A toy boat moves in a horizontal plane with position vector \(\mathbf{r} = x\mathbf{i} + y\mathbf{j}\), where \(\mathbf{i}\) and \(\mathbf{j}\) are the standard unit vectors east and north respectively. The origin of the position vectors is at O. The displacements \(x\) and \(y\) are in metres.
First consider only the motion of the boat parallel to the \(x\)-axis. For this motion
$$x = 8t - 2t^2.$$
The velocity of the boat in the \(x\)-direction is \(v_x\) ms\(^{-1}\).
- Find an expression in terms of \(t\) for \(v_x\) and determine when the boat instantaneously has zero speed in the \(x\)-direction. [3]
Now consider only the motion of the boat parallel to the \(y\)-axis. For this motion
$$v_y = (t - 2)(3t - 2),$$
where \(v_y\) ms\(^{-1}\) is the velocity of the boat in the \(y\)-direction at time \(t\) seconds.
- Given that \(y = 3\) when \(t = 1\), use integration to show that \(y = t^3 - 4t^2 + 4t + 2\). [4]
The position vector of the boat is given in terms of \(t\) by \(\mathbf{r} = (8t - 2t^2)\mathbf{i} + (t^3 - 4t^2 + 4t + 2)\mathbf{j}\).
- Find the time(s) when the boat is due north of O and also the distance of the boat from O at any such times. [4]
- Find the time(s) when the boat is instantaneously at rest. Find the distance of the boat from O at any such times. [5]
- Plot a graph of the path of the boat for \(0 \leq t \leq 2\). [3]