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Physics LibreTexts

13.2: Triangles

  • Page ID
    6869
  • I shall start with a geometric theorem involving triangles, which will be useful as we progress towards our aim of computing orbital elements.


    Figure 13.1.png
    \(\text{FIGURE XIII.1}\)

    Figure \(\text{XIII.1}\) shows three coplanar vectors. It is clearly possible to express \(\textbf{r}_2\) as a linear combination of the other two. That is to say, it should be possible to find coefficients such that

    \[\textbf{r}_2 = a_1 \textbf{r}_1 + a_3 \textbf{r}_3 . \label{13.2.1} \tag{13.2.1}\]

    The notation I am going to use is as follows:

    The area of the triangle formed by joining the tips of \(\textbf{r}_2\) and \(\textbf{r}_3\) is \(A_1\).
    The area of the triangle formed by joining the tips of \(\textbf{r}_3\) and \(\textbf{r}_1\) is \(A_2\).
    The area of the triangle formed by joining the tips of \(\textbf{r}_1\) and \(\textbf{r}_2\) is \(A_3\).

    To find the coefficients in equation \ref{13.2.1}, multiply both sides by \(\textbf{r}_1 \times\):

    \[\textbf{r}_1 \times \textbf{r}_2 = a_3 \textbf{r}_1 \times \textbf{r}_3 . \label{13.2.2} \tag{13.2.2}\]

    The two vector products are parallel vectors (they are each perpendicular to the plane of the paper), of magnitudes \(2A_3\) and \(2A_2\) respectively. (\(2A_3\) is the area of the parallelogram of which the vectors \(\textbf{r}_1\) and \(\textbf{r}_2\) form two sides.)

    \[\therefore a_3 = A_3/A_2 . \label{13.2.3} \tag{13.2.3}\]

    Similarly by multiplying both sides of equation \ref{13.2.1} by \(\textbf{r}_3 \times\) it will be found that

    \[a_1 = A_1/ A_2 . \label{13.2.4} \tag{13.2.4}\]

    Hence we find that

    \[A_2 \textbf{r}_2 = A_1 \textbf{r}_1 + A_3 \textbf{r}_3 . \label{13.2.5} \tag{13.2.5}\]