4. (15 points) A ball was thrown vertically upwards from a balcony. It is known that it fell to the ground after 3 seconds. Knowing that the initial speed of the ball is 5 m/s, determine the height of the balcony. The acceleration due to gravity is $10 \mathrm{~m} / \mathrm{s}^{2}$.
Answer: 30 m.

Solution. The equation of motion of the ball:

$y=0=h_{0}+v_{0} t-\frac{g t^{2}}{2}=h_{0}+5 \cdot 3-\frac{10 \cdot 3^{2}}{2}$. We get $h_{0}=30 \mathrm{m}$.