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A mass falling while attached to a massive pulley.

The pulley and the bucket are treated as separate objects. The bucket can be treated as a , but the pulley must be treated as a .

The pulley and the bucket are each subject to external influences from the rope () and from the earth (gravity). The pulley is also subject to a from the axle, but we will choose the axle as the axis of rotation so the axle produces no torque (it has zero ).

and

The system is subject to external forces from the earth acting on the bucket and the pulley (gravity) and from the axle acting on the pulley (contact force). If we choose the axis of rotation to coincide with the axle of the pulley, then the axle's force and gravity acting on the pulley each produce no torque. We need only consider the effects of gravity acting on the bucket.

Bucket as .

External influences from the earth (gravity) and the rope (tension).

One-Dimensional Motion with Constant Acceleration.

Treat the bucket, pulley and the earth as a single system.

Internal interactions of gravity (conservative) and tension from the rope (non-conservative) plus an external influence from the normal force on the pulley (non-conservative).

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