Exploring Forces — Practice

Question 1

Give two examples each of contact and non-contact forces.

Accepted Answer

✦ Answer: CONTACT: muscular force (pushing a trolley), friction (shoe on road). NON-CONTACT: gravitational force (apple falling), magnetic force (magnet attracting iron).

Question 2

Why does a moving ball eventually stop?

Accepted Answer

✦ Answer: Due to FRICTION between the ball and the ground (and air resistance). Friction opposes motion, gradually slowing the ball until it stops.

Question 3

A 70 kg person stands on Earth and on Moon. Calculate weight in each place.

Accepted Answer

Step 1 — Mass remains constant.
  Mass = 70 kg (same on Earth, Moon, anywhere).

Step 2 — Earth weight.
  W = m × g = 70 × 9.8 = 686 N

Step 3 — Moon weight.
  g_moon = 1.6 m/s² (about 1/6 of Earth's)
  W = 70 × 1.6 = 112 N

Step 4 — Ratio.
  Earth weight / Moon weight = 686/112 ≈ 6.1
  Moon weight is about 1/6 of Earth weight ✓

Step 5 — Significance.
  Astronauts on Moon can jump much higher because they weigh less (though mass is same).

✦ Answer: Mass = 70 kg everywhere. Earth weight = 686 N. Moon weight = 112 N (about 1/6 of Earth's, because Moon's gravity is 1/6).

Question 4

Explain the role of friction in different aspects of daily life — both helpful and harmful — with at least 5 examples and methods to manage friction.

Accepted Answer

Step 1 — Helpful aspects of friction.
  (a) WALKING: Friction between shoes and ground prevents slipping. Without friction (e.g., ice), we slip and fall.
  (b) WRITING: Pen/pencil tip grips paper due to friction. Without friction, pen would just slide.
  (c) HOLDING OBJECTS: Hand grips objects through friction. Without it, things slip away.
  (d) BRAKES: Vehicle brakes use friction (brake pad against wheel disc) to stop. Without friction, vehicles couldn't stop.
  (e) LIGHTING MATCH: Friction between match head and striker generates heat to ignite.
  (f) Striking flint to start fire — ancient use of friction.
  (g) Climbing: rough rocks give grip; smooth glass surfaces are hard to climb.

Step 2 — Harmful aspects.
  (a) WEAR AND TEAR: Engine parts wear out due to friction.
  (b) WASTED ENERGY: Friction converts useful kinetic energy to heat. Cars waste ~20% energy to friction.
  (c) HEAT GENERATION: Can damage parts (overheating in engines).
  (d) RESISTANCE TO MOTION: Boats, planes need extra power to overcome fluid friction.
  (e) FOOT BLISTERS: Friction from ill-fitting shoes.
  (f) Bicycle chain rusts/wears due to friction.
  (g) Door hinges squeak due to friction.

Step 3 — Methods to REDUCE friction.
  • LUBRICATION: Oil, grease in machines
  • POLISHING: Make surfaces smoother
  • BALL BEARINGS: Convert sliding to rolling (much less friction)
  • STREAMLINED SHAPES: For ships, planes, cars (less fluid friction)
  • TEFLON COATINGS: Non-stick pans use Teflon to reduce friction

Step 4 — Methods to INCREASE friction.
  • TREADS on tyres and shoe soles
  • SAND on icy roads (Indian winter regions)
  • CHALK on cricket bat handles
  • ROUGH SURFACES for grip (e.g., gym equipment)
  • POWDER on hands (gymnasts, weightlifters)

Step 5 — Indian context.
  Bullock carts use grease on axles to reduce friction. Indian Railways uses ball bearings in wheels. Brakes use friction. Even Olympic medal Indian wrestlers use friction (mat grip).

Step 6 — Engineering balance.
  Engineers must balance: reducing friction where it hurts (engine parts), and using friction where needed (brakes, tyre grip). This balance is key to all modern machines.

✦ Answer: Friction has DUAL nature. Helpful: walking, writing, gripping, braking, lighting match. Harmful: wear and tear, energy loss as heat, machine overheating, resistance. Reduce friction: lubricants, polishing, ball bearings, streamlining. Increase friction: treads, sand, rough surfaces, chalk. Engineering: balance both — reduce where harmful, increase where useful.

Practice — Exploring Forces

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