How can I calculate the thrust if the submerged volume is not given in the problem?

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The image shows a pendulum in simple harmonic motion the pendulum starts at a and swing to e

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.