Q14 (b) (ii)

Criteria	Marks
Provides correct solution	2
States that the resultant force is $0$, or equivalent merit	1

Sample answer:

Constant speed $\Rightarrow$ resultant force $=0$.

$\frac{5\sqrt3}{2}g=2v+2v^2$

With $g=10$:

$25\sqrt3=2v+2v^2$
$2v^2+2v-25\sqrt3=0$
$v=\frac{-2\pm\sqrt{4+4(2)(25\sqrt3)}}4=4.1798\ldots\text{ or }-5.1798\ldots$

$\therefore\text{speed}=4.1798\ldots\text{ as moving down the slope}\approx4.2\ \mathrm{m\,s^{-1}}\text{ (1 decimal place)}$