Final answer:
The reference information provided does not directly answer the query about the speed at which car doors automatically lock. Typically, doors auto-lock at 15-20 mph, but this can vary by vehicle.
Step-by-step explanation:
The question appears to be related to the principles of physics, specifically those concerning force and motion as it pertains to the operation of car doors or the dynamics involved in opening a door manually. However, there isn't a direct answer provided for the asked question - 'At what speed do the doors automatically lock?' - in the reference information given. Instead, various scenarios about applying force to open doors and conducting car experiments are described. These scenarios talk about the effectiveness of opening doors based on force application, point of force application, and pushing direction. Another unrelated mention is of a car exercise at 50 mph, which focuses on braking distance and time but not on door locking mechanisms.
To properly answer the query, one must note that the speed at which car doors automatically lock can vary by make and model. Typically, modern vehicles have an auto-lock feature that engages when the car reaches a speed of around 15-20 mph. Please consult the vehicle's owner manual or manufacturer for the exact speed as it pertains to a specific car model.