When observed in an inertial frame of reference, all objects that experience no forces will appear to move in a straight line at a constant speed (or remain at rest). This is a fundamental principle of physics known as Newton's First Law of Motion or the Law of Inertia.

Here's why:

* Inertial Frame: An inertial frame is a reference frame where objects at rest remain at rest and objects in motion continue in motion at a constant velocity unless acted upon by a force.

* No Forces: If an object experiences no forces, there is nothing to change its state of motion.

* Straight Line and Constant Speed: In the absence of forces, objects maintain their existing velocity. This means they move in a straight line and at a constant speed.

Examples:

* A ball rolling on a frictionless surface will continue to roll in a straight line at a constant speed.

* A spacecraft in deep space, far from any gravitational influence, will continue moving at a constant velocity.

Note: It's important to remember that even though an object appears to be at rest, it might still be moving at a constant velocity in a different reference frame. For example, a person sitting in a car moving at a constant speed might appear at rest to someone else sitting inside the car, but they are actually moving relative to someone standing on the sidewalk.