Physics · Ch 4 — Laws of Motion
Couple and its Torque
Couple and its Torque
In the discussion of torque above, a single force acting on a body pivoted about a fixed axis was considered -- but in that case there is necessarily a reaction force at the fixed axis itself. In many everyday situations we instead apply TWO equal-and-opposite forces, acting along two different (parallel) lines of action, specifically to produce pure rotation without needing any external pivot reaction -- turning a bicycle handle or a steering wheel, or opening a tap or a bottle cap are everyday examples. Such a pair of equal-magnitude, opposite-direction forces acting along different lines of action is called a COUPLE, and is used to realise purely rotational motion; the rotational effect (moment) of a couple is, like a single force's moment, also called a torque. …
What this figure shows. A diagram showing two parallel force vectors F1 and F2 of EQUAL magnitude but drawn pointing in OPPOSITE directions, acting along two separate parallel lines that are a perpendicular distance r apart. A position vector r12 is drawn from a point on the line of action of F2 to a point on the line of action of F1, and (implicitly, by symmetry) a position vector r21 is drawn the opposite way, from a point on the line of action of F1 to a point on the line of action of F2; the angle each of these position vectors makes with its associated force vector (alpha for r12 with F1, beta for r21 with F2) is marked, since r12 sin(alpha) = r21 sin(beta) = r, the fixed perpendicula …