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Unformatted text preview: FNT #4: Kinetic energy, PE grav and PE mass-spring Damien Martin November 30, 2006 Question 1 Three identical rocks labelled X , Y and Z are thrown from the top of a building with the same initial velocity. Ball X is thrown directly down. Ball Y is thrown directly up. Ball Z is thrown across. Which is going the fastest just before they hit the ground? (You may neglect air resistance). Solution Let us call the ground y = 0. Let us look at a particular ball (say X). We know that it starts at a height y i > 0 and ends up at y f = 0. We know that the initial velocity has some value v i > 0 (headed downward). Writing this as an energy-interaction diagram is ' & $ % PE grav y ' & $ % KE v y i > y f = 0 v i > v f ??? KE + PE grav = 0 1 Because PE grav is decreasing, it must be true that KE is increasing. So let us finish off our diagram. ' & $ % PE grav y ' & $ % KE v y i > y f = 0 v i > v f > v i KE + PE grav = 0 Observation 1: This diagram is the same for all three rocks. Because weThis diagram is the same for all three rocks....
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