Net external torque is the vector sum of all torques exerted on a system by objects outside that system, and it equals the rate of change of the system's angular momentum. When the net external torque is zero, angular momentum is conserved, the core idea behind AP Physics 1 Topic 7.4.
Net external torque is the total twisting effect on a system from everything outside the system. Add up every external torque (each one is a force times its lever arm, with direction) and you get one number that controls one thing, which is how fast the system's angular momentum changes. It's the rotational version of net external force. Just as net force tells you how momentum changes, net external torque tells you how angular momentum changes.
The word external is doing the heavy lifting. Torques that objects inside the system exert on each other always cancel in pairs (Newton's third law), so internal torques can never change the system's total angular momentum. That's why a spinning figure skater pulling in her arms speeds up without any outside push. Her arm muscles exert internal torques only, so her angular momentum stays fixed while her rotational inertia drops, which forces her angular speed up. If the net external torque on a system is zero, angular momentum is conserved, full stop. That conditional statement is the whole engine of Conservation of Angular Momentum problems.
This term lives in Topic 7.4, Conservation of Angular Momentum. Every conservation law in AP Physics 1 follows the same template, where some quantity stays constant if the right external influence is absent. For energy it's external work, for momentum it's net external force, and for angular momentum it's net external torque. The exam doesn't just want you to use L_initial = L_final. It wants you to justify it, and the justification is always the same sentence: the net external torque on the system is zero, so angular momentum is conserved. Knowing when that condition holds (and when it doesn't, like a ball rolling down a ramp with friction supplying an external torque) is what separates a full-credit answer from a half-credit one.
Keep studying AP Physics 1 Unit 7
Conservation of Angular Momentum (Unit 7)
This is the closest relationship on the page. Conservation of angular momentum isn't a separate rule, it's just the special case where net external torque equals zero. The conservation law is the conclusion; zero net external torque is the premise you must state to earn it.
Lever Arm and Torque Arm (Unit 7)
Before you can sum torques, you have to compute each one, and that means force times lever arm (the perpendicular distance from the axis to the force's line of action). A huge force aimed straight at the axis has zero lever arm and contributes nothing to the net external torque.
Rotational Equilibrium (Unit 7)
Rotational equilibrium is what you call it when the net external torque is exactly zero. A balanced seesaw and a freely spinning skater are both in this condition. One stays still and one keeps a constant angular momentum, but the underlying statement is identical.
Angular Acceleration (Unit 7)
For a rigid object with fixed rotational inertia, net external torque equals I times angular acceleration. That's Newton's second law dressed in rotational clothes. The angular-momentum version (torque equals the rate of change of L) is more general because it still works when rotational inertia changes, like the skater pulling in her arms.
Multiple-choice questions love the conditional logic here. A stem describes a system (a child jumping onto a merry-go-round, a collapsing spinning star, a skater extending her arms) and asks whether angular momentum is conserved or what happens to angular speed. Your job is to identify the system, check whether any torque comes from outside it, and reason from there. On free-response questions, the move that earns points is the explicit justification sentence. Write that the net external torque on the system is zero, so angular momentum is conserved, before setting L_initial equal to L_final. This mirrors the reasoning the 2019 short-answer question about blocks on a frictionless surface rewarded for linear momentum, where conservation had to be justified by the absence of an external influence, not just assumed. Watch for traps where torques exist but cancel (net torque zero, L conserved) versus situations like friction on a rolling object, where a single external torque means L is not conserved.
These are exact analogs, and students mix up which conservation law each one governs. Net external force changes a system's linear momentum (p), while net external torque changes its angular momentum (L). They're independent conditions. A system can have zero net external force but nonzero net external torque, like two equal and opposite forces applied at different points on a rod. The rod's center of mass doesn't accelerate, but the rod spins up. Check each condition separately before claiming either conservation law.
Net external torque is the sum of all torques from objects outside the system, and it equals the rate at which the system's angular momentum changes.
If the net external torque on a system is zero, the system's angular momentum is conserved. That sentence is the required justification on FRQs, not an optional extra.
Internal torques always cancel in pairs, so nothing inside a system can change the system's total angular momentum. That's why a skater pulling in her arms spins faster without any outside push.
Zero net external force does not guarantee zero net external torque. Two opposite forces at different points produce no net force but still spin the object.
Net external torque is the rotational analog of net external force, and the conservation logic works exactly the same way for L as it does for p.
It's the vector sum of all torques exerted on a system by external objects, and it equals the rate of change of the system's angular momentum. It shows up in Topic 7.4 as the condition you check before using conservation of angular momentum.
No. Angular momentum is conserved only when the net external torque on the system is zero. A ball rolling down a rough ramp, for example, has friction supplying an external torque, so its angular momentum changes.
Net external force changes linear momentum; net external torque changes angular momentum. They're independent. A pair of equal and opposite forces applied at different points gives zero net force but a nonzero net torque, so the object spins without its center of mass accelerating.
Her muscles exert only internal torques, so the net external torque is essentially zero and her angular momentum L = Iω stays constant. Pulling her arms in decreases her rotational inertia I, so her angular speed ω must increase to keep L the same.
Yes, if you're claiming conservation. Graders look for the explicit justification that the net external torque on the system is zero, so angular momentum is conserved. Skipping that sentence and jumping straight to L_initial = L_final typically costs you the reasoning point.