Why was Copernicus reluctant to publish his ideas?

Which best represents the law of conservation of mass ?

When you push downward on a book at rest on a table, you feel an upward force. does this force depend on friction? defend your answer.

A2.00-m long piano wire with a mass per unit length of 12.0 g/m is under a tension of 8.00 kn. what is the frequency of the fundamental mode of vibration of this wire?