Before we understood that objects have a tendency to maintain their velocity in a straight line unless acted upon by a net force, people thought that objects have a tendency to stop on their own. This happened because a specific force was not yet understood. What was that force?

Frictional force

Explanation:

The passive force that stops a body from its state of uniform motion is the force of friction between the surfaces that have relative motion between them and it always acts in the direction opposite to the relative motion of the body.

Force of friction is mathematically given as:

where:

coefficient of friction between the two surfaces having relative motion

reaction force due to the weight of the body acting on the body normal to the surface