Final answer:
The whole pad of paper moves when pulled slowly due to static friction being greater than inertia. When yanked suddenly, the top sheet tears away due to inertia resisting sudden force and tearing strength being exceeded.
Step-by-step explanation:
When you pull slowly on the top sheet of a pad of paper, the whole pad will move due to the static friction between the pad and the surface it is resting on. This frictional force is evenly distributed across the contact area, allowing the entire pad to move as one unit. In contrast, if you yank suddenly on the sheet, the dynamic friction acts unevenly, and the inertia of the remaining sheets resists the sudden force, causing the top sheet to tear away from the pad. This behavior illustrates Newton's First Law of Motion, where an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. In the case of tearing the sheet, the sudden application of force exceeds the tear strength and the static friction of the pad, resulting in tearing rather than collective movement.