CBSE Important Questions Class 9 Science Chapter 10 – Work and Energy

Class 9 Science Chapter 10 Work and Energy is part of the CBSE Class 9 Science syllabus. It explains the scientific meaning of work, how to calculate it, and the different forms of energy. Students learn about kinetic energy and potential energy and their formulae, the law of conservation of energy, the transformation of energy from one form to another, power and the commercial unit of energy, the kilowatt hour.

These CBSE important questions for Class 9 Science Chapter 10 cover work done by brakes on a car, energy changes in a thermal power plant, the law of conservation of energy, reading a table of potential and kinetic energy, energy changes on a roller coaster, numerical problems on potential and kinetic energy, and what happens to the kinetic energy of a body when it hits the ground. Every question has a clear step-by-step answer in simple English with figures, so students can practise on their own, parents can check the working, and teachers can use them for class tests.

Q1. Harry drives a car weighing 1500 kg. He travels at a speed of 25 m/s and stops in a distance of 40 m, decelerating uniformly.
(i) What is the force exerted on the car by the brakes?
(ii) What is the work done by the brakes?

Answer: (i) First find the acceleration using v² = u² + 2as, with u = 25 m/s, v = 0 and s = 40 m.
0 = (25)² + 2 × a × 40
0 = 625 + 80a, so a = −625/80 = −7.81 m/s²
F = ma = 1500 × (−7.81) = −11718.75 N
So the brakes exert a force of about 11718.75 N. The negative sign shows it is a retarding force.
(ii) Work done, W = F × s = 11718.75 × 40 = 4.687 × 10⁵ J (468750 J). As the brakes take energy away from the car, this work is negative.

Q2. What kind of energy transformations take place in a thermal power plant?

Answer: In a thermal power plant the energy changes in these steps:
1. Coal is burnt, so the chemical energy of coal changes into heat energy.
2. This heat turns water into high-pressure steam.
3. The steam turns the turbines, so heat energy changes into kinetic energy.
4. The spinning turbine drives the generator, which changes kinetic energy into electrical energy.

Q3. Which of the following best describes the law of conservation of energy?
(a) Energy can be created but not destroyed.
(b) Energy can be destroyed but not created.
(c) Energy can neither be created nor destroyed, only transformed from one form to another.
(d) Energy can both be created and destroyed.

Answer: (c) Energy can neither be created nor destroyed, only transformed from one form to another.
Energy only changes from one form into another, and the total energy of a system always stays the same.

Q4. Samaira drops a ball from different heights onto the floor. The table shows the gravitational potential energy (GPE) and kinetic energy (KE) at various points during the fall. Determine:
(a) Which point represents the floor?
(b) Which point represents Samaira's hand?
(c) At which point is the ball moving with maximum speed?

Answer: (a) The floor is where the potential energy is zero, so point A represents the floor.
(b) Samaira's hand is at the top, where the potential energy is maximum (1 J) and the kinetic energy is zero, so point D represents her hand.
(c) The ball moves fastest where the kinetic energy is maximum (1 J), which is at point A, just as it reaches the floor.

Q5. Arun lives on the 17th floor of his building, and he carries a ball having potential energy equal to 200 J. The kinetic energy possessed by the ball when it just touches the ground is
(a) less than PE
(b) equal to PE
(c) greater than PE
(d) equal to the sum of PE and KE

Answer: (b) equal to PE
By the law of conservation of energy, as the ball falls its potential energy keeps changing into kinetic energy. At the moment it touches the ground the potential energy is zero, so all 200 J has become kinetic energy.

Q6. The gravitational potential energy of a freely falling ball from the top of a building keeps on decreasing. Does it violate the law of conservation of energy? Explain.

Answer: No, it does not violate the law of conservation of energy.
As the ball falls, its height decreases, so its potential energy decreases. But at the same time its speed increases, so its kinetic energy increases by exactly the same amount. The potential energy is simply being converted into kinetic energy, and the total mechanical energy of the ball stays constant throughout the fall.

Q7. When the cart starts rolling down along the path WXYZ, the energy transformation from point W to X will be from ________.
(a) potential energy to kinetic energy
(b) kinetic energy to potential energy
(c) gravitational energy to mechanical energy
(d) mechanical energy to gravitational energy

Answer: (a) potential energy to kinetic energy
W is at the top of the path and X is lower down. As the cart comes down, its height decreases so the potential energy falls, and its speed increases so the kinetic energy rises. So potential energy is changing into kinetic energy.

Q8. The potential energy of a tennis ball at the top of a tower is 10 J. If the ball is dropped from the top, then its kinetic energy just before it hits the ground is
(a) 0
(b) 5 J
(c) 8 J
(d) 10 J

Answer: (d) 10 J
At the top the ball has 10 J of potential energy and no kinetic energy. Just before hitting the ground its height is zero, so the potential energy is zero and all of it has changed into kinetic energy. So the kinetic energy is 10 J.

Q9. An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down. (Take g = 9.8 m/s²)

Answer: Potential energy at the top:
E_p = mgh = 40 × 9.8 × 5 = 1960 J
Half-way down (h = 2.5 m):
Potential energy there = 40 × 9.8 × 2.5 = 980 J
The rest has changed into kinetic energy.
Kinetic energy = 1960 − 980 = 980 J

Q10. An object of mass 40 kg is placed at point A, at a height of 50 m from the ground. If 75% of the gravitational potential energy at A becomes kinetic energy when the object reaches point B at a height of 30 m, calculate its speed at point B. (Take g = 10 m/s²)

Answer: Potential energy at A, E_p = mgh = 40 × 10 × 50 = 2 × 10⁴ J (20 kJ)
Kinetic energy at B = 75% of E_p = (3/4) × 2 × 10⁴ = 1.5 × 10⁴ J
Using E_k = ½mv²:
1.5 × 10⁴ = ½ × 40 × v²
v² = (2 × 15000)/40 = 750
v = √750 = 27.4 m/s (approximately)

Q11. A freely falling body stops when it hits the ground. What happens to its kinetic energy?

Answer: The kinetic energy is not destroyed; it changes into other forms of energy.
When the body hits the ground, most of its kinetic energy changes into heat energy (both the body and the ground become slightly warmer) and sound energy (we hear the thud). Some of it is used in deforming the body or the ground, and a little is passed on to the earth. So the total energy is still conserved.

Q12. State the law of conservation of energy and give two daily-life examples. A ball of mass 4 kg is thrown vertically upwards with a velocity of 20 m/s. Find its kinetic energy, potential energy and total energy at a point 15 m above the ground. (Take g = 10 m/s²)

Answer: Law of conservation of energy: Energy can neither be created nor destroyed. It can only change from one form to another, and the total energy always remains constant.
Examples: 1. A ball thrown upwards: its kinetic energy changes into potential energy as it rises. 2. A swinging pendulum: energy changes between potential and kinetic at every swing.
Numerical:
Total energy at the start = ½mv² = ½ × 4 × (20)² = 800 J
At a height of 15 m:
Potential energy = mgh = 4 × 10 × 15 = 600 J
Kinetic energy = total energy − potential energy = 800 − 600 = 200 J
Total energy = 800 J (unchanged)

Related Study Material on Extramarks

• CBSE Important Questions Class 9 Science Chapter 1
• CBSE Important Questions Class 9 Science Chapter 2
• CBSE Important Questions Class 9 Science Chapter 3
• CBSE Important Questions Class 9 Science Chapter 4
• CBSE Important Questions Class 9 Science Chapter 5
• CBSE Important Questions Class 9 Science Chapter 6
• CBSE Important Questions Class 9 Science Chapter 7
• CBSE Important Questions Class 9 Science Chapter 8
• CBSE Important Questions Class 9 Science Chapter 9
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• CBSE Important Questions Class 9 Science Chapter 12