As explained in the pages for demonstrations 16.03 – Ball on turntable, 16.06 – Twirl ball(s) on string and 16.12 – Masses on turntable, for an object to travel in a circular path, it must experience an acceleration perpendicular to its direction of travel, whose magnitude equals v²/r. This centripetal (for “center-seeking”) acceleration points inward, toward the center of the cirular path. In demonstration 16.12 (Masses on turntable), friction provides this acceleration, and in demonstration 16.06 (Twirl ball(s) on string), a shoelace provides it. In this demonstration, a curved strip of steel accelerates the ball that you roll into it, by constraining its motion to a roughly circular path. As soon as the ball exits the opening in the C frame, it no longer suffers a sideways acceleration, and it travels in the straight line that is tangent to its path at the point at which it leaves the frame. This is similar to what happens in demonstration 16.06 if, after you’ve been swinging the ball in a circle, you let go of the shoelace. In both cases, the motion of the ball is in the straight line tangent to its path at the point where it is no longer being accelerated.