Reading & explanation
Why moving does not always mean pushing
Imagine a cart moving on a nearly frictionless straight track. After a push ends, it can continue at constant velocity. Actual carts eventually slow because wheel and surface friction and air resistance still exert external forces. Newton's first law states that a body experiencing zero net external force retains its velocity in an inertial reference frame.
The important word is net
The vector sum of all external forces on a chosen object is the net force. A 6 N push right and a 6 N push left add to zero. Zero net force implies zero acceleration, not necessarily zero velocity. A cart already moving right at 3 m/s retains that velocity if the ideal force balance continues.
An object can be at rest too
The same balanced force diagram can represent a cart initially at rest. Initial velocity matters: zero initial velocity stays zero, while an initial velocity of 3 m/s stays 3 m/s. Distinguish a velocity arrow describing motion from force arrows describing interactions. Velocity is not an external force.
What a simulation leaves out
The Force Builder models a supported block with chosen constant external forces. It does not automatically compute rolling resistance, drag, collisions or a changing kinetic-friction direction. Real claims should state the chosen body, reference frame, initial velocity and forces rather than treating this model as a sensor measurement.