Question Bank — Work, Energy, and Simple Machines

Section A — MCQs (10 questions, 1 mark each)

1. The SI unit of work and energy is:
(a) watt (b) newton (c) joule (d) metre
Answer: (c)

2. Work done by a force on an object is zero when:
(a) force and displacement are in the same direction
(b) force is zero but displacement occurs
(c) displacement is zero
(d) force acts perpendicular to displacement
Answer: (c) and (d) (both correct as per text)

3. When a girl lifts a dumbbell upwards, the work done by her on the dumbbell is:
(a) positive (b) negative (c) zero (d) undefined
Answer: (a)

4. The kinetic energy of an object of mass \(m\) moving with velocity \(v\) is given by:
(a) \(mgh\) (b) \(\frac{1}{2}mv^2\) (c) \(mv\) (d) \(mgh + \frac{1}{2}mv^2\)
Answer: (b)

5. Mechanical advantage of a fixed pulley is:
(a) greater than 1 (b) less than 1 (c) equal to 1 (d) zero
Answer: (c)

Assertion-Reason Questions

6. Assertion (A): The work done by the force of gravity on a freely falling object is positive.
Reason (R): Displacement of the object is in the direction of the gravitational force.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (a)

7. Assertion (A): The mechanical energy of a simple pendulum bob remains constant if friction is neglected.
Reason (R): Potential energy lost by the bob is completely converted into kinetic energy and vice versa.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (a)

8. Power is defined as:
(a) work done (b) rate of doing work (c) force × displacement (d) change in energy
Answer: (b)

9. For an inclined plane of length \(L\) and height \(h\), its mechanical advantage is:
(a) \(L/h\) (b) \(h/L\) (c) 1 (d) 0
Answer: (a)

10. Gravitational potential energy of an object near the Earth’s surface depends on:
(a) mass and height only (b) velocity only (c) mass, height and \(g\) (d) force only
Answer: (c)

Section B — Very Short Answer (6 questions, 2 marks each)

1. Define work done by a constant force. Give its SI unit.
2. When is the work done by a force on an object equal to zero? Give one example from daily life.
3. Differentiate between positive and negative work with one example of each.
4. State the work-energy theorem.
5. What is mechanical advantage? Write its expression for a lever.
6. Why is the work done against friction not stored as potential energy?

Section C — Short Answer (5 questions, 3 marks each)

1. Derive the expression for kinetic energy of an object of mass \(m\) moving with velocity \(v\) using the work-energy theorem.
2. A 5 kg bag is lifted to a height of 2 m. Calculate the work done and the potential energy gained (take \(g = 10\) m s\(^{-2}\)).
3. Explain with a labelled diagram how a fixed pulley changes the direction of effort but does not reduce the force required.
4. Using the example of a ball dropped from height \(h\), show that mechanical energy is conserved (neglect air resistance).
5. A truck of mass 2000 kg moves at 18 km h\(^{-1}\). Calculate its kinetic energy. If its velocity is doubled, what happens to its kinetic energy?

Section D — Long Answer (3 questions, 5 marks each)

1. (a) Draw a labelled diagram of a Class I lever showing fulcrum, effort arm, load arm, effort and load. (2 marks)
(b) Derive the relation between mechanical advantage and the ratio of effort arm to load arm for a lever. (3 marks)

2. A jet aircraft of mass 15 000 kg lands and is stopped by a wire exerting a constant backward force of 367 500 N over 100 m. Using the work-energy theorem, calculate the landing speed of the aircraft. Show all steps. (multi-step numerical)

3. (a) State the law of conservation of mechanical energy. (1 mark)
(b) A child of mass 40 kg slides down a frictionless slide of height 5 m. Calculate the speed at the bottom using energy conservation. If friction does –200 J work, find the new speed. (4 marks)

Section E — Case/Source-Based (2 questions, 4 marks each)

Case 1: In hilly regions, water flowing downhill is used in a traditional watermill (gharat). Water loses potential energy and gains kinetic energy that rotates the grinding wheel.

(i) Identify the energy transformation at the waterwheel. (1 mark)
(ii) If water falls from height 10 m, calculate potential energy lost by 2 kg of water (\(g = 10\) m s\(^{-2}\)). (1 mark)
(iii) Why does the wheel eventually stop if water flow ceases? (1 mark)
(iv) Relate this to the work-energy theorem. (1 mark)

Case 2: A girl carries a box while walking on a horizontal road. She applies an upward force equal to the weight of the box.

(i) Is any work done by this upward force on the box? Give reason. (1 mark)
(ii) Name the type of energy possessed by the box due to its position. (1 mark)
(iii) If the girl lifts the box vertically by 2 m, calculate work done (mass of box = 5 kg, \(g = 10\) m s\(^{-2}\)). (1 mark)
(iv) How does a movable pulley system help reduce the effort needed for similar lifting tasks? (1 mark)