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Spring 2010 - HW09_Solution

# Spring 2010 - HW09_Solution - banuelos(ojb93 Homework 09...

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banuelos (ojb93) – Homework 09 – florin – (58140) 1 This print-out should have 10 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points An object hangs motionless from a spring. When the object is pulled down, the sum of elastic potential energy of the spring and the gravitational potential energy of the object and Earth 1. stays the same. 2. decreases. 3. increases. correct Explanation: If released from its new position, the object accelerates upward and passes the equilibrium point with nonzero velocity. The two forms of potential energy are elastic potential en- ergy of the spring and gravitational potential energy. Even though the latter decreases as the object is pulled down, the sum of the two must increase for the object to be able to gain kinetic energy. 002 10.0 points A conservative force has the potential energy function U ( x ), shown by the graph. A particle moving in one dimension under the influence of this force has kinetic energy 1.0 Joule when it is at position x 1 . x 0 x 1 x 2 x 3 1 0 1 Position Potential Energy (J) Potential Energy vs Position Which of the following is a correct state- ment about the motion of the particle? 1. It moves to the left of x 0 and does not return. 2. It oscillates with maximum position x 2 and minimum position x 0 . correct 3. It cannot reach either x 0 or x 2 . 4. It moves to the right of x 3 and does not return. 5. It comes to rest at either x 0 or x 2 . Explanation: The total energy of the particle is con- served, since only a conservative force acts on it. So at any point on the axis, V ( x ) + U ( x ) = V ( x 1 ) + U ( x 1 ) = 1 . 0 J + ( 1 . 0 J) = 0 , V ( x ) = U ( x ) , where V is the kinetic energy of the particle. Since kinetic energy is bigger than or equal to zero, V ( x ) = U ( x ) 0 U ( x ) 0 J so the particle oscillates between position x 0 and x 2 .

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banuelos (ojb93) – Homework 09 – florin – (58140) 2 003 10.0 points
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