Work and Energy
Q1. Define work in physics.
A. Work is said to be done when a force is applied on a body and the body is displaced in the direction of the force.
Q2. Write the formula for work.
A. Work = Force × Displacement × cosθ
Q3. When is work said to be zero?
A. Work is zero when:
1. No displacement occurs.
2. Force is perpendicular to displacement.
Q4. What is positive work? Give example.
A. When force and displacement are in the same direction, work done is positive. Example: Pushing a moving car forward.
Q5. What is negative work? Give example.
A. When force and displacement are in opposite directions, work is negative. Example: Friction on a moving object.
Q6. What is power?
A. Power is the rate at which work is done. Formula: P = Work / Time
Q7. State the SI unit of work and power.
A. Work – Joule (J), Power – Watt (W)
Q8. Define energy.
A. Energy is the capacity to do work.
Q9. What is kinetic energy? Write formula.
A. Kinetic energy is the energy possessed by a body due to its motion.
Formula: KE = ½ m v²
Q10. What is potential energy? Write formula.
A. Potential energy is the energy possessed by a body due to its position or configuration.
Formula: PE = m g h
Q11. State the work-energy theorem.
A. The work done on an object is equal to the change in its kinetic energy.
Q12. Give an example of work-energy theorem in daily life.
A. A moving car stops when brakes are applied — work done by friction equals loss of kinetic energy.
Q13. What is mechanical energy?
A. The sum of kinetic energy and potential energy of a body is called mechanical energy.
Q14. State the law of conservation of energy.
A. Energy can neither be created nor destroyed; it can only change from one form to another, but total energy remains constant.
Q15. Give an example of conservation of energy.
A. A swinging pendulum — kinetic energy and potential energy continuously change but total energy remains constant.
Q16. What is commercial unit of energy?
A. Kilowatt-hour (kWh) is the commercial unit of energy.
Q17. Convert 1 kWh into joules.
A. 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J
Q18. Why does work done become zero in uniform circular motion?
A. Because the force is always perpendicular to the displacement.
Q19. Give two examples of kinetic energy.
A. 1. Moving car
2. Flowing water
Q20. Give two examples of potential energy.
A. 1. Water stored in a dam
2. Stretched spring
Q21. What is power of a machine?
A. The rate at which a machine does work is its power.
Q22. What is average power?
A. The total work done divided by the total time taken.
Q23. What is instantaneous power?
A. The power at a particular instant of time.
Q24. Why is the kinetic energy always positive?
A. Because mass and the square of velocity are always positive.
Q25. Write relation between work and kinetic energy.
A. Work done = Change in kinetic energy (W = KE₂ - KE₁)
Q26. Why does a stretched bow have potential energy?
A. Because of its changed configuration, it can do work on the arrow.
Q27. Why is work done by gravity on a falling object positive?
A. Because force due to gravity and displacement are in the same direction.
Q28. Give one situation where potential energy changes into kinetic energy.
A. Water falling from a height in a dam.
Q29. Why is energy important in our daily life?
A. All activities require energy, from moving vehicles to operating machines.
Q30. Why is work done zero when a coolie carries a load on his head at a constant speed on a level road?
A. Because the force applied is vertical while displacement is horizontal, so θ = 90° and cosθ = 0.