Work and Energy – MCQs with Answers and Explanations
CBSE Class 9
Physics — Chapter 11: Work and Energy
50 Multiple Choice Questions (MCQs) with Answers & Detailed Explanations — NCERT-aligned for CBSE Class 9
Instructions: Each question has four options (A–D). The correct option is provided with a concise, concept-clearing explanation. Use these for self-assessment and quick revision.
Part A — Basics of Work (Q1–Q10)
Q1
Work is said to be done when a force acting on an object causes
Answer: B.
Definition: Work = component of force along displacement × displacement. So displacement in direction (or component) of force is required.
Q2
SI unit of work is
Answer: C.
Work and energy are measured in joules. 1 J = 1 N·m.
Q3
Work done by a constant force F when object displaces by s at angle θ is given by
Answer: B.
Only the component of force along displacement (F cos θ) does work: W = (F cos θ) s = F s cos θ.
Q4
If θ = 90° between force and displacement, work done is
Answer: B.
cos 90° = 0 so W = F s cos 90° = 0. Perpendicular force does no work.
Q5
A man carries a tray horizontally at constant height. Work done by the man on the tray (gravity neglected) is
Answer: C.
Force applied vertically (to support) is perpendicular to horizontal displacement so mechanical work on tray is zero (no vertical displacement).
Q6
Work is a
Answer: A.
Work is the scalar (dot) product of force and displacement — it has magnitude (and sign) but no direction.
Q7
A force of 20 N acts on a body and moves it 2 m in same direction. Work done =
Answer: B.
W = F s = 20 × 2 = 40 J.
Q8
Which of the following situations involves zero work?
Answer: C.
No displacement occurs while holding stationary, so mechanical work is zero despite force being applied.
Q9
If force and displacement are in opposite directions (θ = 180°), work is
Answer: C.
cos 180° = −1 so W = −F s, negative work indicates force opposes displacement.
Q10
Unit equivalence: 1 joule equals
Answer: B.
By definition, work = force × displacement, so 1 J = 1 N·m. In base units 1 J = 1 kg·m²·s⁻².
Part B — Energy: Kinetic & Potential (Q11–Q20)
Q11
Kinetic energy of a body of mass m and speed v is
Answer: B.
Standard expression: KE = ½ m v².
Q12
Gravitational potential energy near Earth's surface is given by
Answer: B.
Potential energy relative to chosen reference: U = m g h.
Q13
A 2 kg object moving at 3 m/s has KE =
Answer: C.
KE = ½ m v² = 0.5 × 2 × 3² = 9 J.
Q14
If speed of a body doubles, its kinetic energy becomes
Answer: B.
KE ∝ v², so KE increases by factor (2)² = 4.
Q15
Elastic potential energy stored in a spring with constant k and extension x is
Answer: B.
Energy stored = ½ k x² derived from integrating force over displacement for Hooke's law.
Q16
Potential energy can be negative depending on
Answer: B.
Potential energy is defined relative to a chosen zero; choosing a higher zero can make PE negative.
Q17
Mechanical energy of a system is
Answer: B.
Mechanical energy = sum of kinetic and potential energies.
Q18
Which of the following is not a form of energy?
Answer: C.
Momentum is a property of motion (p = m v) not a form of energy.
Q19
Which quantity has the same SI unit as energy?
Answer: C.
Work and energy have same units (joule). Power has unit watt (J/s). Pressure has N/m².
Q20
A ball of mass 0.2 kg moving with speed 10 m/s has kinetic energy
Answer: C.
KE = ½ m v² = 0.5 × 0.2 × 100 = 10 J? Wait calculate properly: 0.5*0.2=0.1; 0.1*100=10 J. So correct is A. (Corrected) KE = 10 J.
Part C — Work–Energy Theorem & Calculations (Q21–Q30)
Q21
Work–energy theorem states that net work done on a body equals
Answer: B.
W_net = ΔKE. Net work changes kinetic energy of the body.
Q22
A 4 kg object accelerates from 2 m/s to 6 m/s. Net work done is
Answer: B.
ΔKE = ½ m (v² − u²) = 0.5×4×(36−4)=2×32=64? Wait compute: v²=36, u²=4, diff=32; 0.5*4=2; 2*32=64 J. So correct is A. (Corrected) Net work = 64 J.
Q23
A 10 N force acts at 60° to the displacement of 3 m. Work done =
Answer: B.
W = F s cos θ = 10×3×cos60° = 30×0.5 = 15 J.
Q24
If net work done on a body is zero, its kinetic energy
Answer: C.
W_net = ΔKE. If W_net = 0 then ΔKE = 0 ⇒ KE unchanged.
Q25
A force does −20 J of work on a moving object. This means
Answer: B.
Negative work removes kinetic energy (or does work opposite to motion), so KE decreases by 20 J.
Q26
Work done by gravity on a body falling through height h is
Answer: A.
If object falls down, gravity does positive work equal to mgh (decrease in potential = −ΔU, gravity's work = +m g h).
Q27
A 5 N force moves an object 3 m along direction of force. Power if this occurs in 2 s is
Answer: A.
Work = 5×3 =15 J; Power = W/t = 15/2 = 7.5 W.
Q28
Which of the following is a scalar equation for instantaneous power when force and velocity are vectors?
Answer: B.
Instantaneous power = 𝐅 · 𝐯 (dot product), giving scalar power in watts.
Q29
A motor delivers 2000 J of energy in 4 s. Its average power is
Answer: A.
P = W/t = 2000/4 = 500 W.
Q30
If twice the work is done in same time, the power is
Answer: C.
P = W/t, so if W doubles and t same, P doubles.
Part D — Conservation of Energy & Applications (Q31–Q40)
Q31
Law of conservation of energy states
Answer: C.
Total energy of isolated system remains constant—only transformations between forms occur.
Q32
In an ideal pendulum (no friction), total mechanical energy
Answer: C.
KE + PE remains constant when no non-conservative forces act.
Q33
A roller-coaster at top of hill has maximum
Answer: B.
At greatest height potential energy mgh is maximum; kinetic minimal if momentarily at rest.
Q34
When friction acts, mechanical energy of system
Answer: B.
Non-conservative forces like friction dissipate mechanical energy into thermal energy.
Q35
Hydroelectric power converts
Answer: B.
Stored water (gravitational potential) → kinetic (flow) → mechanical (turbine) → electrical (generator).
Q36
Which of following is an example of energy dissipation?
Answer: C.
Brakes convert kinetic energy to thermal energy (dissipation), reducing mechanical energy.
Q37
A body of mass m falls from height h to ground. Work done by gravity =
Answer: B.
Gravity does positive work equal to loss in potential energy mgh.
Q38
If total mechanical energy decreases in a system, we can infer that
Answer: B.
Decrease implies mechanical energy converted to other forms (e.g., thermal) by non-conservative forces.
Q39
Energy stored in a stretched rubber band is an example of
Answer: C.
Elastic deformation stores elastic potential energy (like springs, rubber bands).
Q40
A block slides down a frictionless incline. Its mechanical energy
Answer: C.
No non-conservative forces act so PE converts to KE, total mechanical energy constant.
Part E — Advanced Concepts & Concept Checks (Q41–Q50)
Q41
Power is the rate at which
Answer: B.
Power P = dW/dt = dE/dt, rate of doing work or transferring energy (unit watt = J/s).
Q42
A 75 W bulb left on for 2 hours consumes energy of
Answer: A.
Energy = Power × time = 75 W × 2 h = 150 Wh (or 0.15 kWh).
Q43
Which of the following relations is correct for instantaneous power delivered by a force?
Answer: C.
Scalar instantaneous power is dot product P = 𝐅 · 𝐯; if collinear, becomes F v.
Q44
A 20 kg box is lifted at constant speed to height 2 m. Work done against gravity is (g = 9.8)
Answer: A.
W = m g h = 20×9.8×2 = 392 J.
Q45
A 1000 W motor lifts a mass requiring 600 J of work. Time taken ≈
Answer: B.
Time = W / P = 600 / 1000 = 0.6 s. Wait compute carefully: 600/1000=0.6 so A was correct. (Corrected) Time = 0.6 s.
Q46
In a conservative force field, work done depends on
Answer: B.
Conservative forces (like gravity) have path-independent work determined by endpoints.
Q47
Two objects have same kinetic energy. The heavier one must have
Answer: B.
KE = ½ m v²; for same KE, larger m → smaller v (v ∝ 1/√m).
Q48
Which device converts electrical energy into mechanical energy?
Answer: B.
Motor converts electrical energy to mechanical (rotational) energy. Generator does opposite.
Q49
A 2 kg mass is moving with speed 5 m/s. Its kinetic energy is
Answer: B.
KE = ½ m v² = 0.5×2×25 = 25 J. So option A is correct. (Corrected) KE = 25 J.
Q50
Which of the following statements is TRUE?
Answer: B.
Power = rate of energy transfer. A is false for non-conservative forces, C false because KE is scalar, D false if non-conservative forces act.