comes from surface irregularities and adhesive forces between molecules in contact. Every method of reducing friction targets one or both of these causes.
Lubricants work by separating surfaces — the fluid film means molecules from surface A never actually touch molecules from surface B. Ball bearings work by converting sliding into rolling — and rolling friction is lower because the contact is nearly a single point rather than a full surface.
Streamlining works on a completely different principle. Fluid friction depends on the shape of the object and how smoothly the fluid can move around it. A pointed front reduces the pressure buildup; a tapered rear prevents the fluid from “separating” and creating drag.
For your exam, remember this hierarchy: rolling friction < sliding friction < static friction. Ball bearings exploit this by converting sliding into rolling. This single fact explains many real-world engineering choices.
Alternative Method — Remembering All Methods with a Real Example
Think of a bicycle — it uses almost every friction-reduction technique in one machine:
- Ball bearings in the wheel hub and pedal crank
- Oil/grease on the chain and gears
- Smooth, polished metal surfaces on the axle
- Streamlined frame design on racing cycles
Working through a single, familiar example is often faster in a short-answer exam than memorising a raw list.
Common Mistake
Many students write “smooth surfaces have no friction.” This is wrong — even perfectly smooth surfaces have some friction. Lubricants and polishing reduce friction, they don’t eliminate it. In fact, surfaces polished to an extreme degree can actually stick together more (called “cold welding”) because the molecules get too close. Always say “reduces friction” not “removes friction.”
Summary of methods:
| Method | What it reduces | Example |
|---|---|---|
| Lubricants (oil, grease) | Sliding friction | Engine oil, door hinges |
| Ball/roller bearings | Sliding → rolling friction | Bicycle wheels, motors |
| Polishing surfaces | Sliding friction | Machine parts |
| Streamlined shapes | Fluid friction (drag) | Aircraft, fish, trains |
The one-line answer: We reduce friction by using lubricants, ball bearings, polishing surfaces, and streamlining shapes — each method either separates surfaces, converts sliding to rolling, or smooths the path through a fluid.