A force of 7 lb is required to hold a spring stretched 3 in. beyond its natural length. How much work W is done in stretching it from its natural length to 7 in. beyond its natural length?

The focal length of a relaxed human eye is approximately 1.7 cm. when we focus our eyes on a close up object, we can change the refractive power of the eye by about 16 diopters. (a) does the refractive power of our eyes increase or decrease by 16 diopters when we focus closely? explain. (b) calculate the focal length of the eye when we focus closely.

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What is special about a dc circuit ?