Mass is sometimes described as the difficulty of acceleration of an object. How does Newton's second law agree with this description?

Increasing force tends to increase acceleration while increasing mass tends to decrease acceleration. Thus, the greater force on more massive objects is offset by the inverse influence of greater mass. Subsequently, all objects free fall at the same rate of acceleration, regardless of their mass.

If the mass increases, it's harder to accelerate

float.

archimedes' principle

denser than water sinks in water.

denser than mercury would sink in it