In order to use the Pythagorean theorem to find the magnitude of a resultant vector which must be true regarding two initial vectors?

An observer that stands still as the electron moves by him, will observe

An object is dropped from a tower, 400 ft above the ground. the object's height above ground t seconds after the fall is s(t)equals400 minus 16 t squared. determine the velocity and acceleration of the object the moment it reaches the ground. the velocity of the object the moment it reaches the ground is nothing ft/s.

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.