2007f_1

# 2007f_1 - Physics 2101, First Exam, Fall 2007 September 4,...

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Physics 2101, First Exam, Fall 2007 September 4, 2007 Please turn OFF your cell phone and MP3 player! Write down your name and section number in the scantron form. Make sure to mark your answers in the scantron form. You can mark your answers in the exam form too, and keep it for reference. We will show the correct answers at the end of the exam, so you can know the grade you will receive. Feel free to detach, use and keep the formula sheet. No other reference material is allowed during the exam. You may use scientiﬁc or graphing calculators. 1

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1. Consider two vectors, ~ A = (2m) ˆ i + (6m) ˆ j - (3m) ˆ k , and ~ B = (4m) ˆ i + (2m) ˆ j + (1m) ˆ k . The product ~ A · ~ B is. .. (a) (8m) ˆ i + (12m) ˆ j - (3m) ˆ k (b) (12m) ˆ i - (14m) ˆ j - (20m) ˆ k (c) 23m 2 (d) 17m 2 = (e) None of these. The scalar product is ~ A · ~ B = A x B x + A y B y + A z B z = (2m)(4m) + (6m)(2m) + ( - 3m)(1m) = 8m 2 + 12m 2 - 3m 2 = 17m 2 . 2. The value of ˆ i · ( ˆ j × ˆ k ) is: (a) 0 (b) +1 = (c) - 1 (d) 3 (e) 3 ˆ i · ( ˆ j × ˆ k ) = ˆ i · ˆ i = +1 3. The coordinate of a particle as a function of time is given by x ( t ) = (18m / s)t - (3 . 0m / s 2 )t 2 . The particle is momentarily at rest at t =... (a) 0 (b) 2s (c) 3s = (d) 6s (e) 12s The particle is momentarily at rest when its velocity is zero. We get the velocity as a function of time from the derivative of the position: v ( t ) = 18m / s - 6m / s 2 t . The velocity is zero when t = (18m / s) / (6m / s 2 ) = 3s . 2
4. An object is thrown vertically into the air. Which of the following graphs represents the velocity v of the object as a function of the time t ? The positive direction is taken to be upward. An object thrown vertically into the air will have constant negative acceleration a = - g , so the velocity will be a linear function of time with negative slope. It will also have a positive value at t = 0 . The only graph representing such velocity is (c). 5. The graph represents the straight line motion of a car. How far does the car travel between t = 2 s and t = 5 s? (a) 4m (b) 12m (c) 24m (d) 36m = (e) 60m Since Δ x = R v ( t ) dt , the distance traveled can be obtained as the area under the curve between t = 2 s and t = 5 s . The area of the rectangle is Δ x = 3s × 12m / s = 36m . 3

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6. A bullet shot horizontally from a gun: (a) never strikes the ground (b) travels in a straight line (c) strikes the ground much later than one dropped vertically from the same point at the same instant (d) strikes the ground much sooner than one dropped from the same point at the same instant (e) strikes the ground at approximately the same time as one dropped vertically from the same point at the same instant = A bullet shot horizontally will travel with constant vertical acceleration pointing down due to gravity, and horizontal initial velocity, so the vertical component of the initial velocity v 0 y is zero. The vertical motion will be the same as any other projectile
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## This note was uploaded on 05/20/2011 for the course PHYS 2101 taught by Professor Grouptest during the Spring '07 term at LSU.

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2007f_1 - Physics 2101, First Exam, Fall 2007 September 4,...

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