The head of a grass string trimmer has 100 g of cord wound in a light cylindrical spool with inside diameter 3.00 cm and outside diameter 18.0 cm, as in Figure P10.50. The cord has a linear density of 10.0 g/m. A single strand of the cord extends 16.0 cm from the outer edge of the spool. When switched on, the trimmer speeds up from 0 to 2450 rev/min in 0.210 s.

(a) What average power is delivered to the head by the trimmer motor while it is accelerating?

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(b) When the trimmer is cutting grass, it spins at 1960 rev/min and the grass exerts an average tangential force of 8.40 N on the outer end of the cord, which is still at a radial distance of 16.0 cm from the outer edge of the spool. What is the power delivered to the head under load?

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