Because rectilinear motion takes place in one straight line, examples include a train following a straight set of tracks, a baseball thrown in a gravity-free vacuum or a penny that someone drops from the top of a tall building, heading straight to the ground. Rectilinear motion is the opposite of curvilinear motion, which involves movement along a fixed curve.
When studying physics, the most common examples of rectilinear motion use falling objects. Because of the effects of gravity and friction on anything moving in a horizontal direction, having those objects continue to move in a perfectly straight line is extremely unlikely. Even in the case of the train, having a perfectly straight track for the complete duration of the trip is not a realistic scenario. However, tracking a falling object involves a straight line, because gravity forces that object to take the straightest path to the ground.
Other physics problems examine the effects of forces such as gravity on an object's rectilinear motion. One common example involves shooting a projectile up into the air. Whether one shoots the object straight up, perpendicular to the ground, or at an angle, gravity almost immediately begins to take effect, slowing the projectile down and, in the case of the angle, turning the rectilinear path into a curvilinear one.