Schooling · Class 11 · Physics · Work, Energy and Power · Chapter on a page
CBSE Class 11 · Physics · NCERT Physics Part-I
Work, Energy and Power: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
This note's current text has no applied fact-check on record.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- Work done by a constant force F on a…
- Work can be positive (force aids…
- Kinetic energy of a body of mass m…
- The work-energy theorem states that…
- Potential energy is energy stored…
- A conservative force is one for…
- Mechanical energy E = K + U is…
- Formulas and facts
- Work by constant force
- Work by variable force along a…
- K = (1/2) m v²
- Gravitational potential energy (near…
- Elastic potential energy
- Work-energy theorem
- Conservation of mechanical energy…
- Worked examples
- Work done by gravity and a normal…
- Using the work-energy theorem
- Conservation of energy with a spring
- Watch out for
- Forgetting the cos θ factor and…
- Using mgh even when the reference…
- Believing kinetic energy can be…
- Applying conservation of mechanical…
- Confusing energy (joules) with power…
What this chapter is about
This chapter introduces three of the most fundamental quantities in physics: work, energy and power. You have already studied motion and forces in earlier chapters. Now you learn how to connect force and displacement through a single number called work, and how that work changes the energy of a body. Energy is a quantity that can be stored, transferred and transformed, but the total energy of an isolated system stays constant. This principle, called the conservation of energy, runs through every branch of physics.
Key ideas
- Work done by a constant force F on a body that moves through displacement s is W = F s cos θ, where θ is the angle between F and s. Work is a scalar; its SI unit is the joule (J), equal to 1 N m.
- Work can be positive (force aids motion), negative (force opposes motion) or zero (force is perpendicular to motion, as in uniform circular motion where the centripetal force does no work).
- Kinetic energy of a body of mass m moving with speed v is K = (1/2) m v². It is always non-negative.
- The work-energy theorem states that the net work done on a body equals the change in its kinetic energy: W_net = K_f − K_i.
- Potential energy is energy stored due to position or configuration. Gravitational potential energy near Earth's surface is U = mgh (taking the reference level where U = 0). Elastic potential energy of a spring stretched or compressed by x from its natural length is U = (1/2) k x², where k is the spring constant.
- A conservative force is one for which the work done depends only on initial and final positions, not on the path. Gravity and the spring force are conservative; friction is non-conservative because it dissipates energy as heat.
- Mechanical energy E = K + U is conserved when only conservative forces act. When non-conservative forces (like friction) act, mechanical energy decreases.
- Power is the rate of doing work: P = W/t for constant power, or P = dW/dt in general. Also P = F v cos θ. The SI unit is the watt (W), equal to 1 J/s.
Formulas and facts to remember
- Work by constant force: W = F s cos θ (F is magnitude, s is displacement magnitude, θ is angle between them).
- Work by variable force along a straight line: W = ∫ F dx from x_i to x_f.
- Kinetic energy: K = (1/2) m v².
- Gravitational potential energy (near Earth): U = mgh (h above chosen reference).
- Elastic potential energy: U = (1/2) k x² (x is extension or compression).
- Work-energy theorem: W_net = ΔK = (1/2) m v_f² − (1/2) m v_i².
- Conservation of mechanical energy (no non-conservative forces): K_i + U_i = K_f + U_f.
- Power: P = W/t (average), P = dW/dt (instantaneous), P = F v (when F is along v).
- 1 horsepower ≈ 746 W.
Common mistakes
- Forgetting the cos θ factor and always writing W = F s → Remember that only the component of force along the displacement does work.
- Using mgh even when the reference level is not stated → Choose a clear reference level and state it; potential energy is defined relative to that level.
- Believing kinetic energy can be negative → Kinetic energy is (1/2) m v²; since m > 0 and v² ≥ 0, kinetic energy is never negative.
- Applying conservation of mechanical energy when friction acts → Friction is non-conservative; mechanical energy is not conserved. Use work-energy theorem and include the work done by friction.
- Confusing energy (joules) with power (watts) → Energy is the total work done; power is how fast it is done.
Quick revision
- Work = force component along displacement × displacement; unit is joule.
- Work-energy theorem: net work = change in kinetic energy.
- Mechanical energy (K + U) is conserved when only conservative forces act.
- Potential energy depends on the choice of reference level.
- Power = energy transferred per unit time; unit is watt (1 W = 1 J/s).
- In collisions, momentum is always conserved; kinetic energy is conserved only in elastic collisions.