Intermediate physics / by W. Watson.

  • Watson, W. (William), 1868-1919.
Date:
1912
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Licence: In copyright

Credit: Intermediate physics / by W. Watson. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    ordinate. It is also usual to take the abscissae as positive when measured to the right of the point o where the axes intersect, this point being called the origin. The ordinates are usually taken as positive when measured above the point o. Thus the co-ordinates of the point p are both positive. The abscissa of the point Q is negative while the ordinate is positive. In the case of the point R both co-ordinates are negative, while for the point s the abscissa is positive but the ordinate is negative. All points included in the quadrant to the ^- right' of OY and above ox have both co-ordinates positive, and similarly for the other quadrants. Another method of specifying the position of a point on a plane is sometimes convenient. Suppose we have a fixed origin o (Fig. 2) and a fixed line ox, then we can specify the position of the Polar point p if we eive the value of the angle xop and the co-ordinates. ^ . ^ . ° distance op. In this case we are said to use polar co-ordi7iates. If we take ox as the one axis and a line through o perpendicular to ox as the other axis, we can change from polar co-ordinates (r, 6) to rectangular co-ordinates (a?, y) by means of the following relations :— x = r cos 9\ r= J^^^ y = r sin 6 \ tan ^ = - X 2. Metric and British Units of Lengfth, Volume, and Mass.—We have in the preceding section made no statement as to the units in which the co-ordinates of a point are expressed. Of course it is essential that the unit should either be explicitly stated or at any rate understood. In most scientific work the units of length employed are those of the metric system. The reason for this preference is that the subdivisions employed in this system are so Units of i^;iuch more convenient than those of the ordinary British Length. . . system. The following table gives the relations between the more commonly employed units of length of the metric system, together with the values of the British units in terms of the metric units :— 10 millimetres (mm.) = l centimetre (cm.). , 10 centimetres = 1 decimetre (dm.). 10 decimetres = 1 metre (m.). 1000 metres = 1 kilometre (km.).