Work Energy and Simple Machines — Class 9 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 9 Science chapter "Work Energy and Simple Machines" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 9 Science chapter "Work Energy and Simple Machines" — 8 important questions with detailed answers for CBSE…
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NCERT Solutions for Class 9 Science — Work Energy and Simple Machines. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Work Energy and Simple Machines — All 8 Questions Solved
Q1. Define work. When is work zero?
Work is the product of force and displacement in the direction of force.
Formula: W = F × d × cos θ
Where F = force, d = displacement, θ = angle between them
Work is zero when force is perpendicular to displacement (θ = 90°) or displacement is zero.
Q2. Calculate work done by a 50 N force displacing an object 10 m in the direction of force.
Step 1: F = 50 N, d = 10 m, θ = 0°
Step 2: W = F × d × cos(0°) = 50 × 10 × 1 = 500 J
Q3. Define kinetic energy and potential energy.
Kinetic energy is energy possessed by an object due to its motion.
Formula: KE = ½mv²
Potential energy is energy stored due to position or configuration.
Formula: PE = mgh (gravitational PE)
Both measured in Joules (J).
Q4. A 2 kg object moves at 3 m/s. Find its kinetic energy.
Step 1: m = 2 kg, v = 3 m/s
Step 2: KE = ½mv² = ½ × 2 × 3² = 1 × 9 = 9 J
Q5. Define power and write its formula.
Power is the rate of doing work or the rate of energy transfer.
Formula: P = W/t = E/t
Where W = work (J), E = energy (J), t = time (s)
Unit: Watt (W) = J/s
Q6. Calculate power if 100 J of work is done in 5 seconds.
Step 1: W = 100 J, t = 5 s
Step 2: P = W/t = 100/5 = 20 W
Q7. State the Law of Conservation of Mechanical Energy.
In the absence of external forces, the total mechanical energy (KE + PE) of a system remains constant.
Formula: KE₁ + PE₁ = KE₂ + PE₂
When an object falls, PE converts to KE, keeping total energy constant.
Q8. A ball of mass 1 kg at height 5 m is dropped (g = 10 m/s²). Find PE and KE at start and velocity at ground.
Step 1: Initial PE = mgh = 1 × 10 × 5 = 50 J, Initial KE = 0
Step 2: At ground, PE = 0, KE = 50 J (conserved)
Step 3: ½mv² = 50 → v² = 100 → v = 10 m/s
Work Energy and Simple Machines — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. What is the SI unit of work?
- A. Newton
- B. Joule
- C. Watt
- D. Pascal
Answer: B. Joule — The SI unit of work is Joule (J), which is defined as the work done when a force of one Newton displaces an object by one meter in the direction of the force.
MCQ 2. A boy pushes a wall with a force of 50 N but fails to move it. How much work is done by the boy?
- A. 50 J
- B. 0 J
- C. 100 J
- D. 25 J
Answer: B. 0 J — Work is done only when there is a displacement in the direction of the applied force. Since the wall does not move, the displacement is zero, and thus no work is done.
MCQ 3. A coolie lifts a luggage of 15 kg from the ground and puts it on his head 1.5 m above the ground. Calculate the work done by him. (Take g = 10 m/s²)
- A. 22.5 J
- B. 225 J
- C. 150 J
- D. 15 J
Answer: B. 225 J — Work done against gravity is given by W = mgh. Here, m = 15 kg, g = 10 m/s², h = 1.5 m. So, W = 15 × 10 × 1.5 = 225 J.
MCQ 4. What is the energy possessed by a body due to its motion called?
- A. Potential energy
- B. Chemical energy
- C. Kinetic energy
- D. Heat energy
Answer: C. Kinetic energy — Kinetic energy is the energy an object possesses due to its motion. The faster an object moves, the more kinetic energy it has.
MCQ 5. An object of mass 'm' is moving with a velocity 'v'. Its kinetic energy is:
- A. mv
- B. mgh
- C. 1/2 mv²
- D. Fd
Answer: C. 1/2 mv² — The formula for kinetic energy (KE) of an object of mass 'm' moving with velocity 'v' is KE = 1/2 mv².
MCQ 6. A stone is dropped from a certain height. What happens to its potential and kinetic energy just before it hits the ground?
- A. PE increases, KE decreases
- B. PE decreases, KE increases
- C. Both PE and KE increase
- D. Both PE and KE decrease
Answer: B. PE decreases, KE increases — As the stone falls, its height decreases, so potential energy (mgh) decreases. Simultaneously, its velocity increases, converting the lost potential energy into kinetic energy (1/2 mv²).
MCQ 7. A 10 kg object is raised to a height of 5 m. Calculate the potential energy gained by the object. (Take g = 9.8 m/s²)
- A. 490 J
- B. 50 J
- C. 500 J
- D. 98 J
Answer: A. 490 J — Potential energy (PE) = mgh. Here, m = 10 kg, g = 9.8 m/s², h = 5 m. So, PE = 10 × 9.8 × 5 = 490 J.
MCQ 8. Which energy transformation occurs in an electric bulb?
- A. Chemical to light and heat
- B. Electrical to sound
- C. Electrical to light and heat
- D. Mechanical to electrical
Answer: C. Electrical to light and heat — An electric bulb converts electrical energy into light energy and also produces heat as a byproduct, making it warm to the touch.
MCQ 9. According to the Law of Conservation of Energy:
- A. Energy can be created and destroyed
- B. Energy can be created but not destroyed
- C. Energy can be destroyed but not created
- D. Energy can neither be created nor destroyed
Answer: D. Energy can neither be created nor destroyed — The Law of Conservation of Energy states that energy can be transformed from one form to another, but the total amount of energy in an isolated system remains constant.
MCQ 10. What is the SI unit of power?
- A. Joule
- B. Watt
- C. Newton
- D. Kilogram
Answer: B. Watt — The SI unit of power is Watt (W), which is defined as one Joule of work done per second (1 W = 1 J/s).
MCQ 11. A machine does 100 J of work in 10 seconds. What is its power?
- A. 10 W
- B. 1000 W
- C. 100 W
- D. 1 W
Answer: A. 10 W — Power (P) = Work done (W) / Time taken (t). Here, W = 100 J, t = 10 s. So, P = 100 / 10 = 10 W.
MCQ 12. An electric motor of 200 W operates for 5 hours daily. How much electrical energy does it consume in a day in commercial units?
- A. 1 kWh
- B. 1000 Wh
- C. 0.5 kWh
- D. 200 Wh
Answer: A. 1 kWh — Energy consumed = Power × Time. Power = 200 W, Time = 5 hours. Energy = 200 W × 5 h = 1000 Wh. Since 1 kWh = 1000 Wh, the energy consumed is 1 kWh.
MCQ 13. Which of the following is NOT a simple machine?
- A. Lever
- B. Pulley
- C. Bicycle
- D. Inclined plane
Answer: C. Bicycle — A bicycle is a complex machine made up of several simple machines (like levers, wheel and axle, gears), whereas levers, pulleys, and inclined planes are fundamental simple machines.
MCQ 14. What is the primary function of a simple machine?
- A. To generate energy
- B. To create work
- C. To reduce effort or change direction of force
- D. To increase mass of an object
Answer: C. To reduce effort or change direction of force — Simple machines are used to make work easier by multiplying force, multiplying distance, or changing the direction of the applied force (effort).
MCQ 15. If a simple machine allows a 50 N effort to lift a 200 N load, what is its mechanical advantage (MA)?
- A. 0.25
- B. 4
- C. 150
- D. 250
Answer: B. 4 — Mechanical Advantage (MA) = Load / Effort. Here, Load = 200 N, Effort = 50 N. So, MA = 200 / 50 = 4.
MCQ 16. In a pulley system, if the effort moves 10 meters and the load moves 2 meters, what is the velocity ratio (VR) of the system?
- A. 0.2
- B. 5
- C. 8
- D. 12
Answer: B. 5 — Velocity Ratio (VR) = Displacement of Effort / Displacement of Load. Here, Displacement of Effort = 10 m, Displacement of Load = 2 m. So, VR = 10 / 2 = 5.
MCQ 17. An ideal machine has an efficiency of:
- A. Less than 100%
- B. 100%
- C. More than 100%
- D. Varies
Answer: B. 100% — An ideal machine is a theoretical machine where there are no energy losses due to friction or other factors. Therefore, its output work equals its input work, resulting in 100% efficiency.
MCQ 18. A machine has a mechanical advantage of 3 and a velocity ratio of 4. What is its efficiency?
- A. 75%
- B. 133%
- C. 25%
- D. 12%
Answer: A. 75% — Efficiency (η) = (Mechanical Advantage / Velocity Ratio) × 100%. Here, MA = 3, VR = 4. So, η = (3 / 4) × 100% = 0.75 × 100% = 75%.
MCQ 19. A see-saw is an example of which class of lever?
- A. Class 1 lever
- B. Class 2 lever
- C. Class 3 lever
- D. Inclined plane
Answer: A. Class 1 lever — In a Class 1 lever, the fulcrum is located between the effort and the load. A see-saw perfectly illustrates this arrangement, with the pivot point as the fulcrum.
MCQ 20. Which simple machine consists of a sloping surface that allows objects to be moved up or down with less force?
- A. Lever
- B. Pulley
- C. Wheel and axle
- D. Inclined plane
Answer: D. Inclined plane — An inclined plane is a flat, sloped surface that reduces the force needed to move an object vertically, distributing the effort over a longer distance.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.