Chapter 7: Work, energy & simple machine Quick Revision notes | Class 9th Science (Exploration) notes

1

Work Done by a Constant Force

Work done on an object by a constant force = Force applied × Displacement in the direction of the force.

W = F × s (work in joules, F in newtons, s in metres)

Fig 7.2 — Lifting bags: bigger force OR bigger distance = proportionally more work

Lifting 3 bags (one by one) to same height → 3× work vs lifting 1 bag
Lifting all 3 bags together (3× force) → same task → 3× work
Lifting 1 bag to 3× the height → 3× work

📏 SI Unit of Work — Joule (J)

1 J = 1 N × 1 m. So 1 joule = work done when 1 newton force displaces an object by 1 metre in the direction of the force.

1 J = 1 kg m² s⁻²

Even with a non-constant force, work done = area under the Force–Displacement graph between initial & final positions!

2

When is Work Done Equal to Zero?

Work done = 0 when:

Force = 0 (no force applied)
Displacement = 0 — e.g. pushing a rigid wall — you feel tired (muscles use internal energy) but scientifically zero work is done on the wall!
Force is perpendicular to displacement — e.g. a girl carrying a box horizontally applies an upward force to balance weight, but box moves horizontally → no work done by that force

Fig 7.5 — Pushing a wall: displacement = 0 → Work = 0

Fig 7.6 — Force ⊥ displacement → Work = 0

3

Positive & Negative Work Done

Type	Condition	Example
Positive Work	Displacement in SAME direction as force	Boy pushing a wheelchair forward
Negative Work	Displacement OPPOSITE to force direction	Goalkeeper stopping a moving ball

Positive work on the wheelchair

Negative work on the ball

Example 7.2: Goalkeeper’s hand moves back 15 cm stopping a ball, force = 200 N.
W = 200 N × (−0.15 m) = −30 J (negative — force opposite to displacement)

Note: While describing work done, always specify the FORCE (or agency) doing the work AND the object on which it’s done!

4

The Work-Energy Theorem

An object with the capacity to do work is said to possess energy. When positive work is done on an object, it gains energy; it can then transfer that energy to another object.

Work done on an object = Change in its Energy

James Prescott Joule — SI unit of work & energy named after him

Fig 7.13 — Slingshot & archery: stored energy → kinetic energy

SI unit of energy = same as work = joule (J). Energy can transfer as mechanical work, heat, radiation, electricity, sound, or via nuclear reactions!

Example — Carrom shot: Striker → hits white coin (does positive work, white coin gains energy; striker does negative work on itself by Newton’s 3rd law) → white coin hits black coin (positive work on black coin, negative work on white coin).

5

Forms of Energy

Fig 7.10 — Different forms of energy

Form	Description
Mechanical	Energy due to motion or position of objects
Thermal	Energy that makes things warm or hot
Light	Energy that allows us to see
Sound	Energy of vibrations of air/other molecules
Electrical	Energy related to position/motion of charges
Nuclear	Energy stored in the nuclei of atoms
Chemical	Energy stored in fuels/food (chemical bonds)

Energy converts between forms: Electrical→Light (bulb), Electrical→Thermal (heater), Chemical→Mechanical (muscles), Mechanical→Sound (bell)!

6

Mechanical Energy — Kinetic Energy

Mechanical energy = energy an object has due to its motion or position.

Kinetic energy (KE) = energy possessed by an object due to its motion. An object at rest has zero KE.

K = ½ m v² (SI unit: joule, J)

Positive work on object → velocity ↑ → KE ↑
Negative work on object → velocity ↓ → KE ↓
No work done (W=0) → velocity unchanged → KE constant
KE has no direction — it’s a scalar!

Example 7.4: If velocity doubles (v→2v), KE becomes 4× the original (KE ∝ v²)!

Example 7.6: Jet aircraft (mass 15000 kg) lands, wire exerts 367500 N over 100 m to stop it. Using work-energy theorem: landing velocity = 70 m/s = 252 km/h.

Fig 7.12 — Jet aircraft landing on a carrier

7

Potential Energy

Potential energy (PE) = energy stored by an object due to its deformation (stretched/compressed) OR due to the relative positions of objects in a system (gravitational, magnetic, electric).

Fig 7.15 — Systems of magnets & electric charges can also store PE

🌍 Gravitational Potential Energy

Object of mass m raised to height h above ground (PE = 0 at ground):

U = mgh

Greater height → deeper depression when a ball falls into sand → greater PE! (Activity 7.1)

Example 7.7: Ball of mass 200 g thrown 10 m high, g = 10 m/s².
PE = mgh = 0.2 kg × 10 m/s² × 10 m = 20 J

8

Conservation of Mechanical Energy

Mechanical Energy = Kinetic Energy + Potential Energy

When an object moves due to gravity alone (no friction/air resistance), its mechanical energy stays constant — as PE decreases, KE increases by the same amount, and vice versa!

Top: All PE, Zero KE→ Middle: Half PE, Half KE→ Bottom: All KE, Zero PE

🔔 Pendulum Demo (Activity 7.2)

At extreme point P: Only PE (KE=0)
At lowest point Q: Only KE (PE=0)
At other extreme R: Only PE again — reaches nearly the same height!

In real life, the pendulum eventually stops due to energy loss from friction & air resistance.

Example 7.8 (Slide): Using PE→KE conservation: ½mv² = mgh → v = √(2gh). Velocity depends ONLY on height h — not on the shape of the slide or the mass of the child!

Fig 7.22 — Ball roller coaster: PE↔KE conversion (later points are lower due to friction losses)

Example 7.9 (Escape ramp): Truck (10000 kg, 72 km/h) stopped by sand (50000 N) on a 30° ramp. Using energy conservation → minimum ramp length = 20 m.

Fig 7.21 — Escape ramp for trucks

9

Power

Power = rate at which work is done. Doing the same work faster (or more work in the same time) requires more power.

P = W / t (SI unit: watt, W = 1 J/s)

James Watt — unit “watt” named after him

1 horsepower (hp) = 746 W — old unit comparing engine power to actual horses!

Example 7.10: Weightlifter lifts 75 kg by 2 m in 5 s.
W = mgh = 75×10×2 = 1500 J → P = 1500/5 = 300 W

Example 7.11: Car (1000 kg) 0→72 km/h (20 m/s) in 10 s.
W = ΔKE = ½×1000×20² − 0 = 200000 J → P = 200000/10 = 20000 W

10

Simple Machines — Pulley

Simple machines make work feel easier by changing the magnitude or direction of the applied force — but they DON’T reduce the total work needed!

Effort = force we apply. Load = force to be overcome.

Mechanical Advantage = Load / Effort

🔗 Pulley

A wheel with a groove that guides a rope. A fixed pulley doesn’t reduce force needed — it only changes the direction (pull down instead of lift up). Mechanical advantage = 1.

A movable pulley / pulley system CAN give mechanical advantage > 1 — lift heavier loads with smaller effort (used in elevators, cranes).

11

Simple Machines — Inclined Plane

An inclined plane helps move a heavy load to a height using a smaller force — but over a larger distance.

Fig 7.26(a) — Lifting straight up: large force

Fig 7.26(b) — Pushed up ramp: smaller force

Fig 7.27(a) — Steeper plank: more force

Fig 7.27(b) — Gentler plank: less force

Mechanical Advantage = L / h (L = length of incline, h = height)

Since L > h always → mechanical advantage of an inclined plane is always > 1. Longer/gentler the ramp → smaller the effort needed (but you push it over a longer distance — total work stays the same)!

Example 7.12: Ramp: height 30 cm, width 40 cm → length = 50 cm (3-4-5 triangle).
Mechanical advantage = 50/30 = 1.67

This is why hill roads wind around in gentle slopes instead of going straight up — and why an inclined ladder is easier to climb than a vertical one!

12

Simple Machines — Lever

A lever = rigid bar that rotates about a fixed point. Three parts:

Fulcrum — the fixed point about which the lever rotates
Load — the force to be overcome (with its load arm — distance from fulcrum)
Effort — the force applied (with its effort arm — distance from fulcrum)

Fig 7.31 — A heavier stapler lifted using a lighter eraser!

Effort × Effort arm = Load × Load arm → Mechanical Advantage = Effort arm / Load arm

Increasing the effort arm → larger force applied to the load with smaller effort. But effort has to move a LARGER distance — total work stays the same!

Activity 7.5 — A beam balance is a lever! n₁×L₁ = n₂×L₂

Class	Arrangement	Examples
Class I	Fulcrum in between Load & Effort	Tongs, scissors, crowbar, pliers, seesaw
Class II	Load in between Fulcrum & Effort	Lemon squeezer, wheelbarrow, bottle opener
Class III	Effort in between Fulcrum & Load	Tweezers, broom, hammer, oar

Table 7.2 — Everyday examples of the 3 lever classes

Example 7.13 (Seesaw): AC=EC=2m, BC=DC=1m. Child of 15 kg sits at A; where should a 30 kg child sit?
15×2 = 30×L → L = 1 m → sits at seat D

Fig 7.35 — A spoon as a lever to open a lid

Fig 7.36 — Gharat/Panchakki: PE of water → KE → grinds grain!

Machines don’t create energy — they only help us use it more effectively. In every case, conservation of mechanical energy holds: work put in = useful work done on the load (ignoring friction).

13

Key Formulas Cheat-Sheet

Quantity	Formula	SI Unit
Work	W = F × s	joule (J)
Work-Energy Theorem	W = ΔEnergy	joule (J)
Kinetic Energy	K = ½mv²	joule (J)
Potential Energy	U = mgh	joule (J)
Mechanical Energy	K + U (conserved, no friction)	joule (J)
Power	P = W / t	watt (W)
Mechanical Advantage	Load / Effort	no unit
Inclined Plane MA	L / h	no unit
Lever MA	Effort arm / Load arm	no unit

1 hp = 746 W · g = 10 m/s² (used in examples) · 1 kWh (household unit) ≈ 3.6 × 10⁶ J

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At a Glance — Full Chapter Recap

Work is done by a force when it displaces an object in the direction of the force
An object with the capacity to do work possesses energy
Work-Energy Theorem: Work done on an object/system = change in its energy
Kinetic energy = energy due to motion; Potential energy = energy due to deformation/position
Mechanical energy (KE + PE) is conserved when only gravity acts (no friction)
Power = rate of doing work
Simple machines (pulley, inclined plane, lever) make work easier by changing force magnitude/direction — but never reduce total work done

✨ Notes prepared by @edugrown ✨