Physics · Class 11 Science
Ch 5Work, Energy and Power — Class 11 Physics, concept-first.
Work, energy and power are among the most widely used ideas in the whole of physics, because they let us describe how one form of motion or configuration changes into another without having to track every force in full vector detail at every instant.
Key concepts
Hover a concept to preview it and jump to its most relevant Q&A.
Work Done by a Constant Force
When a force acting on a body remains constant in magnitude throughout the displacement (with a fixed angle between the force and the direction of motion), the small work done for a small displacement is .
Most relevant Q&A
In previous exams
How often this chapter’s concepts have been examined — real appearance data, never estimated.
Chapter contents
The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.
Introduction to Work, Energy and Power
Work, energy and power are among the most widely used ideas in the whole of physics, because they let us describe how one form of motion or configuration changes into another without having to track e…
Work Done by a Constant Force
When a constant force acts on a body while the body undergoes a displacement , the work done by that force is defined as the scalar (dot) product of the two vectors: where , , and is the angle between…
Work Done by a Variable Force
The simple formula applies only when the force stays exactly constant, in both magnitude and direction, throughout the whole displacement.
Kinetic Energy and the Work-Energy Theorem
Kinetic energy is defined as the energy a body possesses purely because of its motion: where is the body's mass and is its speed.
Power
Power answers a question work alone does not: not merely how much work is done, or how much energy is transferred, but how quickly.
Conservative and Non-Conservative Forces
A force is called conservative if the work it does on a body moving from one point to another depends only on those two points -- the starting point and the ending point -- and not at all on the parti…
Potential Energy, and the Potential Energy of a Spring
Potential energy, , is energy a body possesses because of its position or its state of configuration, rather than because of its motion, and it can be defined only for a conservative force.
Conservation of Mechanical Energy
The total mechanical energy of a body is defined as the sum of its kinetic and potential energy at any instant, .
Motion in a Vertical Circle
A body moving in a vertical circle -- unlike one moving in a horizontal circle at constant speed -- does not move at constant speed at all, because gravity now has a component along the direction of m…
Elastic Collisions in One Dimension
A collision between two bodies is called perfectly elastic if, in addition to the total momentum of the system being conserved (as it always is in any collision, so long as no external force acts duri…
Inelastic Collisions and the Coefficient of Restitution
In any real collision, momentum is always conserved (so long as no significant external force acts during the very short time of contact), but kinetic energy is conserved only in the idealised, perfec…
Collisions in Two Dimensions
When the colliding bodies are not confined to a single straight line -- a glancing or oblique collision, such as one billiard ball striking another slightly off-centre -- the outgoing velocities gener…
Summary
This chapter developed WBCHSE Unit 4's account of work, energy and power in the following order. Work done by a constant force is , a scalar; for a variable force it generalises to the integral , the…
Sample & Board Papers
Sample papers and previous-year board questions for this subject.
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- Q1If there be perfectly elastic collision between two bodies of the same mass moving along a straight line, the two bodies will (a) stay stati…Preview
- Q2The number of joules in 1 kg-m is (a) 9.8 (b) 980 (c) 1000 (d) 10⁵Preview
- Q3The displacement of a body of mass 3 kg under the action of a force is S = t²/3 metre. The work done in time 2s by the same force in (J) is…Preview
- Q4Which one of the kinematic equations states the work-energy principle? **OR** If A⃗ = 0.4î + 0.3ĵ + ck̂ be a unit vector, then what is the v…Preview
- Q5A particle moves from a point r⃗₁ = î + 2ĵ in metre to another point r⃗₂ = 2î + 4ĵ in metre under the action of a force F⃗ = 2î + 3ĵ in newt…Preview
More questions
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- Example 1A force of $20\ \text{N}$ acts on a block, making an angle of $60^\circ$ with the horizontal, and displaces the block through $4\ \text{m}$…Free
- Example 2A block of mass $2\ \text{kg}$, initially at rest on a frictionless horizontal surface, is acted on by a variable force $F(x) = 3x^2\ \text{…Free
- Example 3A car of mass $1000\ \text{kg}$ moving at $20\ \text{m/s}$ is brought to rest by its brakes over a distance of $50\ \text{m}$. Using the wor…Free
- Example 4A pump lifts $200\ \text{kg}$ of water per minute through a height of $10\ \text{m}$. Find the power the pump must deliver (take $g = 9.8\ \…Preview
- Example 5A spring of force constant $k = 200\ \text{N/m}$ is compressed by $0.1\ \text{m}$ from its natural length. Find the elastic potential energy…Preview
- Example 6A ball of mass $0.2\ \text{kg}$ is dropped from a height of $5\ \text{m}$. Using conservation of mechanical energy (and neglecting air resis…Preview
- Example 7A stone of mass $0.5\ \text{kg}$ tied to a string of length $1\ \text{m}$ is whirled in a vertical circle. Find (a) the minimum speed the st…Preview
- Example 8A ball of mass $2\ \text{kg}$ moving at $3\ \text{m/s}$ collides elastically, head-on, with a stationary ball of mass $1\ \text{kg}$. Find t…Preview
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- Q9Explain why the work done by a force is a scalar quantity, even though both the force and the displacement it acts through are vector quanti…Free
- Q10Starting from Newton's second law, derive the work-energy theorem for a particle moving under a variable force acting along a straight line.Free
- Q11A porter raises a suitcase of mass $15\ \text{kg}$ vertically through $1.5\ \text{m}$ onto a trolley, and then wheels the trolley $10\ \text…Free
- Q12Distinguish between a conservative and a non-conservative force, giving one everyday example of each, and explain why the work done by a con…Preview
- Q13Starting from the work-energy theorem and the definition of potential energy, show that the total mechanical energy of a system remains cons…Preview
- Q14A block slides along a rough horizontal surface and gradually comes to rest. Using the work-energy theorem, explain why its kinetic energy d…Preview
- Q15A shell of mass $0.02\ \text{kg}$ is fired with a velocity of $200\ \text{m/s}$ from a gun of mass $5\ \text{kg}$, initially at rest and fre…Preview
- Q16Define the coefficient of restitution for a collision between two bodies, in terms of their velocities of approach and separation, and expla…Preview
- Q17A pendulum bob of mass $m$, attached to a string of length $L$, is released from rest with the string held horizontal. Using conservation of…Preview
- Q18Two identical balls, each of mass $m$, undergo a perfectly inelastic, head-on collision, one moving with speed $v$ and the other initially a…Preview
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- Q19A particle of mass $1\ \text{kg}$, initially at rest at $x = 0$, moves along the $x$-axis under a force $F(x) = (10 - 2x)\ \text{N}$ (with $…Free
- Q20A spring of force constant $k = 400\ \text{N/m}$ is compressed by $0.2\ \text{m}$ and used to launch a block of mass $1\ \text{kg}$ up a fri…Free
- Q21Ball $A$, of mass $3\ \text{kg}$, moving at $4\ \text{m/s}$, collides elastically, head-on, with ball $B$, of mass $1\ \text{kg}$, initially…Free
- Q22A bullet of mass $0.01\ \text{kg}$, moving horizontally at $300\ \text{m/s}$, embeds itself in a stationary wooden block of mass $2\ \text{k…Preview
- Q23A ball of mass $2\ \text{kg}$, moving at $5\ \text{m/s}$ along the $x$-axis, strikes an identical, stationary ball of mass $2\ \text{kg}$ in…Preview
- Q24A bucket of water of mass $2\ \text{kg}$ is whirled in a vertical circle of radius $1.2\ \text{m}$. Find (a) the minimum speed the bucket mu…Preview
- Q25A lift motor raises a total load (lift cabin plus passengers) of mass $1500\ \text{kg}$ through a height of $20\ \text{m}$ in $25\ \text{s}$…Preview
- Q26A car of mass $1000\ \text{kg}$, moving at $72\ \text{km/h}$, is brought to rest entirely by friction over a distance of $40\ \text{m}$ on a…Preview