All 10 short questions of 9th Class Physics Chapter 5, Work, Energy and Power, with exam-ready answers in the book's words. Punjab Board new syllabus.
Exam-ready answers. Learn the meaning, then write them in your own words.
Zero. Work needs a displacement: W = F × S. If the object stays at rest, S = 0, so W = F × 0 = 0. For example, a man pushing hard against a wall does no work on it, however tired he gets.
Kinetic energy depends on mass as well as speed: Ek = ½mv². A car is many times heavier than a motorcycle, so even at a lower speed its ½mv² can be larger.
Answerthe slower car has more kinetic energy
So F₂ delivers greater power, even though it does less work, because it does its work faster.
Still 4500 J. Gravitational potential energy is Ep = mgh: it depends on her mass and the height of the stairs, not on her speed. Running faster only increases her power (she does the same work in less time).
The product of a force and the distance moved in the direction of the force: W = F × S.
One joule of work is done when a force of 1 N moves a body 1 m in its own direction: 1 J = 1 N m.
To raise it at constant speed, a force equal to its weight, mg, must act through the height h. Work done = mg × h, and this is stored as gravitational potential energy:
The ratio of useful output energy to total input energy, usually given as a percentage: (useful output ÷ total input) × 100%.
A system can never be 100% efficient because in every energy change some energy is wasted, mostly as heat produced by friction (and sometimes as sound). No device turns all its input into useful output.
The rate of doing work: P = W ÷ t.
The power of a system that does one joule of work per second: 1 W = 1 J s⁻¹.
| Renewable | Non-renewable |
|---|---|
| Replaced naturally after use | Used up; cannot be replaced quickly |
| Will not run out | Limited stock |
| Solar, wind, hydroelectric, tidal, geothermal | Coal, oil, natural gas, nuclear fuel |
