Question

Physics students study a piano being pulled across a room on a rug. They know that when it is at rest, it experiences a gravitational force of 2200 N. When it is being pulled with 1200 N it is sliding across the room in equilibrium, but the students feel like they had to pull harder than 1200 N at first to begin motion.

Which best describes the friction acting on the piano?

The static and kinetic frictional forces are equal, so the static frictional force is equal to 1200 N.
The kinetic frictional force is greater than the static frictional force, so the kinetic frictional force is greater than 1200 N.
The static frictional force is greater than the kinetic frictional force, so the static frictional force is greater than 1200 N.
The static and kinetic friction are both equal to the gravitational force of 2200 N.

Answer 1

The static frictional force is greater than the kinetic frictional force, so the static frictional force is greater than 1200 N.

Reason: while sliding in equilibrium the kinetic friction equals the pull = 1200 N. The fact that you had to pull harder initially to start motion means the maximum static friction (µsN) is larger than the kinetic friction (µkN), so the static frictional limit is greater than 1200 N.