Final answer:
Both the 4 kg and 2 kg bricks, when dropped from the Leaning Tower of Pisa, will hit the ground in 2.1 seconds because the acceleration due to gravity acts equally on all objects irrespective of their mass.
Step-by-step explanation:
According to the principles of physics demonstrated by Galileo's experiment, when two objects of different masses are dropped from the same height without any air resistance, they should hit the ground at the same time. This is because the acceleration due to gravity is constant for all objects on Earth's surface, irrespective of their mass. In the scenario provided, Galileo is standing on the leaning tower of Pisa and drops two granite bricks, one weighing 4 kg and the other weighing 2 kg. If he releases them simultaneously and without air resistance, they will both take the same amount of time to hit the ground. Therefore, if the 4 kg brick took 2.1 seconds to reach the ground, the 2 kg brick would also take 2.1 seconds to hit the ground.