Final answer:
The force of friction is equal to the applied force when pushing an object at a constant speed on a horizontal surface, confirming Newton's first law that the net external force must be zero for an object to maintain constant velocity.
Step-by-step explanation:
The relationship between the force of friction and the applied force when pushing an object at a constant speed on a horizontal surface can be understood through Newton's laws of motion. If an object is moving at a constant speed, the net force acting on it must be zero. This means that if the force of friction is known to be 90.0 N, the applied force must be equal and opposite to maintain constant velocity. Therefore, the applied force must also be 90.0 N to balance the friction. In terms of directional values, if we consider the applied force in the positive direction (e.g., +90 N), the frictional force opposite the motion would be equal in magnitude but opposite in direction (e.g., -90 N), thus confirming Newton's first law of motion that states that an object in motion will remain in constant velocity unless acted upon by a net external force.