a. The box experiences static friction because the box is moving at a constant velocity. Static friction is the force that opposes the initiation of motion between two surfaces that are in contact and at rest relative to each other.
b. The four forces acting on the box are:
1. Force of gravity (weight), which acts downwards towards the center of the earth, and has a magnitude of mg, where m is the mass of the box and g is the acceleration due to gravity.
2. Normal force, which is the force exerted by the floor on the box, perpendicular to the surface of contact, and has a magnitude of mg when the box is at rest or moving at a constant velocity.
3. Force applied by the student, denoted as FSB, which is directed to the right and has a magnitude that is equal to or greater than the force of friction.
4. Force of friction, denoted as f, which opposes the motion of the box and is directed to the left, opposite to the direction of the applied force. The magnitude of the frictional force is equal to the magnitude of the force applied by the student, as long as the box is moving at a constant velocity.