Work, Energy and Power — Class 11 MCQs with Answers

Work, Energy and Power — important questions & MCQs with answers (Class 11 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   SI unit of work and energy:

   ANewtonBPascalCWattDJoule
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   Answer: Joule

   Explanation: Work and energy share the SI unit joule (1 J = 1 N·m), same as PE and KE. Watt is the unit of power (J/s), not energy — that mix-up is the tempting wrong pick here.

2. Q2 of 6Easy

   Work W = ?

   AF · d cos θBF + dCF − dDF · d sin θ
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   Answer: F · d cos θ

   Explanation: Work is the dot product of force and displacement, W = F·d·cosθ, picking out only the component of force along the displacement. Writing F·d sinθ instead confuses it with the cross product used for torque.

3. Q3 of 6Medium

   KE of 2 kg body at 5 m/s:

   A25 JB10 JC50 JD100 J
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   Answer: 25 J

   Explanation: Kinetic energy is KE = ½mv² = ½ × 2 kg × (5 m/s)² = ½ × 2 × 25 = 25 J. Forgetting to square the velocity gives 10 J (½ × 2 × 5), a common slip when the numbers look easy to multiply straight through.

4. Q4 of 6Medium

   PE of 5 kg body at height 10 m (g = 10):

   A250 JB50 JC500 JD1000 J
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   Answer: 500 J

   Explanation: Gravitational PE is U = mgh = 5 kg × 10 m/s² × 10 m = 500 J, using g = 10 m/s² as stated. Multiplying only mg (= 50) without the extra factor of h gives 50 J, missing the height entirely.

5. Q5 of 6Hard

   Pump lifts 100 L of water per second to height 10 m. Power (water density 1000 kg/m³, g = 10):

   A10 kWB1000 WC100 WD100 kW
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   Answer: 10 kW

   Explanation: Mass flow rate is 100 L/s × 1000 kg/m³ = 100 kg/s, since 100 L = 0.1 m³, so power is P = (dm/dt)gh = 100 × 10 × 10 = 10000 W = 10 kW. Forgetting to convert litres to kg via the density would give a value ten times too small.

6. Q6 of 6Hard

   Work-energy theorem: Net work =

   AHeatBTotal energyCChange in KEDMass × velocity
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   Answer: Change in KE

   Explanation: The work-energy theorem says net work done on a body equals its change in kinetic energy, W_net = ΔKE — not the total energy, which also includes PE and other stored terms.

Work, Energy and Power notes: key concepts (Class 11 Physics)

Work as the transfer of energy, the work-energy theorem linking net work to change in kinetic energy, potential energy for gravity and springs, the distinction between conservative and non-conservative forces, conservation of mechanical energy, power, and the classification of collisions by what is conserved.

Work done by a constant force

The product of force, displacement and the cosine of the angle between them. Work is a scalar even though both force and displacement are vectors.

Positive, negative and zero work

Work is positive when the force has a component along the displacement, negative when opposed, and zero when the force is perpendicular to the motion.

Work done by a variable force

Found as the area under the force-displacement graph, or the integral of F dx. This is how spring work and non-uniform force problems are handled.

Kinetic energy

The energy a body has by virtue of its motion, equal to ½mv². It is always positive and can also be written as p²/2m in terms of momentum.

Work-energy theorem

The NET work done by all forces equals the change in kinetic energy. It holds whether forces are conservative or not, which makes it very general.

Potential energy

Energy stored by virtue of position or configuration. Only changes in potential energy are physically meaningful; the zero level is a free choice.

Gravitational potential energy

Near the Earth's surface it is mgh, measured from any chosen reference level. It depends only on the vertical height, not on the path taken.

Elastic potential energy

Energy stored in a stretched or compressed spring, ½kx². It depends on the square of the deformation, so doubling the stretch quadruples the stored energy.

Conservative force

A force whose work depends only on the endpoints, not the path, and is zero around a closed loop. Gravity and the spring force are the standard examples.

Non-conservative force

A force whose work depends on the path taken, so mechanical energy is not conserved. Friction is the archetype, converting mechanical energy into heat.

Conservation of mechanical energy

When only conservative forces do work, the sum of kinetic and potential energy stays constant. Friction anywhere in the problem breaks this.

Power

The rate of doing work. Average power is total work over total time; instantaneous power is the dot product of force and velocity.

Elastic collision

Both momentum AND kinetic energy are conserved. For equal masses in one dimension the bodies simply exchange velocities.

Inelastic collision

Momentum is conserved but kinetic energy is not, some of it going into deformation and heat. Real collisions between everyday objects are inelastic.

Coefficient of restitution

The ratio of relative speed of separation to relative speed of approach. It is 1 for a perfectly elastic collision and 0 when the bodies stick together.