# hour 1 - University of Minnesota Name School of Physics and...

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Unformatted text preview: University of Minnesota Name: School of Physics and Astronomy Student ID No: Physics 1301, Section 200, Fall 2008 TA Name: Hour Exam 1: Problem 1 (Friday, September 19, 2008) [Please do the problem on the front and back of this sheet of paper.] A physics professor, equipped with a rocket backpack, steps out of a hovering helicopter at an altitude of 575 m with zero initial velocity. For 8.0 s, she falls freely. At that time, she fires her rockets and slows her rate of descent (decelerates) at 15 m/s2, until her rate of descent reaches 5 m/s. At that time, she adjusts her rockets so that her acceleration is zero and she descends the remaining distance to the ground at 5.00 m/s. Neglect air resistance. [Hint: This is a onedimensional motion problem.] (a) The professor's initial altitude is 575 m. What is her altitude at the end of her free fall? (5 points) (b) The professor's initial velocity is zero. What is her velocity at the end of her free fall? (5 points) (c) Once the professor fires her rockets, how long does it take for her to decelerate to 5 m/s? (5 points) (d) What is the total time for the professor's trip from the helicopter to the ground? (5 points) (e) Draw a graph showing the professor's altitude as a function of time. Assume that time is zero at the moment the professor steps out of the helicopter. (5 points) University of Minnesota Name: School of Physics and Astronomy Student ID No: Physics 1301, Section 200, Fall 2008 TA Name: Hour Exam 1: Problem 2 (Friday, September 19, 2008) [This problem is similar to the "Optional Homework Problem", Chapter 1, Problem 73. Please do the problem on the front and back of this sheet of paper.] Our Sun is a typical star. The mass of the Sun is 1.989 x 1030 kg. Assume that a particular galaxy consists of 2.00 x 1011 Suntype stars. (a) What is the total mass of Suntype stars in this galaxy? (5 points) (b) About 75% of the mass of the Sun is hydrogen and 25% of the mass of the Sun is helium. If the stars in this galaxy are all the same as the Sun, what are the masses of hydrogen and helium in this galaxy? (5 points) (c) The mass of the hydrogen atom is 1.67 x 1027 kg. The mass of the helium atom is 4.00 times the mass of the hydrogen atom. How many hydrogen atoms are present in this galaxy? (5 points) (d) How many helium atoms are present in this galaxy? [5 points] (e) Assume that this galaxy also contains dark matter with a total mass equal to the total visible mass. If the mass of each dark matter particle is 100 times the mass of a hydrogen atom, how many dark matter particles are present in this galaxy? (5 points) University of Minnesota School of Physics and Astronomy Physics 1301, Section 200, Fall 2008 Hour Exam 1: Multiple Choice (Friday, September 19, 2008) [Please mark the answers to the problems below on the bubble sheet. Each question is 5 points.] 1. A liter is the volume of a cube that is 10 cm on each side. A cubic meter is how many liters? (a) 100 (b) 101 (c) 102 (d) 103 (e) 104 2. You are playing "go fetch" with your dog. After you throw the stick, the dog runs 20.0 m in 1.00 s and then returns 15.0 m with the stick in 1.5 s. What is the dog's average velocity? (a) 1.0 m/s (b) 2.0 m/s (c) 3.0 m/s (d) 8.0 m/s (e) 14 m/s 3. The position of a stone dropped from a cliff is described approximately by the function y = 5t2, where y is the height in meters (positive downward) and t is the time in seconds. What is the velocity of this stone at t = 3 s? (a) 5 m/s (b) 10 m/s (c) 15 m/s (d) 20 m/s (e) 30 m/s 4. A cheetah can accelerate from 0 to 96 km/h in 2.0 s. What is the acceleration of the cheetah in terms of the acceleration of gravity g = 9.81 m/s2. (a) 0.6 g (b) 0.8 g (c) 1.2 g (d) 1.4 g (e) 1.6 g 5. A crazy physicist climbs to the top of a high tower and drops two bowling balls with a one second interval between the drops. Which of the following statements are true? I. The acceleration of the first bowling ball is always greater than that of the second bowling ball until the first one hits the ground. II. The velocity of the first bowling ball is always greater than the velocity of the second bowling ball until the first one hits the ground. III. When the two balls are both falling the separation between the balls decreases with time. (a) Statement I only. (b) Statement II only. (c) Statement III only. (d) Statements I and II only. (e) Statements II and III only. ...
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