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?
he 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

Answer:

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

Explanation:

Frictional force is a force that acts against the motion of any object sliding on a surface, and it is due to the contact between the molecules on the surface of the object and the molecules of the surface on which the object is sliding.

There are two types of frictional forces:

- Static friction: this occurs when the object is at rest. When the students push the piano and the piano is still at rest, this force acts in the opposite direction to the students' push. In order to be able to move the piano, the students must apply a force which is greater than the maximum value of the static force, which is given by:

$\mu_s mg$

where $\mu_s$ is the coefficient of static friction and (mg) is the weight of the piano.

- Kinetic friction: this occurs when the object is already moving. Its direction is opposite to the direction of motion of the piano, and its magnitude is given by

$\mu_k mg$

where $\mu_k$ is the coefficient of kinetic friction and (mg) the weight of the piano.

Generally, $\mu_k < \mu_s$, which means that the static friction is always greater than the kinetic friction (this is why it is usually harder to make an object start moving rather than keep it in motion along a surface).

If we apply all of this to this problem, we see that:

- When the piano is sliding, it is pulled with a force of 1200 N and it is in equilibrium. This means that the net force is zero, so the force of kinetic friction is equal in magnitude to 1200 N.

- Since the static friction is always greater than the kinetic friction, this also means that the static friction is greater than 1200 N, and this is why the students need a force greater than 1200 N to put the piano in motion.

Answer 2

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