Final answer:
The force opposing the direction of motion of a 150lb crate dragged across a factory floor is the frictional force, which is an external force opposing motion.
Step-by-step explanation:
The force that opposes the direction of motion when a worker drags a 150lb crate across a factory floor is the frictional force. Friction is an external force that acts opposite to the direction of motion and runs parallel to the surface between two objects. In this case, the frictional force counteracts the worker's effort to move the crate by dragging it along the floor. The frictional force is dependent on the materials in contact and the normal force, which is equal to the weight of the object when resting on a horizontal surface. Equation 6.1 and Equation 6.2 in physics textbooks model the relationship between friction, the materials involved, and the normal force.