A stroke engine has a slider-crank mechanism that converts rotational motion from the crank into linear motion of the slider or vice versa. This mechanism consists of three main parts: the crank, the connecting rod, and the slider.

When an external force is exerted, it sets the crank into a rotational movement. This, in turn, instigates the motion of the connecting rod, leading to what is referred to as a general plane motion. This process involves two key points - point A on the connecting rod and point B on the crank. Both these points experience translational motion. In addition to this, point B also experiences a rotational motion in relation to point A. This dual motion at point B adds complexity to the mechanics of the stroke engine.

To understand the movements of points A and B, different frames of reference can be used. The absolute translational motion of both points can be examined using a stationary or fixed frame, while the rotational motion of point B relative to point A can be studied with a translating x'y' frame attached to point A.

The absolute linear velocity of point B can then be calculated as the vector sum of two components - the absolute linear velocity of point A and the relative velocity of point B with respect to point A. The latter term signifies the relative velocity of point B due to its rotational motion. This motion's direction is always perpendicular to segment AB, which connects points A and B.