Net torque is the vector sum of every torque acting on an object about a chosen pivot. A nonzero net torque changes the object's rotational motion, producing angular acceleration via Στ = Iα; a net torque of zero means the rotation rate stays constant (rotational equilibrium).
Net torque is the rotational version of net force. You take every individual torque acting on an object (each one equal to force times lever arm, τ = rF sin θ), assign a sign based on direction (counterclockwise vs. clockwise), and add them up about the same pivot point. That sum is the net torque.
Why does it matter what the sum is? Because net torque is what actually changes rotation. Just like net force causes linear acceleration through F = ma, net torque causes angular acceleration through Στ = Iα, where I is rotational inertia. If the net torque is zero, the object either doesn't rotate or keeps spinning at constant angular velocity. If it's nonzero, the object's spin speeds up or slows down. One huge detail that trips people up is that net torque depends on your choice of pivot. The same forces can give different individual torques about different points, so always state your axis before summing.
Net torque anchors Topics 7.2 (Torque and Angular Acceleration) and 7.3 (Angular Momentum and Torque) in Unit 7 of AP Physics 1. It's the bridge between forces and rotational motion. Every rotational dynamics problem on the exam, from pulleys to hinged beams, comes down to writing Στ = Iα about a smart pivot. It also connects directly to the oscillations content in this unit. A simple pendulum oscillates because gravity exerts a restoring torque about the pivot, and that torque is what drives the SHM behavior behind the period equation T_p = 2π√(ℓ/g). Net torque is also your gateway to angular momentum, since a zero net external torque is the condition for angular momentum conservation.
Keep studying AP Physics 1 Unit 7
Newton's Second Law for Rotation (Unit 7)
Στ = Iα is literally the definition of what net torque does. Net torque plays the role of net force, rotational inertia plays the role of mass, and angular acceleration plays the role of linear acceleration. If you can write F = ma, you can write this.
Lever Arm (Unit 7)
Each torque in the sum is force times lever arm, the perpendicular distance from the pivot to the force's line of action. Picking a pivot that makes an unknown force's lever arm zero kills that torque term entirely, which is the single best trick for equilibrium problems.
Equilibrium (Unit 7)
Rotational equilibrium means net torque equals zero about any point. The 2024 beam-and-string FRQ runs on exactly this idea. You set the counterclockwise torques equal to the clockwise torques to solve for the string tension or hinge force.
Angular Velocity and Angular Momentum (Unit 7)
Net torque is the rate of change of angular momentum. Zero net external torque means angular momentum is conserved, which is why a spinning skater speeds up when she pulls her arms in. No torque needed, just a smaller rotational inertia.
Net torque shows up in two main flavors. First, nonzero net torque problems, like the 2021 short FRQ with two pulleys sharing an axle. A hanging mass creates torque through a string, and you use Στ = Iα to find the angular acceleration, often combined with Newton's second law on the hanging object. Watch out when two radii are involved, because the same force produces different torques at different distances from the axle. Second, zero net torque (static equilibrium) problems, like the 2024 long FRQ with a uniform beam attached to a wall by a hinge and held by a string. There you choose the hinge as the pivot, set counterclockwise torques equal to clockwise torques, and remember that the beam's weight acts at its center of mass, a lever arm of L/2. MCQs love qualitative versions, like asking which configuration of forces produces the greatest net torque, or whether net torque can be zero while net force is not. Always state your pivot and your sign convention before summing.
Net force and net torque are independent conditions, and an object can have one without the other. A couple (two equal, opposite forces along different lines) gives zero net force but nonzero net torque, so the object spins without its center of mass accelerating. Flip it around and a single force through the center of mass gives nonzero net force but zero net torque about that center. Full equilibrium on the AP exam requires both ΣF = 0 and Στ = 0, and FRQs regularly make you apply both equations to the same object.
Net torque is the signed sum of all torques about one chosen pivot, with counterclockwise and clockwise torques carrying opposite signs.
A nonzero net torque produces angular acceleration through Στ = Iα, the rotational version of Newton's second law.
Zero net torque means rotational equilibrium, so the object either stays still or rotates at constant angular velocity.
Net torque depends on the pivot you choose, so picking a pivot where an unknown force acts (like a hinge) eliminates that force from the torque equation.
Zero net force does not guarantee zero net torque; two equal and opposite forces along different lines still make an object spin.
When net external torque is zero, angular momentum is conserved, which is the setup for collision and spinning-skater problems.
Net torque is the sum of all torques acting on an object about a chosen pivot, where each torque equals force times lever arm (τ = rF sin θ). It determines the object's angular acceleration through Στ = Iα.
No. Two equal and opposite forces acting along different lines give zero net force but a nonzero net torque, so the object rotates while its center of mass stays put. Equilibrium problems on the AP exam require checking ΣF = 0 and Στ = 0 separately.
Torque is what one individual force contributes about a pivot; net torque is the signed sum of all of them. Only the net torque tells you the angular acceleration, just like only the net force tells you linear acceleration.
Yes, individual torques change when you change the pivot because lever arms change. That's actually useful. On the 2024 hinged-beam FRQ, choosing the hinge as the pivot makes the unknown hinge force's torque zero, leaving one equation you can actually solve.
Net torque is the rate of change of angular momentum, so zero net external torque means angular momentum is conserved. This is the foundation of Topic 7.3 and of every spinning-skater or rotational-collision problem.