Work and Energy
About 6 min · Science · English
Time worked out from the lesson's 431 words at 150 words a minute, plus a minute for each question.
Work and Energy is a foundational mechanics topic that can appear in both the Science section (direct application) and Achievers section (multi-concept problems). This chapter bridges kinematics and dynamics by introducing the energy perspective—a powerful alternative to Newton's laws for solving motion problems.
Key ideas
- Work is done when a force causes displacement in its direction. If force and displacement are perpendicular (like centripetal force in circular motion), work done is zero. Work is a scalar quantity measured in joules.
- Kinetic Energy (KE) is the energy possessed by a body due to its motion. It depends on mass and the square of velocity, so doubling velocity quadruples kinetic energy. A moving bullet, flowing river, and spinning wheel all have kinetic energy.
- Potential Energy (PE) is the energy stored in a body due to its position or configuration. Gravitational PE increases with height above ground; elastic PE stores when you compress a spring or stretch a rubber band.
- Conservation of Energy states that energy can neither be created nor destroyed, only transformed from one form to another. The total mechanical energy (KE + PE) of a system remains constant in the absence of friction or air resistance.
- Power measures the rate of doing work or the rate of energy transfer. A more powerful engine does the same work in less time. Power determines how quickly energy is converted from one form to another.
Quick reference
- Work is done only when force causes displacement in its direction; work is zero if force ⊥ displacement.
- KE = ½ mv² depends on the square of velocity; PE = mgh depends linearly on height.
- Total mechanical energy (KE + PE) is conserved when only conservative forces (gravity, spring force) act.
- Work-energy theorem: Net work done = Change in kinetic energy.
- Power = Work/Time; measured in watts; 1 kW = 1000 W; 1 kWh = 3.6 million joules.
Check yourself: 3 questions
1. A boy pushes a wall with a force of 50 N for 2 minutes but the wall does not move. How much work is done by the boy on the wall?
2. A 2 kg ball is thrown vertically upward and reaches a maximum height of 5 m above the point of release. What is the potential energy of the ball at the highest point? (Take g = 10 m/s²)
3. A car of mass 1000 kg moving at 20 m/s is brought to rest by applying brakes. What is the kinetic energy that must be dissipated to stop the car?
This lesson is cut from the topic's note, not rewritten. Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain. The questions are from Shishya's validated question bank.
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