Why does a heavier object fall faster

In a vacuum (no air resistance), it doesn't. All falling objects, regardless of mass, accelerate at the same rate.

However, when air resistance is taken into account, heavier objects indeed fall faster than lighter objects, provided they have the same shape and size. For example, a lead ball falls faster than a styrofoam ball.

To understand why, first look at what factors affect air resistance:

D = ½ρv²CA

where ρ is air density,

v is velocity,

C is drag coefficient,

and A is cross sectional area.

As falling objects accelerate, they eventually reach a maximum velocity where air resistance equals weight. This is called terminal velocity.

D = W

½ρv²CA = mg

v = √ (2mg / (ρCA))

If we increase m while holding everything else constant, v increases. So two objects with the same size and shape but different masses will have different terminal velocities, with the heavier object falling faster.

Before you ask why, you need to know whether.

On the moon, or any other airless body, all objects fall together, no matter how much each one weighs. We've known this for a good 500 years.

On Earth, if one object falls slower, it's only because the air caught it and held it back.