In order to pull a sled across a level field at a constant velocity you have to exert a constant force. Doesn’t this violate Newton’s first and second laws of motion, which imply that no force is required to maintain a constant velocity? Explain this seeming contradiction.

The first law states that an object moves with constant velocity unless resisted by an external force.

The second law states that the rate of the change of momentum is directly proportional to the applied force and the direction of the force.

Friction is the resistance provided by one surface to another surface during the movement of the surface.

In this question, friction is one of the leading forces acting on the sled while pulling it. However, friction acts in the opposite direction of the object’s motion which signifies friction cancels and counteracts the forces that cause motion. Hence, it eventually leads to smaller acceleration and reduced net force. Hence, that is one of the main reasons for applying a constant force.

Thus, this violates the first and the second laws of the motion.