A stone of mass 0.50 kg is dropped from a height of 20 m above the ground. Calculate its speed just before it hits the ground. (g = 10 m s -2, air resistance is negligible.)

The stone is dropped, so its initial velocity $u = 0 \text{ m/s}$.
The height $h = 20 \text{ m}$.
The acceleration due to gravity $g = 10 \text{ m/s}^2$.
We need to find the final speed $v$.

Since air resistance is negligible, we can use the equation:

The speed of the stone just before it hits the ground is $\boxed{20 \text{ m/s}}$.

7. Step 1: Identify the given values for the lens problem. The object distance $u = 30 \text{ cm}$. The focal length of the converging lens $f = 15 \text{ cm}$. We need to find the image distance $v$.

The lens formula is given by:

To subtract the fractions, find a common denominator, which is 30 cm.

The position of the image formed is $\boxed{30 \text{ cm}}$. Since $v$ is positive, the image is real and formed on the opposite side of the lens from the object.