NCERT Solutions for Class 9th Science Chapter 7 Work, Energy, and Simple Machines
Updated on 2026-09-19
About this chapter
Work done by a constant force = force × displacement in the direction of the force, W = F × s. Its SI unit is the joule: 1 J = 1 N × 1 m = 1 kg m 2 s –2 . Work is zero when F = 0, when s = 0, or when the force is perpendicular to the displacement. Work is positive when the displacement is along the force and negative when it opposes the force. Work has no direction — only a sign. For a force that varies, the work done is the area under the force–displacement graph . Work–energy theorem: work done on an object (or system) = change in its energy. This holds even when the forces are not constant, and it is the tool that gives every other formula in the chapter. Kinetic energy K = ½mv 2 — the energy of motion. Gravitational potential energy U = mgh near the Earth's surface — the energy of posi
- Chapter opener
- .1 Work Done by a Constant Force
- .2 The Work-Energy Theorem
- .4.1 Kinetic energy
- .4.2 Potential energy
- .4.3 Conservation of mechanical energy
- .6.2 Inclined plane
- .6.3 Lever
- End-of-chapter exercise
- Project work
Quick revision
| Term | What it means | Formula or key rule | Where it appears in the chapter |
|---|---|---|---|
| Work | Force acting on an object while the object is displaced along that force | W = F × s; SI unit joule (J), 1 J = 1 N m | Section 7.1, Eqs. 7.1 and 7.2; Fig. 7.2 lifting wheat bags |
| Zero work | No force, no displacement, or force perpendicular to displacement | W = 0 even though you feel tired | Section 7.1.1; Fig. 7.5 pushing a wall, Fig. 7.6 carrying a box |
| Negative work | Force opposite to the displacement — the force takes energy away | W = F × (– s) | Section 7.1.2; Example 7.2, goalkeeper stopping a ball |
| Force–displacement graph | Area under the graph gives the work done, even for a varying force | Area = work; 10 N × 1 m = 10 J | Fig. 7.4, page 118; Question 11, Fig. 7.37 |
| Energy | Capacity to do work; joule (J) is its SI unit too | Mechanical, thermal, light, sound, electrical, chemical, nuclear | Section 7.3, Fig. 7.10 |
| Work–energy theorem | Work done on an object appears as a change in its energy | work done = change in energy (Eq. 7.3) | Section 7.2; used to derive Eqs. 7.5, 7.6 and 7.8 |
| Kinetic energy | Energy an object has because it is moving | K = ½mv2 (Eq. 7.6); W = ½m(v2 – u2) | Section 7.4.1; Examples 7.4, 7.5, 7.6 |
| Potential energy | Energy stored by deformation, or by the relative positions of objects in a system | U = mgh near the Earth's surface (Eq. 7.8) | Section 7.4.2; Activity 7.1, Example 7.7 |
| Conservation of mechanical energy | K + U stays constant when only gravity acts | mgh at the top = ½mv2 at the bottom | Section 7.4.3; Fig. 7.19, Activity 7.2, Examples 7.8, 7.9 |
| Power | Rate at which work is done | P = W/t (Eq. 7.11); 1 W = 1 J s–1; 1 hp = 746 W | Section 7.5; Examples 7.10, 7.11 |
| Mechanical advantage | How much a machine multiplies the force you apply | MA = load / effort (Eq. 7.12) | Section 7.6 |
| Pulley | Grooved wheel guiding a rope; a fixed pulley only changes the direction of the effort | MA of a fixed pulley = 1 | Section 7.6.1; Figs. 7.23–7.25 |
| Inclined plane | Ramp that raises a load with a smaller force over a longer path | MA = L/h (Eq. 7.13); F′ × L = mgh | Section 7.6.2; Activity 7.3, Example 7.12 |
| Lever | Rigid bar turning about a fulcrum; three classes by the order of fulcrum, load and effort | effort × effort arm = load × load arm (Eq. 7.15); MA = effort arm / load arm | Section 7.6.3; Activities 7.4 and 7.5, Table 7.2, Example 7.13 |
| Gharat / panchakki | Himalayan watermill: the potential energy of falling water turns a wheel that drives a grinding stone | U of water → K of water → rotation of wheel → grinding | Bridging Science and Society, page 136, Fig. 7.36 |
Exercises
- Chapter opener — Think It Over Page 116
- Chapter opener — In-text Questions Page 116
- .1 Work Done by a Constant Force — Pause and Ponder Page 1197
- .2 The Work-Energy Theorem — In-text Questions Page 1207
- .2 The Work-Energy Theorem — Pause and Ponder Page 1217
- .4.1 Kinetic energy — In-text Questions Page 1217
- .4.1 Kinetic energy — Pause and Ponder Page 1237
- .4.2 Potential energy — Activity 7.1: Let us investigate Page 1257
- .4.2 Potential energy — Pause and Ponder Page 1267
- .4.3 Conservation of mechanical energy — Activity 7.2: Let us experiment Page 1277
- .4.3 Conservation of mechanical energy — Pause and Ponder Page 1297
- .6.2 Inclined plane — In-text Questions Page 1317
- .6.2 Inclined plane — Activity 7.3: Let us experiment Page 1317
- .6.2 Inclined plane — Pause and Ponder Page 1327
- .6.3 Lever — Activity 7.4: Let us investigate Page 1337
- .6.3 Lever — In-text Questions Page 1337
- .6.3 Lever — Activity 7.5: Let us experiment Page 1347
- .6.3 Lever — Pause and Ponder Page 1357
- .6.3 Lever — What If … Page 1357
- End-of-chapter exercise — Revise, Reflect, Refine Page 136 – 138
- Project work — The Journey Beyond Page 139