Science · Class 8 · Chapter 5

Exploring Forces

Aim: Build a clear, working understanding of this chapter's key ideas, connected to real NCERT examples, worked problems, and everyday situations.
  • Force — A push or pull on an object; SI unit newton (N).
  • Contact force — Acts only when objects are touching.
  • Friction — Opposes motion between touching surfaces.
  • Non-contact force — Acts even without touching.
  • Gravitational force — Earth's pull on objects; always attractive.
  • Weight — Force of Earth's pull on an object.
  • Mass — Amount of matter in an object; constant everywhere.
  • Buoyant force (Upthrust) — Upward force a liquid exerts on an object.
40 min lesson

Chapter 5 · Concept 1 of 8

What Is a Force?

  • Moving a box by pushing, pulling, or lifting all involve applying a force to it.
  • Force — A push or pull applied on an object. Its SI unit is the newton (N).

Chapter 5 · Concept 2 of 8

Effects of Force

  • Start motion — Make a resting object move.
  • Change speed — Speed something up or slow it down.
  • Change direction — Turn an object's path.
  • Change shape — Squash, stretch, or bend an object.

Chapter 5 · Concept 3 of 8

Force Is an Interaction

  • Two objects, always — A force always involves at least two objects interacting — a hand pushing a table means both the hand and the table experience a force. The force disappears the moment the interaction stops.

Chapter 5 · Concept 4 of 8

Contact Forces

  • Muscular force — The force from your muscles — walking, lifting, pushing — and even internally, like chewing or your heartbeat.
  • Friction — Opposes motion between two surfaces in contact. Rougher surfaces mean more friction — and liquids and gases resist motion too, which is why planes and ships are streamlined.

Chapter 5 · Concept 5 of 8

Magnetic and Electrostatic Force

  • Magnetic force — A magnet acts on another magnet or magnetic material without touching it. Like poles repel, unlike poles attract.
  • Electrostatic force — A charged object acts on another charged or uncharged object. Rubbing certain materials builds up static charge — like charges repel, unlike charges attract.

Chapter 5 · Concept 6 of 8

Gravitational Force

  • The Earth attracts every object toward itself — this is gravitational force.
  • Only ever attractive — Unlike magnetic or electrostatic force, gravity can never push, only pull — it's why a thrown ball always falls back down.

Chapter 5 · Concept 7 of 8

Weight vs Mass

  • Location | Weight of a 1 kg mass
  • Earth — 10 N
  • Moon — 1.6 N
  • Mars — 3.8 N
  • Venus — 9 N
  • Jupiter — 25.4 N

Chapter 5 · Concept 8 of 8

Why Things Float or Sink

  • Buoyant force (upthrust) — A liquid pushes an immersed object upward, while gravity pulls it down. If gravity wins, it sinks; if the two balance, it floats.
  • Archimedes' Principle — An object immersed in a liquid experiences an upward force equal to the weight of the liquid it displaces.

Quick Recap

Check Your Understanding

  1. 1Match each type of force to its example: Muscular force, Magnetic force, Frictional force, Gravitational force, Electrostatic force.
  2. 2True or False: (i) A force is always needed to change an object's speed. (ii) Friction increases a rolling ball's speed. (iii) There's no force between two nearby charged objects.
  3. 3Two balloons, each rubbed with a woollen cloth, are brought near each other. What happens, and why?