Rotational work

Also called Work done by a torque

The energy a torque transfers into or out of a rigid system as that system turns, equal to the torque times the angular displacement in radians. It is measured in joules and it is the rotational counterpart of force times distance.

Turning something takes energy in the same way pushing it does, and the bookkeeping has the same shape with rotational quantities swapped in.

W=τΔθW = \tau\,\Delta\theta

EK 6.2.A.1 states the condition: a torque can transfer energy into or out of an object or rigid system if the torque is exerted over an angular displacement. No turning, no work, however large the torque. A hand holding a wrench still against a stuck bolt does none.

What each sheet prints. The AP Physics 1 and AP Physics 2 tables of information print W=τΔθW = \tau\Delta\theta. Both AP Physics C sheets print the integral form W=τdθW = \int \tau \cdot d\theta instead, which is what a torque that changes with angle needs.

Radians, or the number is wrong. τ\tau is in newton metres and Δθ\Delta\theta has to be in radians. Because the radian is a ratio of two lengths and carries no dimensions, the product comes out in joules with nothing left over. Substitute degrees and the answer is off by a factor of 180/π180/\pi with no unit mismatch to warn you.

From a graph. EK 6.2.A.3: work done on a rigid system by a given torque can be found from the area under the curve of a graph of torque as a function of angular position. Same reading as force against displacement, one axis relabelled.

Sign works the same way too. A torque acting against the rotation, a bearing dragging on an axle, does negative work and removes energy.

Where the energy lands is rotational kinetic energy, and a rolling object splits it with the translational kind.

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