Why is it easier to slide a heavy box over a floor that it is to start it sliding in the first place?

This is because of the inertia of the object.

Explanation:

This is because of the inertia of the object.

When you push a static object you must overcome the static friction force of the object. Once you have overcome the static friction the inertia law demands that the object tend to conserve its motion. That is the reason why you need less force when the object is already in motion. In other words, the inertia gained by the object with the initial force, "helps" you with the work of moving the object. This is also the reason why the kinetic friction of an object in motion over a surface is lower than the static friction.

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