Question

With what speed must an object be thrown vertically upwards to reach a height of 91.5 m? (Take $a=9.8m{s}^{-2}$)

• A. $52.3m{s}^{-1}$
• B. $49.2m{s}^{-1}$
• C. $42.3m{s}^{-1}$
• D. $45.7m{s}^{-1}$

Answer 1

The correct option is C $42.3m{s}^{-1}$

Let the initial velocity be 'u'.
On reaching the maximum height, final velocity $v=0$.
Height, s = 91.5 m and acceleration, $a=-9.8m{s}^{-2}$
Using the third equation of motion,
$v^2=u^2+2as$,
$0^2=u^2-2\times 9.8\times 91.5$
$\Rightarrow u=42.3m{s}^{-1}$
Therefore the object should be thrown vertically up with a speed of $42.3m{s}^{-1}$