This MCQ module is based on: Contact Forces and Friction
TOPIC 16 OF 50
Contact Forces and Friction
🎓 Class 8
Science
CBSE
Theory
Ch 5 — Coal and Petroleum
⏱ ~29 min
🌐 Language: [gtranslate]
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Recap and Set-up
In Part 1, we saw that a force is a push or pull, measured in newton (N). In real life, forces show up in many forms. Scientists group them into two big families depending on whether the two objects must actually touch or not: contact forces and non-contact forces. This part explores contact forces in detail.
5.3 Contact Forces
A contact force is a force that one object applies on another only while the two are in physical contact. Pushing a table, pulling a rope, kicking a ball — these all need touch. Two important contact forces in our daily life are muscular force and friction.
5.3.1 Muscular Force
Our muscles contract and relax to pull on bones. This internal pull lets us push doors, lift bags, pedal bicycles, write in our notebooks, and kick footballs. The force our muscles produce is called muscular force. Animals also use muscular force — a bullock pulling a plough, a horse pulling a carriage, a mule carrying loads up a hillside.
Muscular Force: The force applied by the muscles of a human or animal body. It acts only when the muscles are in contact with something (directly, through bones, or through a rope/handle).
5.3.2 Friction — The Force That Resists Motion
Roll a marble on a smooth tiled floor. It rolls for a while and then stops — even though nothing blocks its path. Why does it stop? A hidden force is opposing its motion. This force is called friction.
Friction: The contact force that opposes the relative motion (or tendency of motion) between two surfaces that are touching each other.
Friction arises because no surface is perfectly smooth. Even glass, when seen under a microscope, has tiny bumps and valleys. When two surfaces slide over each other, their microscopic bumps catch on each other and resist the motion.
🔬 Activity 5.4 — Book on Different FloorsL3 Apply
🤔 Predict first: On which surface will it be hardest to push the book — polished tiles or a woollen carpet?
- Place the same hard-cover book on a polished tile floor. Push it with your finger and feel how easily it slides.
- Now place the book on a thick carpet or rug. Push with the same finger.
- Finally, place the book on a sheet of sand-paper. Try again.
Observation: The book is easiest to slide on tiles, harder on carpet, hardest on sand-paper. The rougher the surface, the greater the friction between book and surface.
Types of Friction
Not all friction behaves the same way. Depending on whether an object is still, sliding, or rolling, scientists name three types:
Static Friction
Acts on an object that is at rest when you try to push or pull it. It stops the object from starting to slide until the applied force is large enough.
Sliding Friction
Acts between two surfaces that are sliding against each other. Usually less than static friction — that's why a box needs a strong shove to start but less to keep going.
Rolling Friction
Acts when one surface is rolling over another (wheel on road, ball bearings in a fan). It is the smallest — so wheels and bearings make machines efficient.
Did you know? Ancient Egyptians and Indians discovered very early that logs placed under heavy stones reduced friction from sliding to rolling. This trick let them move enormous stone blocks to build pyramids and temples. The modern version is the ball bearing inside every fan, bicycle, and car wheel.
5.4 Friction — Friend and Foe
Friction is sometimes a helpful friend and sometimes a troublesome foe. The same force that lets us walk also wastes energy in machines.
Friction as a Friend (Useful Friction)
- Walking: We grip the ground with our feet. Without friction (think of a frozen pond), we slip!
- Holding things: Friction between your fingers and a pencil lets you write. A soapy wet pen slips out of your hand.
- Braking: Friction between brake pads and wheels stops cycles, cars, and trains safely.
- Striking a match: Friction between the match head and the rough strip on the matchbox creates heat, igniting the chemical.
- Writing: Friction between pen tip and paper deposits ink. On a plastic sheet, ink rolls off.
Friction as a Foe (Harmful Friction)
- Wear and tear: Shoes, tyres, pencil nibs, and machine parts slowly wear out because of friction.
- Wasted energy: A lot of the fuel burned by a vehicle is lost as heat in moving parts.
- Heating of machines: Machines get hot and sometimes damaged; we add oil and fans to keep them cool.
- Slower motion: Friction slows down objects, so more fuel or force is needed to keep them moving.
Reducing and Increasing Friction
Because friction can be useful or harmful, we sometimes want more of it and sometimes less.
| How to REDUCE friction | How to INCREASE friction |
|---|---|
| Polish surfaces smooth | Make surfaces rough (sandpaper, knurling) |
| Apply lubricants — oil, grease, graphite powder | Use rubber grips on handles |
| Use ball bearings / wheels instead of sliding | Use tyres with deep treads for road grip |
| Streamline shape to reduce fluid drag | Spread rugs/mats on slippery floors |
| Float on air/water cushion (hovercraft) | Sprinkle sand on icy roads |
5.5 Fluid Friction (Drag)
Not only solid surfaces cause friction. Liquids and gases — which together we call fluids — also resist motion through them. This type of friction is called fluid friction or drag.
Try pushing your hand through water in a bucket — you feel resistance. Stick your hand out of a moving car's window — the air pushes back. Fish, dolphins, airplanes, bullet trains, and racing cars face the same challenge.
Streamlined Shapes
To reduce drag, nature and engineers give moving objects a streamlined shape — pointed at the front, tapered at the back. Fish have it, birds have it, airplanes have it, and so do bullet trains and sports cars.
Sometimes, though, we want drag. A parachute opens into a wide canopy to catch lots of air and slow down a falling skydiver safely.
🎯 Match It — Reduce or Increase Friction? L3 Apply
Click each scenario to reveal whether we want to reduce or increase friction.
1. Oiling the chain of a bicycle
2. Tyres of cars having deep grooves
3. Using ball bearings in a ceiling fan
4. Wrapping a cricket bat handle with rubber
5. Designing an aeroplane with a pointed nose
6. Rough side of a matchbox
📋 Competency-Based Questions
Rhea's father is packing for a long car journey. He checks the tyres — he notices two of them are very smooth. He also feels the car engine making a screeching sound and the mechanic mentions the engine oil needs to be changed. On the way, there is a sudden rain shower and the road becomes wet.
Q1. L2 Understand Why are smooth tyres dangerous on a wet road?
Answer: Smooth tyres have very little grip (less friction) with the road. On a wet road the water makes things worse — the car can slide without stopping when brakes are applied. Tyres need deep treads to increase friction.
Q2. L3 Apply The mechanic says, "Oil reduces friction inside the engine." Is this good or bad for the engine?
Answer: B. Oil forms a thin layer between moving parts and replaces harsh sliding friction with smooth liquid layers — parts last longer and the engine uses less fuel.
Q3. L1 Remember Arrange these types of friction in decreasing order: rolling, static, sliding.
Order: Static > Sliding > Rolling. That is why a heavy trolley is hardest to start (static), a bit easier once sliding, and much easier on wheels (rolling).
Q4. L4 Analyse A coin dropped into honey falls slowly but the same coin falls quickly through air. Explain.
Analysis: Both honey and air are fluids, so both exert fluid friction (drag). But honey is a much thicker fluid than air, so its drag on the coin is very large — the coin falls slowly. Air offers little drag, so the coin falls quickly.
Q5. L5 Evaluate A student says, "Friction should be removed completely from all machines so they run forever." Evaluate this claim.
The claim is wrong. Machines also need friction in some places — for example, brakes must have friction to stop the machine, and conveyor belts need friction to carry goods. Removing all friction would make it impossible to control the machine or transfer forces through belts, pulleys and gears.
🔗 Assertion–Reason Questions
Assertion (A): It is easier to drag a heavy box on wheels than to slide it on the floor.
Reason (R): Rolling friction is less than sliding friction.
Answer: A. Wheels convert sliding friction into the much smaller rolling friction, so less force is needed.
Assertion (A): Aeroplanes and bullet trains have streamlined bodies.
Reason (R): Streamlined shapes reduce fluid friction (drag) and save fuel.
Answer: A. A streamlined shape lets air glide around the vehicle, cutting drag and saving energy.
Assertion (A): Walking on a highly polished marble floor is difficult.
Reason (R): Polished surfaces have very low friction.
Answer: A. Walking needs friction between foot and ground to push off. On smooth marble the friction is so low that feet slip.
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