2 The clockwork Universe

2.1 Mechanics and determinism 1/4

2.1 Mechanics and determinism 2/4

2.1 Mechanics and determinism 3/4

2.1 Mechanics and determinism 4/4

2.2 Energy and conservation 1/2

» 2.2 Energy and conservation 2/2

3 The irreversible Universe

4 The intangible Universe

5 The uncertain Universe

6 Closing items

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Other titles in the Physical World series

Describing motion

Predicting motion

Classical physics of matter

Static fields and potentials

Dynamic fields and waves

Quantum physics: an introduction

Quantum physics of matter

2.2 Energy and conservation

Part 1 of 2 | Part 2

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Perhaps the simplest form of energy is kinetic energy : the energy associated with motion. If a particle has mass m and speed v, its kinetic energy is given by the formula

Suppose the particle hits a wall and is brought to a sudden halt. It then has no speed and no kinetic energy, but the initial energy has not been lost. Rather, it has been converted into other forms of energy, such as those associated with sound and heat.

The conservation of energy can be illustrated by considering a stone that is thrown vertically upwards. The stone starts out with a certain amount of kinetic energy, but as it climbs it slows down and its kinetic energy decreases. What happens to this energy? The answer is that there is another form of energy called potential energy, which in this case is associated with the downward pull of gravity and increases as the stone climbs. On the upward part of its journey, the stone's kinetic energy is gradually converted into potential energy until, at the top of its flight, the stone is momentarily at rest. At this point, the stone has no kinetic energy and its potential energy is at its highest. On the way down, potential energy is converted back into kinetic energy, as the stone loses height and gains speed. Assuming that no other forms of energy are involved, by the time the stone returns to its initial height, all of its initial kinetic energy is recovered and the stone is once again travelling at its initial speed. Figure 1.11 shows how the kinetic and potential energies of the stone vary during its up-and-down flight. The total energy, formed by adding the kinetic and potential energies together, is also shown. You can see quite clearly that energy is converted from one form to another while the total energy remains fixed.

Figure 1.11 A stone is thrown vertically upwards and falls down again. The graph shows how the kinetic energy, potential energy and total energy vary as a stone travels up and down again. (For convenience, the potential energy is taken to be zero when the stone is launched, and when it is caught again.) Click here for larger image (7.85kb)

One of the consequences of the conservation of energy is that it makes sense to think of storing energy in order to have a ready supply whenever required. Figure 1.12 shows several examples of energy storage in action.

Figure 1.12 Some examples of energy storage

a) A hydroelectric scheme in which the gravitational potential energy of water descending from a high lake is used to drive generators that produce electricity. Click here for larger image (15.20kb)

(b) Petrol, a liquid from which it is easy to extract chemical energy. Click here for larger image (9.73kb)

(c) An electrical dry cell which stores electrical energy. Click here for larger image (4.98kb)

Continue on to 3 The irreversible Universe, part 1 of 3

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