An object falls freely from 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 given values and the unknown. The object falls freely, so its initial velocity is $u = 0 \text{ m/s}$. The total height from which the object starts is $25 \text{ m}$. The height of the roof is $5 \text{ m}$. The acceleration due to gravity is $g = 10 m/s^2$. We need to find the final velocity $v$ when the object strikes the roof.

Step 2: Calculate the displacement. The object falls from a height of $25 \text{ m}$ to a height of $5 \text{ m}$. The displacement $s$ is the distance fallen: $s = 25 m - 5 m = 20 \text{ m}$

Step 3: Use the appropriate kinematic equation. Since we have initial velocity ($u$), acceleration ($a=g$), displacement ($s$), and we need to find the final velocity ($v$), we use the equation: $v^2 = u^2 + 2as$ Substitute the known values: $v^2 = (0 m/s)^2 + 2(10 m/s^2)(20 m)$ $v^2 = 0 + 400 m^2/s^2$ $v^2 = 400 m^2/s^2$

Step 4: Solve for $v$. $v = \sqrt{400 m^2/s^2}$ $v = 20 \text{ m/s}$

The velocity with which the object strikes the roof is $\boxed{20.0 m/s}$.