Final answer:
Increasing an object's mass affects both static and kinetic friction, but static friction is affected more because it's generally higher than kinetic friction. More force is needed to overcome static friction to start the object moving than kinetic friction to keep it moving.
Step-by-step explanation:
When you increase the mass of an object, both static friction and kinetic friction are affected because the frictional forces are proportional to the normal force, which is in turn proportional to the mass of the object. However, because static friction is usually greater than kinetic friction, the effort required to initiate movement will increase more significantly than the effort required to maintain movement once the object is in motion. To illustrate, adding mass to a crate requires more force to overcome the static friction to start its movement, and it also increases the kinetic friction, but to a lesser extent since kinetic friction coefficients are lower than static friction coefficients.