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In everyday speech, holding a heavy bag for ten minutes is hard work. In physics it is no work at all. That gap between the two meanings is why this chapter feels easy and still costs marks. This post covers what the 2026–27 syllabus asks, explains each idea with a worked calculation, and ends with a routine you can use this week.
What does the 2026–27 chapter include?
The new NCERT book *Exploration* calls this Chapter 7, Work, Energy, and Simple Machines. The CBSE Class IX syllabus for 2026–27 lists work done by a constant force, the work-energy theorem, kinetic and potential energy, conservation of energy, power, and simple machines with mechanical advantage (pulley, inclined plane, lever). Simple machines are new compared with the 2025–26 syllabus, which listed work, energy and power without them. Check the syllabus for your exam year.
When is work done, and when is it negative?
Work done by a constant force is force × displacement in the direction of the force: W = F × s, measured in joules (1 J = 1 N × 1 m).
Zero work: pushing a wall that does not move (s = 0), or carrying a bag across a flat floor, where your upward force is at right angles to the motion.
Positive work: force and displacement in the same direction, as when you push a wheelchair forward.
Negative work: force opposite to displacement. NCERT's goalkeeper stops a ball with 200 N while her hands move back 15 cm, so she does 200 × (−0.15) = −30 J of work on the ball.
The work-energy theorem links this to energy: the work done on an object equals the change in its energy.
How do you calculate kinetic and potential energy?
Kinetic energy is K = ½mv². Potential energy of an object raised to height h near the Earth's surface is U = mgh.
Example 1: why speed matters so much. A 1000 kg car moves at 54 km/h.
Convert: 54 km/h = 15 m/s.
K = ½ × 1000 × 15² = 112,500 J.
At 108 km/h (30 m/s): K = ½ × 1000 × 30² = 450,000 J, four times as much. Doubling speed quadruples kinetic energy because v is squared.
Example 2: a falling ball. A 2 kg ball is dropped from 5 m. Take g = 10 m/s² and ignore air resistance.
At the top: U = 2 × 10 × 5 = 100 J, K = 0.
Halfway down (2.5 m): U = 50 J, so K = 50 J. Speed: ½ × 2 × v² = 50, so v = √50 ≈ 7.1 m/s.
Just before landing: U = 0, K = 100 J, so v = √100 = 10 m/s.
The total stays at 100 J throughout. This is the conservation of mechanical energy, and it gives the same answer as v² = 2gh from the motion chapter. NCERT's slide example makes the same point: the speed at the bottom of a frictionless slide depends only on its height, not on its shape or on the child's mass.
What is power?
Power is the rate of doing work: P = W/t, in watts (1 W = 1 J/s).
Example 3: climbing stairs. A 50 kg student climbs to a floor 12 m higher in 30 s. Take g = 10 m/s².
Work against gravity: W = mgh = 50 × 10 × 12 = 6000 J.
Power: 6000/30 = 200 W.
Walking up in 60 s takes the same 6000 J but only 100 W.
How do simple machines make a task easier?
The force you apply is the effort; the force to be overcome is the load. Mechanical advantage (MA) = load ÷ effort.
Example 4: a ramp. You push a 30 kg box up a smooth ramp 3 m long onto a platform 1 m high. Take g = 10 m/s².
Load = weight = 300 N.
With no friction, effort × ramp length = load × height, so effort = 300 × 1/3 = 100 N.
MA = 300/100 = 3, which is also L/h = 3/1.
Work done by you: 100 N × 3 m = 300 J. Lifting straight up: 300 N × 1 m = 300 J. The same.
Example 5: a lever. A crowbar has an effort arm of 1.2 m and a load arm of 0.3 m. MA = 1.2/0.3 = 4, so a 400 N rock needs only 100 N of effort, applied over four times the distance.
A machine trades a smaller force for a longer distance. It never reduces the total work, which is why NCERT stresses that machines "do not create energy".
Which mistakes come up most?
Counting work when force and displacement are perpendicular.
Dropping the minus sign for work done by friction or braking.
Forgetting to square v in ½mv², or to convert km/h to m/s first.
Writing power as force × time. It is work ÷ time.
Thinking a ramp or lever saves work. It saves force.
If you can follow the theory but freeze on numericals, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning, so you can check which step you skipped. When you move to Class 11, our post on the work-energy theorem for JEE problems shows how the same idea scales up.
A practice routine
Redo NCERT Examples 7.5 (the 0.2 kg cricket ball at 154.8 km/h, about 185 J) and 7.10 (the weightlifter, 300 W).
For three chapter-end questions, write the energy at two points before calculating anything.
Measure your own stair-climbing power: time yourself up one flight and estimate the height.
Build NCERT's seesaw or ruler lever and check that effort × effort arm = load × load arm.
Today, time your climb up one flight of stairs and work out your power in watts.
