All objects, neglecting air resistance, fall with the same acceleration towards the Earth's center due to the force exerted by the Earth's gravity. This experimentally determined fact is unexpected because we are so accustomed to the effects of air resistance and friction that we expect light objects to fall slower than heavier ones. People believed that a heavier object had a greater acceleration when falling until Galileo Galilei (1564–1642) proved otherwise. We now know this is not the case.

The motion of an object under the influence of gravitational force is called a free-fall. Free-fall motion is not only associated with objects dropped from rest, but with all objects moving freely under gravity's influence. The acceleration of objects under a free-fall is constant, and is known as acceleration due to gravity, g. The magnitude of acceleration due to gravity is 9.8 m/s². Because of the constant acceleration, kinematic equations of motion can be used to predict the dynamics of objects under free-fall, provided that the air effects are neglected.

In the real world, however, air resistance is always present, and it opposes the motion of objects. The momentum required to oppose the motion of heavier objects is larger compared to lighter objects. Therefore, heavier objects fall faster towards the ground during free-fall in the presence of air resistance.

This text is adapted from Openstax, University Physics Volume 1, Section 3.5: Free-fall.