Physics
External Torque
Quick fact
A tiny force applied at a large distance from the pivot can produce the same torque as a huge force applied close to the pivot—this is why a long wrench makes loosening a bolt effortless.
Why this is interesting
You know a door opens more easily when you push far from the hinge—but why does that distance matter? The hidden force behind that ease is external torque.
Read the full explanation
Understanding External Torque
Imagine trying to open a stubborn jar lid. You grip the lid and twist. The twisting effect you apply is torque. When that torque comes from your hand (outside the lid system), it's an external torque. External torque is simply a twist applied to an object from outside, around a pivot point. It depends on two things: the strength of the force and the distance from the pivot (the lever arm). If you push a door at the handle (far from hinges), you need less force than pushing near the hinges. That's external torque at work: torque = force × lever arm (when force is perpendicular).
A deeper explanation
External torque is the rotational counterpart of external force. Just as a net external force causes linear acceleration, net external torque causes angular acceleration. The key equation is τext = I α, where τext is the sum of all external torques, I is rotational inertia, and α is angular acceleration. Internal torques—forces that objects exert on each other within a system—always cancel in pairs, so they cannot change the system's total angular momentum. Only external torques can. This is why a spinning figure skater speeds up by pulling arms in: the torque from her muscles is internal and doesn't change her angular momentum, but the moment of inertia decreases, so angular velocity increases. External torque is central to designing machines, predicting satellite rotations, and understanding why a wrench works better with a longer handle.