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A thrown ball knocks over a wicket; a raised flowerpot can smash something if it falls. Where did they get the ability to do that?
- An object that can do work is said to possess energy . A thrown ball gains energy from the work done in throwing it; a raised pot from the work done in lifting it.
- When positive work is done on an object, it gains energy; it can then transfer that energy to another object.
- Work-energy theorem: work done on an object appears as a change in its energy. $$\text{work done on an object} = \text{change in its energy}$$
- It holds for a system of objects and even for non-constant forces. The SI unit of energy is the same as of work — the joule (J) .
- Energy — the capacity of an object to do work.
- Work-energy theorem — the work done on an object (or system) is equal to the change in its energy.
- The SI unit of work and energy, the joule , is named after James Prescott Joule . He studied how mechanical energy and thermal energy are related and can be converted from one to the other, helping build a unified understanding of energy.
In carrom, the striker hits the white coin, which hits the black coin. Who does work, and how does energy change at each collision?
The moving striker pushes the white coin in its direction of displacement, so the striker does positive work on the white coin (its energy increases). By Newton's third law the white coin does negative work on the striker (its energy decreases).
Likewise, the white coin does positive work on the black coin (energy increases), and the black coin does negative work on the white coin (energy decreases).
- Doing mechanical work is one way to transfer energy, but not the only one. Energy can also be transferred as heat (from a hotter to a colder object), by radiation (the Sun to the Earth), in electric circuits , via sound waves , and in nuclear reactions .
