Science · Class 9
Work and Energy
Work, energy, kinetic and potential energy, and power.
3 sections15–25 min read6 MCQs · 2 examples
Chapter Overview
Work is done when force displaces an object in the direction of force. Energy is capacity to do work — kinetic and potential forms. Power is rate of doing work; commercial unit of energy is kWh.
Work & Energy
Work
W = F·s cos θ (J = N·m)
Kinetic energy
KE = ½mv²
Potential energy
PE = mgh
Conservation
Total mechanical energy constant if no non-conservative work
Power
Power
P = W/t (Watt = J/s)
kWh
1 kWh = 3.6 × 10⁶ J
Solved Examples
Step-by-step solutions — read each step before checking the final answer.
Kinetic energy
2 kg object moving at 3 m/s. Find KE.
- 1KE = ½mv² = ½(2)(9) = 9 J
Answer
9 J
Power
600 J work in 2 min. Power in watts?
- 1t = 120 s
- 2P = 600/120 = 5 W
Answer
5 W
Key Points to Remember
- ✓ Work zero if force ⊥ displacement
- ✓ 1 kW = 1000 W
- ✓ Sound energy, heat, light are other energy forms
Exam Tips
- • Check angle between F and s
- • Convert minutes to seconds for power
- • Use energy conservation to simplify problems
Practice MCQs — Work and Energy
Test your understanding with topic-wise multiple choice questions. Explanations appear after each answer.
Question 1 of 6Score: 0/0
SI unit of work is: