Final answer:
The kinetic energy of the brick when it starts to fall from the top of the roof is 39.2 J, and when it reaches the ground, it is 39.17 J.
Step-by-step explanation:
To determine the kinetic energy of the brick when it starts to fall, we need to calculate its gravitational potential energy at the top of the roof. Gravitational potential energy is given by the formula PE = mgh, where m is the mass of the brick (1 kg), g is the acceleration due to gravity (9.8 m/s^2), and h is the height of the roof (4 m). So, the potential energy at the top is PE = 1 kg * 9.8 m/s^2 * 4 m = 39.2 J.
When the brick reaches the ground, all of its potential energy is converted into kinetic energy. The formula for kinetic energy is KE = 1/2 * mv^2, where m is the mass of the brick (1 kg) and v is its velocity (8.85 m/s). So, the kinetic energy at the ground is KE = 1/2 * 1 kg * (8.85 m/s)^2 = 39.17 J.