5.7 Going Deeper: Enrichment & Exam Preparation

Forces All Around You

Back to the Bicycle Ride Sonali and Ragini’s ride was a parade of forces: muscular force on the pedals, the wind’s push, friction slowing them on rough road and gripping their tyres on turns, gravity speeding them downhill, and the electrostatic crackle when wool brushes a balloon at the party afterward. Once you can name the forces, ordinary life turns into a physics lab.

Quick Reference

Force Type Acts…
Muscular Contact When muscles push/pull (walking, lifting)
Friction Contact Opposite to motion; from surface irregularities
Magnetic Non-contact Between magnets/magnetic materials (attract or repel)
Electrostatic Non-contact Between charged bodies (attract or repel)
Gravitational Non-contact Earth’s pull; always attractive
Quantity Meaning Unit Changes?
Mass Amount of matter g / kg Same everywhere
Weight Earth’s gravitational pull newton (N) Varies with gravity
Buoyant force Liquid’s upward push newton (N) Depends on liquid & object

Memory Tricks & One-Page Revision

Revision Card

  • Force = push or pull, needs two interacting objects, unit newton (N).
  • A force can: start motion · change speed · change direction · change shape.
  • Contact: muscular, friction. Non-contact: magnetic, electrostatic, gravitational. (Mnemonic: “MaG-E-G act at a distance.”)
  • Friction opposes motion and comes from surface bumps. Smoother → less friction → easier to slip.
  • Like repels, unlike attracts — true for both magnetic poles and electric charges. Gravity only attracts.
  • Mass (kg) is constant; weight (N) varies. Moon weight ≈ 1/6 of Earth’s; mass unchanged.
  • Sink if weight > buoyant force; float if they’re equal.

Spot the Mistake

Find the Flaw

  1. “Friction speeds up a rolling ball.” — False: friction acts opposite to motion, so it slows the ball down.
  2. “The weight of this object is 5 kg.” — Wrong unit: kilograms measure mass; weight is in newtons.
  3. “On the Moon the object’s mass becomes one-sixth.” — False: its weight becomes ~1/6; its mass stays the same.

Test Your Reflexes

A quick-fire drill: an example of a force appears. Is it a Contact force or a Non-contact force? Build a streak!

Contact or non-contact?
Does it need the objects to touch?
Score 0 · Streak 0

Exam Corner: CBSE-Style Practice

Mixed Practice (objective, short, long, HOTS)

Objective type (1 mark each)

  1. The SI unit of force is the ______.
  2. Friction always acts in a direction ______ to the motion of an object.
  3. The upward force a liquid exerts on an object is called ______.

Short answer (2 marks each)

  1. List the four effects a force can have on an object.
  2. Give one difference between mass and weight, with their units.

Long answer (3 marks each)

  1. Classify these forces as contact or non-contact and give one example of each: muscular, friction, magnetic, electrostatic, gravitational.
  2. A ball thrown straight up slows, stops, and falls back. Name the force acting and its direction (i) going up, (ii) at the top, (iii) coming down.

HOTS (Higher Order Thinking)

  1. Three identical-looking objects float at different depths in water. What does this tell you about their weights, and which force balances gravity for the floating ones?
  2. A ball released from point P on a ramp stops at A on the floor. Suggest one change so it stops before A, and one so it stops after A. (Hint: think friction.)

Assertion–Reason (Choose: (a) both true, R explains A; (b) both true, R doesn’t explain A; (c) A true, R false; (d) A false, R true.)

  1. Assertion (A): An object weighs less on the Moon than on the Earth.   Reason (R): The Moon’s gravitational pull is weaker than the Earth’s.
  1. newton (N).
  2. opposite.
  3. buoyant force (upthrust).
  4. A force can start motion, change speed, change direction, and change shape.
  5. Mass is the amount of matter (unit kg/g) and is the same everywhere; weight is the Earth’s gravitational pull (unit N) and varies from place to place.
  6. Contact: muscular (lifting a bag), friction (a ball stopping). Non-contact: magnetic (magnet and pin), electrostatic (rubbed balloon and hair), gravitational (a falling fruit).
  7. The force is gravity, always directed downward. (i) Going up: gravity acts down, opposing motion, so the ball slows. (ii) At the top: gravity still acts down; the ball is momentarily at rest. (iii) Coming down: gravity acts down, along the motion, so the ball speeds up.
  8. Different depths mean the objects have different weights (heavier ones sink deeper). For a floating object, the buoyant force balances gravity (the two are equal).
  9. To stop before A: increase friction (rougher floor) or release from a lower point. To stop after A: reduce friction (smoother floor) or release from higher up P. (Either change to the friction or the starting height is acceptable.)
  10. (a) — Both true and R explains A: weaker Moon gravity means a smaller pull, hence less weight (mass stays the same).

Glossary

Key Terms

  • Force: a push or pull from the interaction of two objects; unit newton (N).
  • Contact force: acts only on touching (muscular, friction).
  • Friction: force opposing motion between surfaces in contact.
  • Non-contact force: acts from a distance (magnetic, electrostatic, gravitational).
  • Gravitational force / gravity: the Earth’s attractive pull on objects.
  • Weight: the gravitational force on an object (N); varies with gravity.
  • Mass: the amount of matter (kg); constant everywhere.
  • Buoyant force (upthrust): the upward force a liquid exerts on an object.