B- Frictional force.
Friction forces are opposing forces acting in a direction that is opposite to the direction of the applied force or to the direction of motion.
- The friction force acting on a body when it is still doesn’t move is the static friction force.
- If the applied force exceeds the friction force so the body starts to move the friction between the body and the surface will decrease, the friction force acting on the box during its motion is called the Dynamic (kinetic) friction force.
- Static friction is greater than dynamic (kinetic) force.