Work, Energy and Power is a core and high-weightage chapter in Class 11 Physics that builds a strong foundation in mechanics. This chapter covers key concepts such as work done by a force, kinetic and potential energy, work–energy theorem, conservative and non-conservative forces, law of conservation of energy, power, and collisions. These topics are frequently tested in school exams and competitive exams like JEE and NEET.
NCERT Solutions for Class 11 Physics Chapter 5 – Work, Energy and Power are prepared strictly according to the latest CBSE syllabus and exam pattern. The solutions are explained in simple, step-by-step language with clear derivations, diagrams, and solved numericals, helping students develop conceptual clarity and strong problem-solving skills.
NCERT Solutions for Class 11 Physics Chapter 5 – Work, Energy and Power
Q.
The sign of work done by a force on a body is important to understand. State carefully if the following quantities are positive or negative:
(a) work done by a man in lifting a bucket out of a well by means of a rope tied to the bucket.
(b) work done by gravitational force in the above case,
(c) work done by friction on a body sliding down an inclined plane,
(d) work done by an applied force on a body moving on a rough horizontal plane with uniform velocity,
(e) work done by the resistive force of air on a vibrating pendulum in bringing it to rest.
Q.
Two identical ball bearings in contact with each other and resting on a frictionless table are hit head-on by another ball bearing of the same mass moving initially with a speed V. If the collision is elastic, which of the following figure is a possible result after collision?
Q.
A 1 kg block situated on a rough incline is connected to a spring of spring constant 100 N m–1 as shown in Fig. 6.17. The block is released from rest with the spring in the unstretched position. The block moves 10 cm down the incline before coming to rest. Find the coefficient of friction between the block and the incline. Assume that the spring has a negligible mass and the pulley is frictionless.
Q.
Two inclined frictionless tracks, one gradual and the other steep meet at A from where two stones are allowed to slide down from rest, one on each track (Fig. 6.16).Will the stones reach the bottom at the same time? Will they reach there with the same speed? Explain. Given θ1 = 30°, θ2 = 60°, and h = 10 m, what are the speeds and times taken by the two stones?
Q.
A bullet of mass 0.012 kg and horizontal speed 70 m s–1 strikes a block of wood of mass 0.4 kg and instantly comes to rest with respect to the block. The block is suspended from the ceiling by means of thin wires. Calculate the height to which the block rises. Also, estimate the amount of heat produced in the block.
Q.
A family uses 8 kW of power. (a) Direct solar energy is incident on the horizontal surface at an average rate of 200 W per square meter. If 20% of this energy can be converted to useful electrical energy, how large an area is needed to supply 8 kW? (b) Compare this area to that of the roof of a typical house.
Q.
The blades of a windmill sweep out a circle of area A.
(a) If the wind flows at a velocity v perpendicular to the circle, what is the mass of the air passing through it in time t?
(b) What is the kinetic energy of the air?
(c) Assume that the windmill converts 25% of the wind’s energy into electrical energy, and that A = 30 m2, v = 36kmh-1 and the density of air is 1.2 kg m–3. What is the electrical power produced?
Q.
Q.
The bob of a pendulum is released from a horizontal position. If the length of the pendulum is 1.5 m, what is the speed with which the bob arrives at the lowermost point, given that it dissipated 5% of its initial energy against air resistance?
Q.
A pump on the ground floor of a building can pump up water to fill a tank of volume 30 m3 in 15 min. If the tank is 40 m above the ground, and the efficiency of the pump is 30%, how much electric power is consumed by the pump?
Q.
A body of mass 2 kg initially at rest moves under the action of an applied horizontal force of 7 N on a table with coefficient of kinetic friction = 0.1.
Compute the
(a) Work done by the applied force in 10 s,
(b) Work done by friction in 10 s,
(c) Work done by the net force on the body in 10 s,
(d) Change in kinetic energy of the body in 10 s, and interpret your results.
Q.
A molecule in a gas container hits a horizontal wall with speed 200 m s–1 and angle 300 with the normal, and rebounds with the same speed. Is momentum conserved in the collision? Is the collision elastic or inelastic?
Q.
A rain drop of radius 2 mm falls from a height of 500 m above the ground. It falls with decreasing acceleration (due to viscous resistance of the air) until at half its original height; it attains its maximum (terminal) speed, and moves with uniform speed thereafter. What is the work done by the gravitational force on the drop in the first and second half of its journey? What is the work done by the resistive force in the entire journey if its speed on reaching the ground is 10 ms–1?
Q.
An electron and a proton are detected in a cosmic ray experiment, the first with kinetic energy 10 keV, and the second with 100 keV. Which is faster, the electron or the proton? Obtain the ratio of their speeds. (electron mass = 9.11 × 10–31 kg, proton mass = 1.67 × 10–27 kg, 1 eV = 1.60 × 10–19 J).
Q.
Q.
A body is initially at rest. It undergoes one-dimensional motion with constant acceleration. The power delivered to it at time t is proportional to
(i) t1/2
(ii) t
(iii) t3/2
(iv) t2
Q.
Answer the following:
(a) The casing of a rocket in flight burns up due to friction. At whose expense is the heat energy required for burning obtained? The rocket or the atmosphere?
(b) Comets move around the sun in highly elliptical orbits. The gravitational force on the comet due to the sun is not normal to the comet’s velocity in general. Yet the work done by the gravitational force over every complete orbit of the comet is zero. Why?
(c) An artificial satellite orbiting the earth in very thin atmosphere loses its energy gradually due to dissipation against atmospheric resistance, however small. Why then does its speed increase progressively as it comes closer and closer to the earth?
(d) In Fig. 6.13(i) the man walks 2 m carrying a mass of 15 kg on his hands. In Fig.6.13 (ii), he walks the same distance pulling the rope behind him. The rope goes over a pulley, and a mass of 15 kg hangs at its other end. In which case is the work done greater?
Q.
The potential energy function for a particle executing linear simple harmonic motion is given by V(x) = kx
2/2, where k is the force constant of the oscillator. For k = 0.5 Nm
–1, the graph of V(x) versus x is shown in Fig. 6.12.
Show that a particle of total energy 1 J moving under this potential must ‘turn back’ when it reaches x = ± 2 m.
Q.
Given in Fig. 6.11 are examples of some potential energy functions in one dimension. The total energy of the particle is indicated by a cross on the ordinate axis. In each case, specify the regions,
if any, in which the particle cannot be found for the given energy. Also, indicate the minimum total energy the particle must have in each case. Think of simple physical contexts for which these potential energy shapes are relevant.
Q.
A trolley of mass 200 kg moves with a uniform speed of 36 km h-1 on a frictionless track. A child of mass 20 kg runs on the trolley from one end to the other (10 m away) with a speed of 4 ms–1 relative to the trolley in a direction opposite to the its motion, and jumps out of the trolley. What is the final speed of the trolley? How much has the trolley moved from the time the child begins to run?
NCERT Solutions for Class 11 Physics Chapter 5 – Work, Energy and Power
Q1. What is work? State the condition for work to be done.
Answer:
Work is said to be done when a force applied on a body produces displacement in the direction of the force.
If there is no displacement, no work is done.
Q2. Define kinetic energy and write its expression.
Answer:
Kinetic energy is the energy possessed by a body due to its motion.
Kinetic Energy (KE) = ½mv², where m is mass and v is velocity.
Q3. What is potential energy? Give an example.
Answer:
Potential energy is the energy possessed by a body due to its position or configuration.
Example: A stretched spring or a body kept at a height.
Q4. State the work–energy theorem.
Answer:
The work done by the net force acting on a body is equal to the change in its kinetic energy.
Q5. What is power? Write its SI unit.
Answer:
Power is defined as the rate of doing work.
SI unit of power is watt (W).
Q6. Define average power and instantaneous power.
Answer:
Average power is the total work done divided by the total time taken.
Instantaneous power is the power at a particular instant of time.
Q7. What is mechanical energy?
Answer:
Mechanical energy is the sum of kinetic energy and potential energy of a body.
Q8. State the law of conservation of energy.
Answer:
Energy can neither be created nor destroyed. It can only be transformed from one form to another.
The total energy of an isolated system remains constant.
Q9. What is a conservative force? Give one example.
Answer:
A force is called conservative if the work done by it depends only on the initial and final positions and not on the path followed.
Example: Gravitational force.
Q10. Why is friction called a non-conservative force?
Answer:
Friction is called a non-conservative force because the work done by friction depends on the path followed.
Q11. Define elastic and inelastic collisions.
Answer:
In an elastic collision, both momentum and kinetic energy are conserved.
In an inelastic collision, momentum is conserved but kinetic energy is not conserved.
Q12. What is the significance of potential energy curves?
Answer:
Potential energy curves help in determining equilibrium positions, stability of motion, and allowed regions of motion of particles.
Q13. What is escape velocity?
Answer:
Escape velocity is the minimum velocity required by a body to escape the gravitational pull of the Earth without returning back.
Q14. Write two differences between work and power.
Answer:
Work depends on force and displacement, whereas power depends on work done per unit time.
Work does not depend on time, but power directly depends on time.
Q15. Give two applications of the law of conservation of energy.
Answer:
Hydroelectric power generation.
Motion of roller coasters.
FAQs: Class 11 Physics Chapter 5 – Work, Energy and Power
Q1. Is Work, Energy and Power important for exams?
Yes, it is a high-weightage and fundamental chapter in mechanics.
Q2. Which topics are most important in this chapter?
Work–energy theorem, conservation of energy, and collisions.
Q3. Are numericals asked from this chapter?
Yes, energy and collision-based numericals are very common.
Q4. Are derivations important here?
Yes, derivations related to work–energy theorem and energy conservation are frequently asked.
Q5. How do NCERT Solutions help?
They provide NCERT-aligned, exam-ready explanations with solved numericals.