A ship A is moving Westwards with a speed of 10 kmph and a ship B 100 km South of A, is moving Northwards with a speed of 10 kmph. The time after which the distance between them becomes the shortest is:
1. 0 h
2. 5 h
3. $5\sqrt{2}$h
4. $10\sqrt{2}$h

Two particles A and B, move with constant velocities v₁ and v₂ respectively. At the initial moment, their position vectors are r₁ and r₂ respectively. The condition for particles A and B for their collision will be

1. $\frac{{\mathrm{r}}_1-{\mathrm{r}}_2}{\left|{\mathrm{r}}_1-{\mathrm{r}}_2\right|}=\frac{{\mathrm{v}}_2-{\mathrm{v}}_1}{\left|{\mathrm{v}}_2-{\mathrm{v}}_1\right|}$

2. ${\mathrm{r}}_1·{\mathrm{v}}_1={\mathrm{r}}_2·{\mathrm{v}}_2$

3. ${\mathrm{r}}_1\times {\mathrm{v}}_1={\mathrm{r}}_2\times {\mathrm{v}}_2$

4. ${\mathrm{r}}_1-{\mathrm{r}}_2={\mathrm{v}}_1-{\mathrm{v}}_2$