Air resistance
Explanation
According to the principles of gravitation, in a vacuum (where there is no air), all objects fall with the same acceleration due to gravity ($g$), regardless of their mass, shape, or density. This was famously demonstrated by Galileo. However, in a fluid medium like air, falling objects experience an opposing force known as fluid friction or air resistance (drag).
Detailed Analysis:
- Difference in mass and Difference in density - Incorrect: While mass and density influence the terminal velocity of an object falling through a fluid, they are not the primary reason for the difference in fall time. If the same experiment were conducted in a vacuum, the paper and the stone would hit the ground simultaneously despite differences in mass and density.
- Difference in gravitational pull - Incorrect: The acceleration due to gravity ($g$) acting on both objects is approximately the same. Gravitational acceleration is independent of the mass of the falling body ($g = \frac{GM}{R^2}$).
- Air resistance - Correct: The sheet of paper has a large surface area relative to its weight. As it falls, it encounters significant air resistance, which acts as an upward force opposing gravity. This reduces the net downward acceleration of the paper significantly more than that of the stone, causing the paper to fall slower.
Key Takeaway:
The variation in the speed of falling objects in the atmosphere is due to air resistance. In the absence of air resistance (vacuum), all objects fall at the same rate irrespective of their mass.