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Friction is a force that opposes the relative motion between two surfaces in contact. There are two main types related to this context: static and kinetic friction. While the conveyor belt was moving, the forces keeping the bags still were acting through friction. Frictional forces arise due to the microscopic roughness of surfaces. These roughness peaks catch and resist movement.
A key aspect of friction is its dependency on the normal force. In our problem, the static friction initially holds the objects in place. Once the belt stops, this friction needs to be strong enough to prevent slipping.
As the belt stops, it's critical for us to focus on the static friction because it ensures that bag A stays atop suitcase B without slipping. At the point when it might slip, this friction becomes its maximum value.

The normal force is a perpendicular contact force exerted by a surface against an object sitting on it. It's essential in scenarios involving friction because it directly affects the magnitude of the frictional force.
In our exercise, the normal force comes into play for both suitcase B and bag A. Suitcase B experiences a normal force from the conveyor belt itself. This force equals the combined weight of suitcase B and bag A since both rest on the belt. Mathematically, the normal force, denoted as \( N \), was calculated to be \( 294.3 \, \text{N} \).
For bag A, the normal force is simply its own weight. This force is the basis for calculating friction since the frictional force is the product of the coefficient of friction and the normal force.

Equilibrium occurs when the forces acting on an object are balanced, resulting in no net force and, consequently, no acceleration. In this problem, as bag A stays atop suitcase B, it achieves equilibrium both vertically and horizontally.
Vertically, equilibrium is due to the normal force counteracting the gravitational force or weight. Horizontally, static friction needs to counter any horizontal forces that might cause bag A to slip.
In our exercise, establishing equilibrium for bag A is vital. The static friction must balance the kinetic friction presented by suitcase B as it interacts with the conveyor belt stopping. This ensures that bag A does not slip off suitcase B.

The static friction coefficient, denoted as \( \mu_s \), represents how much frictional force can be resisted before motion starts. It's a ratio without units that reflects the material properties and surface interactions.
In our scenario, bag A does not slip on suitcase B, indicating that the static friction force is sufficient. We calculated this coefficient as \( 0.9 \) using the equation: \( \mu_s = \frac{f_k}{W_A} \), where \( f_k \) is the kinetic friction force exerted by suitcase B on the conveyor belt, and \( W_A \) is the weight of bag A.
This coefficient is significant because it tells us how effectively the bags resist slipping against each other when the conveyor belt stops. Understanding and calculating the static friction coefficient ensures objects remain stationary relative to each other under external forces. It's crucial in designing safe and efficient systems.