Chapter 5 - Work, Energy and Power

The words work, energy and power are used casually in everyday life, but each has a precise meaning in Physics. Work, Energy and Power Class 11 introduces an energy-based method for analysing motion.

In many situations, energy methods are shorter and clearer than resolving every force and calculating acceleration.

Work done by a force

A force does work when it produces displacement. Only the component of force along the displacement contributes to work.

Work may be:

• Positive when force supports displacement

• Negative when force opposes displacement

• Zero when force is perpendicular to displacement or no displacement occurs

For a constant force, work is calculated using the scalar product of force and displacement.

For a variable force, work is obtained by integration or by finding the area under a force-displacement graph.

Kinetic energy and the work-energy theorem

Kinetic energy is the energy associated with motion.

The work-energy theorem states that the net work done on an object equals the change in its kinetic energy.

This theorem is useful when several forces act over a known displacement but time is not given.

Potential energy and conservative forces

Potential energy is associated with the configuration or position of a system.

Students study gravitational potential energy near Earth and elastic potential energy stored in a spring.

For a conservative force:

• Work depends only on initial and final positions

• Work around a closed path is zero

• Potential energy can be defined

Gravity and ideal spring force are conservative. Friction is generally treated as non-conservative.

Conservation of mechanical energy

When only conservative forces perform work, the sum of kinetic and potential energy remains constant.

Energy may change from one form to another, but the total mechanical energy remains unchanged.

When friction acts, mechanical energy is not conserved by itself. Some energy is transferred into thermal or other internal forms.

Springs and variable force

An ideal spring follows Hooke’s law within its elastic limit. Its restoring force is proportional to displacement and acts towards equilibrium.

Students learn how work done by a spring is related to the area under its force-displacement graph and how elastic potential energy is stored.

Power

Power measures the rate at which work is done or energy is transferred.

Two machines may perform the same work but have different power if they take different times.

Students study average power and instantaneous power.

Collisions

The chapter covers elastic and inelastic collisions in one and two dimensions.

Momentum is conserved in an isolated collision. Kinetic energy is conserved only in an elastic collision.

Students should not assume that kinetic energy is conserved in every collision.

Motion in a vertical circle

An object moving in a vertical circle experiences changing speed and radial forces.

Students combine energy conservation with the centripetal-force requirement to determine speeds and tensions at different points.

Common difficulties

Students often:

• Use the total force instead of its component along displacement

• Confuse work done by a force with work done on a system

• Apply mechanical-energy conservation when friction acts

• Forget that spring force changes with displacement

• Assume momentum and kinetic energy are both conserved in every collision

• Confuse power with energy

Learning on Teachoo

Teachoo provides chapter notes, derivations, force-displacement graphs, NCERT solutions, spring questions, collision problems, MCQs and competency-based questions.

The solutions identify the system and explain whether to use the work-energy theorem, mechanical-energy conservation or momentum conservation.

Frequently Asked Questions

What is work in Physics?

Work is the energy transferred when a force produces displacement. It depends on the component of force along the displacement.

When is mechanical energy conserved?

Mechanical energy is conserved when only conservative forces perform work on the system.

Is momentum conserved in an inelastic collision?

Yes, if the system is isolated. Kinetic energy, however, is not conserved in an inelastic collision.

Can work be negative?

Yes. Work is negative when the force component acts opposite to the displacement.

What is the difference between energy and power?

Energy represents the capacity to perform work. Power represents how rapidly work is done or energy is transferred.