Collision courses. Initially two particles are at positions $x_{1}=5 \mathrm{~cm}, y_{1}=0 ;$ and $x_{2}=0, y_{2}=10 \mathrm{~cm}$, with $\mathrm{v}_{1}=-4 \times$
$10^{4} \hat{\mathbf{x}} \mathrm{cm} / \mathrm{s}$, and $\mathbf{v}_{2}$ is along $-\hat{\mathrm{y}}$ as in Fig. $3.27$.
(a) What must be the value of $v_{2}$ if they are to collide? Ans. $-8 \times 10^{4} \hat{\mathbf{y}} \mathrm{cm} / \mathrm{s}$
(b) What is the value of $v_{r}$, the relative velocity? Ans. $4 \times 10^{4}(2 \hat{\mathbf{y}}-\hat{\mathbf{x}}) \mathrm{cm} / \mathrm{s}$
(c) Establish a general criterion for recognizing a collision course for two objects in terms of their positions $\mathbf{r}_{1}, \mathbf{r}_{2}$ and velocities $\mathbf{v}_{1}, \mathbf{v}_{2^{+}}$