0.2 Tg physical science - chapter contexts  (Page 7/10)

Refer to the chapter on units. By showing that the units $\mathrm{kg}·m·s^{-2}$ are equal to J, and mixing units and energy calculations will assist learners to be more watchful when solving problems to convert given data to SI base quantities and units.

Kinetic energy is the energy an object has because of its motion. The kinetic energy of an object can be determined by using the equation:

$E_k=\frac{1}{2}{\mathrm{mv}}^2$

In words, mechanical energy is defined as the sum of the gravitational potential energy and the kinetic energy, and as an equation:

$E_M=E_P+E_K=\mathrm{mgh}+\frac{1}{2}{\mathrm{mv}}^2$

Both the laws of conservation of energy and conservation of mechanical energy are states. To solve problems the latter is applied in the form:

$\begin{array}{}{U}_{\mathrm{top}}=U_{\mathrm{bottom}}\\ E_{P\mathrm{top}}+E_{K\mathrm{top}}=E_{P\mathrm{bottom}}+E_{K\mathrm{bottom}}\end{array}$

To assess their degree of understanding of the content and concepts, learners are advised to engage in studying the worked examples and do the set problems.

Waves and sound and electromagnetic radiation

Transverse pulses

Transverse pulses on a string or spring are discussed, but first the questions are asked: What is a medium? What is a pulse? The following terms related to transverse pulse are introduced, defined and explained: position of rest, pulse length, amplitude and pulse speed. When a transverse pulse moves through the medium, the particles in the medium only move up and down. This important concept is illustrated by a position vs. time graph. When learners engage in doing the investigation, drawing a velocity-time graph and studying the worked example, they will get to grips with the concepts. When two or more pulses pass through the same medium at the same time, it results in constructive or destructive interference. This phenomenon is explained by superposition, the addition of amplitudes of pulses.

Transverse waves

A transverse wave is a wave where the movement of the particles of the medium is perpendicular to the direction of propagation of the wave. Concepts addressed include: wavelength, amplitude, frequency, period, crests, troughs, points in phase and points out of phase, the relationship between frequency and period, i.e. $f=\frac{1}{T}$ and $T=\frac{1}{f}$ , the speed equation, $v=f\lambda$ .

Longitudinal waves

In a longitudinal wave, the particles in the medium move parallel to the direction in which the wave moves. It is explained how to generate a longitudinal wave in a spring. While transverse waves have peaks and troughs, longitudinal waves have compressions and rarefactions. A compression and a rarefaction is defined, explained and illustrated. Similar to the case of transverse waves, the concepts wavelength, frequency, amplitude, period and wave speed are developed for longitudinal waves. Graphs of particle position, displacement, velocity and acceleration as a function of time are presented. Problems set on the equation of wave speed for longitudinal waves, $v=f\lambda$ , concludes this section.

Sound waves

Sound is a longitudinal wave. The basic properties of sound are: pitch, loudness and tone. Illustrations are used to explain the difference between a low and a high pitch and a soft and a loud sound. The speed of sound depends on the medium the sound is travelling in. Sound travels faster in solids than in liquids, and faster in liquids than in gases. The speed of sound in air, at sea level, at a temperature of formula ${21}^{°}C$ and under normal atmospheric conditions, is $344m·s^{-1}$ . Frequencies from 20 to 20 000 Hz is audible to the human ear. Any sound with a frequency below 20 Hz is known as an infrasound and any sound with a frequency above 20 000 Hz is known as an ultrasound.