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Physics 131

Exam 2

March 1, 2005

INSTRUCTIONS: Write your NAME and your LECTURE (01: 8:30/Eskildsen, 02: 10:40/Goussiou, 03: 3:00/Bunker) on the front of the blue exam booklet. The exam is closed book, and you may have only pens/pencils and a calculator (no stored equations or programs and no graphing). Show all of your work in the blue book. For problems II-V, an answer alone is worth very little credit, even if it is correct ­ so show how you get it. Please enter the multiple-choice answers in the stamped form on the first inside page of the exam. Suggestions: Draw a diagram when possible, circle or box your final answers, and cross out parts which you do not want us to consider.

I. Multiple Choice Questions 1. Four identical blocks are moving on a surface for which the coefficient of kinetic friction between each block and the surface is µk. The velocity of each block is indicated by the vector on the block. For which block is the force of friction between the surface and the block greatest? A) 1 B) 2 C) 3 D) 4 E) The force of friction is the same for all blocks.

2. Which of the following statements is true? A) Friction is a conservative force and does negative work. B) Potential energy may be defined by the equation U(x) = ­dF(x)/dx C) The work done by a conservative force between two points depends on the path taken between those points. D) A conservative force cannot change a body's total energy. E) The work done by a conservative force while a body moves at constant velocity must be zero. 3. The nuclear properties of different isotopes of the same element can vary dramatically, as it is for example seen in the neutron absorption cross sections of Boron (B) which differ by a factor of 7µ105 between 10B and 11B. If 10B has a rest mass of 1.66214µ10-26 kg, 11B has a rest mass of 1.82754µ10-26 kg and the neutron rest mass is mn = 1.67493µ10-27 kg, how much is the binding energy of the neutron added in the reaction 10B + n 11B? A) 11.743 MeV B) 939.72 MeV C) 9325.4 MeV D) 10253 MeV E) 20519 MeV Page 1 of 3

Physics 131

Exam 2

March 1, 2005

4. A block of mass m is pushed up against a spring, compressing it a distance x, and is then released. The spring projects the block along a frictionless horizontal surface, giving the block a speed v. The same spring projects a second block of mass 4m, giving it a speed 3v. What distance was the spring compressed in the second case? A) x B) 2x C) 3x D) 4x E) 6x

5. Negative work means A) the kinetic energy of the object increases. B) the applied force is variable. C) the applied force is perpendicular to the displacement. D) the applied force is opposite to the displacement. E) nothing; there is no such thing as negative work.

Problems II. A ramp is tilted at 30± above the horizontal. At the bottom of the ramp, a 2.5 kg block is given a velocity up the ramp of 2 m/s. The kinetic coefficient of friction between the ramp and the block is µk = 0.1. a) How far up the ramp does the block go? (Not the vertical height, but the distance along the ramp.) b) When the block slides down again, how fast will it be going at the bottom of the ramp? c) What is the total work done on the block by friction? d) What is the lowest value of static friction µs such that the block "sticks" at the highest point? =30º

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Physics 131

Exam 2

March 1, 2005

III. A 400-g mass ball at the end of a string is moving in a vertical circle of radius 50 cm. If the speed of the ball at the bottom of the circle is 5 m/s and neglecting air resistance, find: (a) The speed of the ball at the top of the circle (b) The tension of the string at the bottom of the circle (c) The tension of the string at the top of the circle. IV. A 2kg mass block rests on a vertical spring of spring constant 100 N/m. The spring (with the block on top) is compressed by 50 cm and then released from rest. (a) Neglecting air resistance, what is the speed of the block when the spring reaches its equilibrium position? (b) Neglecting air resistance, how high above the equilibrium position of the spring will the block reach? (c) The block falls then back down on the spring. If on the way down there is an average drag force (i.e., air resistance) of 10 N, what is the speed of the block as it reaches the equilibrium position of the spring again? V. A 1.25 kg ball is connected by means of two massless strings to a vertical, rotating rod as shown in the figure. The strings are tied to the rod and are taut. The tension in the upper string is 32 N. (a) Draw the free-body diagram for the ball. (b) What is the tension in the lower string? (c) What is the magnitude of the net force on the ball? (d) What is the speed of the ball?

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