An object falls freely fr a height of 25m onto the roof of a building 5m high. Calculate the velocity with which the object strikes the roof (g=10ms-¹)

Step 1: Identify the quantities.
The object falls from a height of $25\,\mathrm{m}$ (above ground) onto the roof at $5\,\mathrm{m}$ (above ground).
Thus, distance fallen $s=25-5=20\,\mathrm{m}$.
Initial velocity $u=0\,m\,s^{-1}$ (freely falling from rest).
Acceleration $a=g=10\,m\,s^{-2}$.

Step 2: Use the kinematic equation.
$v^{2}=u^{2}+2as$

Step 3: Substitute the values.
$v^{2}=0^{2}+2\times10\,m\,s^{-2}\times20\,\mathrm{m}$
$v^{2}=400\,m^{2\,s^{-2}}$

Step 4: Solve for $v$.
$v=\sqrt{400}\,m\,s^{-1}$
$v=20\,m\,s^{-1}$

20 \ \mathrm{m,s^{-1}}