Ch 4Work, Energy and Power — Class 11 Physics, concept-first.

In everyday language, the word work covers almost any kind of effort, physical or mental -- studying for an exam, carrying groceries, or straining against an immovable wall are all casually called 'wo…

In everyday speech, "work" describes almost any physical or mental effort -- pushing against an immovable wall until you are exhausted certainly feels like work.

When the force acting on a body stays constant in both magnitude and direction throughout the motion (though the angle between and the path can still be fixed by the geometry), the small work done for…

Many real forces are not constant -- a spring pulls harder the more it stretches, and the force needed to compress a gas increases as the volume shrinks.

Energy is defined as the capacity to do work. This is why work and energy carry the same dimensional formula, , and the same SI unit, the joule (J) -- doing work on a system is precisely how energy fl…

Kinetic energy is the energy possessed by a body purely by virtue of its motion. Every moving object has kinetic energy; an object at rest has none.

The work-kinetic energy theorem connects the work done on a body directly to the change produced in its kinetic energy, and can be derived cleanly for the simplest case of a constant force.

Kinetic energy can also be expressed directly in terms of an object's linear momentum, , which is often the more convenient form when momentum -- rather than velocity -- is the known quantity (as in c…

Potential energy is the energy a body possesses on account of its position or configuration relative to its surroundings -- it exists precisely because the forces acting on the body (gravity, a spring…

Near the Earth's surface, gravity is essentially a constant force, (taking "up" as positive). To lift a body of mass through a height at constant velocity, an external force equal in magnitude but opp…

A stretched or compressed spring stores energy too, called elastic potential energy. Consider a mass on a smooth horizontal surface attached to a spring whose other end is fixed to a wall, with markin…

Not every force lets us define a potential energy in the first place -- only conservative forces do. A force is called conservative if the work done in moving a body between two points depends only on…

The law of conservation of energy states that energy can neither be created nor destroyed; it can only be transformed from one form to another, and the total energy of an isolated system stays constan…

Motion in a vertical circle is a classic and important application of both Newton's second law and energy conservation together.

Power measures how fast work is done, or equivalently, how fast energy is delivered. Two engines might ultimately do the same total amount of work, but the more "powerful" one does it in less time --…

Power is defined as the rate at which work is done, or energy is delivered:

Since , power is a scalar quantity, with dimensional formula (energy's divided by time's ). Its SI unit is the watt (W), named for James Watt, the inventor of the practical steam engine: one watt is t…

To connect power directly to force and velocity, start from the definition of work done by a force for a displacement :

A collision is any brief, forceful interaction between two (or more) bodies -- carom, billiards and marbles are everyday examples -- and collisions can happen with or without the bodies physically tou…

In any collision, the total linear momentum and the total energy of the system are always conserved, but the total kinetic energy need not be -- because the impact and any deformation of the bodies ca…

Consider two bodies of masses and moving along the same straight line (the positive -direction) on a frictionless surface, with initial velocities (so catches up to ) and final velocities after an ela…

In a perfectly (completely) inelastic collision, the two bodies stick together permanently on impact and move off as one, sharing a single common velocity .

Even though momentum is exactly conserved in a perfectly inelastic collision, kinetic energy is not -- the sticking-together process necessarily dissipates some of it as heat, sound, and permanent def…

Real collisions -- unlike the idealised perfectly elastic case -- generally lose some kinetic energy, and the amount lost is captured by a single dimensionless number, the coefficient of restitution (…

This unit's results, gathered together for quick revision:

The concept map for this unit traces one continuous chain of reasoning. It starts from work, defined as the dot product of force and displacement, and shows this is equivalent, via the work-kinetic en…

This is the chapter-end evaluation set, structured in five parts that build in depth. Part I (Multiple Choice Questions) contains fifteen objective questions, several drawn directly from AIPMT and IIT…

Sample papers and previous-year board questions for this subject.