Once a crate is sliding, what force is required to keep it moving at constant velocity?

To keep a sliding crate moving at a constant velocity, the force applied must be equal to and opposite the dynamic friction force acting on the crate. When an object is moving at constant velocity, according to Newton's first law of motion, the net force acting on it must be zero. This means that the applied force must balance out all opposing forces.

In this situation, the primary opposing force is the dynamic (or kinetic) friction force, which is the friction that occurs when the crate is in motion. To maintain that constant velocity, the applied force needs to counteract this dynamic friction exactly. If the applied force were greater, the crate would accelerate. If it were less, the crate would slow down and eventually stop due to friction. Therefore, the correct answer is that the applied force must be equal to and opposite to the dynamic friction force to ensure that the net force remains zero, allowing the crate to continue moving at a steady speed.