The unit vectors i\mathbf{i}i and j\mathbf{j}j are due east and due north respectively. At midday a boat A A\,A is 9 km east of a fixed origin O O\,O and is moving with constant velocity (−9i+8j)(-9\mathbf{i} + 8\mathbf{j})(−9i+8j) km h−1^{-1}−1. At the same time, another boat B B\,B is 18 km north of O O\,O and is moving with uniform velocity (−7i+4j)(-7\mathbf{i} + 4\mathbf{j})(−7i+4j) km h−1^{-1}−1.
Show that, at time t t\,t hours after midday, the position vector of A A\,A is [(9−9t)i+8tj][(9 - 9t)\mathbf{i} + 8t\mathbf{j}][(9−9t)i+8tj] km and find a similar expression for the position vector of B B\,B at this time.
Hence show that, at time ttt, the position vector of B B\,B relative to A A\,A is [(2t−9)i+(18−4t)j][(2t - 9)\mathbf{i} + (18 - 4t)\mathbf{j}][(2t−9)i+(18−4t)j] km
By using your answer to part (b), or otherwise, show that the boats would collide if they continued at the same velocities and find the time at which the collision would occur.
363 exam-style questions on Edexcel A Level Maths Further Kinematics, covering 8.1 Vectors in Kinematics, 8.2 Vector Methods with Projectiles, 8.3 Variable Acceleration in One Dimension, 8.4 Differentiating Vectors, and 8.5 Integrating Vectors. Each one has a worked solution and a mark scheme showing where the marks go.