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Revision as of 17:09, 11 October 2010 (edit) Ian (Talk | contribs) (New page: The expression for the position vector <math>\mathbf{r}</math> has been obtained in part b). <math>\mathbf{r}=\left( 40t \right)\mathbf{i}+\left( -2t+400 \right)\mathbf{j}</math>) ← Previous diff		Current revision (17:57, 27 March 2011) (edit) (undo) Ian (Talk | contribs)
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	The expression for the position vector <math>\mathbf{r}</math> has been obtained in part b).		The expression for the position vector <math>\mathbf{r}</math> has been obtained in part b).
-	<math>\mathbf{r}=\left( 40t \right)\mathbf{i}+\left( -2t+400 \right)\mathbf{j}</math>	+	<math>\mathbf{r}=\left( 40t \right)\mathbf{i}+\left( {-{t}^{2}}+400 \right)\mathbf{j}</math>
		+
		+	If the aeroplane is due east of the origin its position in the north-south direction must be zero, which means the <math>\mathbf{j}</math> component of the position vector must be zero.
		+
		+	<math>\begin{align}
		+	& {-{t}^{2}}+400=0 \\
		+	& \\
		+	& t=20\text{ s} \\
		+	\end{align}</math>

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Current revision

The expression for the position vector \displaystyle \mathbf{r} has been obtained in part b).

\displaystyle \mathbf{r}=\left( 40t \right)\mathbf{i}+\left( {-{t}^{2}}+400 \right)\mathbf{j}

If the aeroplane is due east of the origin its position in the north-south direction must be zero, which means the \displaystyle \mathbf{j} component of the position vector must be zero.

\displaystyle \begin{align} & {-{t}^{2}}+400=0 \\ & \\ & t=20\text{ s} \\ \end{align}