PHYSICS 111 SPRING EXAM 2: March 7, 2017; 8:15-9:45 pm

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1 PHYSICS 111 SPRING 017 EXAM : March 7, 017; 8:15-9:45 pm Name (printed): Recitation Instructor: Section # INSTRUCTIONS: This exam contains 0 multiple-choice questions plus 1 extra credit question, each worth 3 points. Choose one answer only for each question. Choose the best answer to each question. Answer all questions. Allowed material: Before turning over this page, put away all materials except for pens, pencils, erasers, rulers and your calculator. There is a formula sheet attached at the end of the exam. Other copies of the formula sheet are not allowed. Calculator: In general, any calculator, including calculators that perform graphing, is permitted. Electronic devices that can store large amounts of text, data or equations (like laptops, e-book readers, smart phones) are NOT permitted. Devices (including calculators and smart watches) with WiFi technology are NOT permitted. If you are unsure if your calculator is allowed for the exam, ask your TA. How to fill in the bubble sheet: Use a number pencil. Do NOT use ink. If you did not bring a pencil, ask for one. You will continue to use the same bubble sheet you already used for Exam 1. Bubble answers -4 on the bubble sheet for this exam. If you did not take the first exam, write and fill in the bubbles corresponding to: - Your last name, middle initial, and first name. - ««Your ID number (the middle 9 digits on your ISU card) ««- Special codes K to L are your recitation section. Always use two digits (e.g. 01, 09, 11, 13). Please turn over your bubble sheet when you are not writing on it. If you need to change any entry, you must completely erase your previous entry. Also, circle your answers on this exam. Before handing in your exam, be sure that your answers on your bubble sheet are what you intend them to be. You may also copy down your answers on a piece of paper to take with you and compare with the posted answers. You may use the table at the end of the exam for this. When you are finished with the exam, place all exam materials, including the bubble sheet, and the exam itself, in your folder and return the folder to your recitation instructor. No cell phone calls allowed. Either turn off your cell phone or leave it at home. Anyone answering a cell phone must hand in their work; their exam is over. Best of luck, Dr. Soeren Prell

2 . A spring scale is tied to two ropes that run over pulleys as shown in the figure. The pulleys are ideal (massless and frictionless) and the masses of the strings and the scale are negligible. What force does the spring scale read? A).0 N B) 0.5 N C) 0.0 N D) 1.0 N E) More than.0 N The scale reads the tension in the ropes. It is 1.0 N. This would be the same, if one end of the scale would be tied to a wall. 3. A 10-kg block is held at rest on a frictionless horizontal table and is connected by a horizontal string to a 63-kg block that is hanging over the edge of the table. What is the magnitude of the acceleration of the 10-kg block when it is released? A) 8.1 m/s B) 9.0 m/s C) 8.5 m/s D) 7.5 m/s E) 6.6 m/s ( 63 kg)g T ( 63 kg)a; T ( 10 kg)a ( 63 kg)g ( 10 kg)a ( 63 kg)a a ( 63 kg ) g 8.5 m/s 73 kg

3 4. A 6.00-kg box is held at rest by two light wires that form 30 angles with the vertical. An external force of magnitude F acts vertically downward on the box. The tension exerted by each of the two wires is denoted by T. A free-body diagram, showing the four forces that act on the box, is shown in the figure. If the magnitude of force F is 410 N, what is the magnitude of the tension T? A) 376 N B) 71 N C) 470 N D) 35 N E) 188 N T cos( 30 o ) w F 0 T F + w cos 30 o 410 N + (6 9.8 N) cos 30 o 71 N 5. Two frisky otters slide down frictionless hillsides of the same height but different slopes. The slope of the hill of otter 1 is 30 (with the horizontal), while the slope of the hill of otter is 60. If both start from rest, which otter is moving faster when she reaches the bottom of her hill? A) Otter 1 is moving faster. B) Otter is moving faster. C) Both otters have the same speed at the bottom. D) The otter that took the shorter time is moving faster. E) The heavier otter is moving faster, no matter which hill she used. PE f + KE f PE 0 + KE 0 mgy f + 1 mv f mgy mv 0; 1 mv f mg(y 0 y f ) v f g( Δy) If the initial height is the same the final speed will be the same for the two otters.

4 6. An ideal spring with a force constant of 15 N/m is initially compressed by 3.0 cm from its uncompressed position. By how much is the potential energy stored in the spring changed when the spring is compressed by an additional 4.0 cm? A) 0.01 J B) J C) J D) 0.04 J E) J 1 PE f PE 0 1 kx f 1 kx 0 1 k x f x 0 ( ) J ( 15 N/m ) ( 0.07 m) 0.03 m 7. The horizontal force that an animal exerts on a large fruit it has found is observed and shown in the graph in the figure. If the fruit was initially sliding on the frictionless ground at 5.5 cm/s when the animal first touched it, by how much did the animal change its kinetic energy during this encounter? A) 50 J B) 0 J C) 5 J D) 1.5 J E) J The change in kinetic energy of the fruit is equal to the work done on the fruit. The work done on the fruit is the area under the force (F) versus distance (x) curve.

5 8. A person carries a.00-n pebble through the path shown in the figure, starting at point A and ending at point B. The total time from A to B is 6.75 min. How much work did gravity do on the rock between A and B? A) 30.0 J B) 56.0 J C) 30.0 J D) 56.0 J E) 36.0 J W g PE PE 0 PE f mg( y 0 y f ) (.0 N) ( 1.5 m 16.5 m) 30 J 9. Person X pushes twice as hard against a stationary brick wall as person Y. Which one of the following statements is correct? A) Both do positive work and person X does four times the work of person Y. B) Both do positive work and person X does twice the work of person Y. C) Both do the same amount of positive work. D) Both do zero work. E) Both do negative work. Without displacement no work is done. 30. A railroad car collides with and sticks to an identical railroad car that is initially at rest. After the collision, the kinetic energy of the system A) is one fifth as much as before. B) is one quarter as much as before. C) is one third as much as before. D) is half as much as before. E) is the same as before. mv 0 (m)v f v f v 0 ; KE f KE 0 1 ( m)v f 1 mv 0 v f v 0 / 4 1 v 0 v 0

6 31. Give the x coordinate of the center of mass for the system of four point masses located at the positions shown in the figure. A).0 m B).3 m C).5 m D).7 m E).9 m (8 kg) 0 m x CM + (4 kg) ( 3 m) + (6 kg) ( 5 m) + ( kg) ( m ) ( ) kg 46 kg m 0 kg.3 m 3. A car goes around a circular curve on a horizontal road at constant speed. What is the direction of the friction force on the car due to the road? A) perpendicular to the curve inward B) tangent to the curve opposite to the direction of the car's motion C) tangent to the curve in the forward direction D) perpendicular to the curve outward E) There is no friction on the car because its speed is constant. Friction provides the centripetal or radial force to keep the car on the curve. It points inward to the center of the circular curve.

7 33. A planet has four times the Earth's mass and twice its radius. If a jar of peanut butter weighs 1 N on the surface of the Earth, how much would it weigh on the surface of this planet? A) 6.0 N B) 36 N C) 4 N D) 3.0 N E) 1 N W p W E G mm p R p G mm E R E G m 4M E ( R E ) G mm E R E 1 W p W E 34. When a fan is turned off, its angular speed decreases from 13.7 rad/s to 6.3 rad/s in 10.0 s. What is the magnitude of the average angular acceleration of the fan? A) 1. rad/s B) 0.86 rad/s C) 0.74 rad/s D) 11 rad/s E) 0.37 rad/s ω ω 0 + αδt α ω ω 0 Δt ( 6.3 rad/s) 13.7 rad/s 10.0 s 0.74 m/s 0.74 m/s

8 35. A Ferris wheel has a period of 10 seconds, a radius of 15 meters, a mass of 1,000 kg and rotates counterclockwise. What is the centripetal (radial) force on a person with mass of 80 kg in a Gondola at half the maximum height (see figure)? A) 784 N B) 0 N C) 3.3 N D) 117 x 10 3 N E) 4 N F C mrω mr π T ( 80 kg) ( 15 m) π 10 s 3.3 N 36. A uniform rod is.0 m long. It is hinged to a wall at its left end, and held in a horizontal position at its right end by a vertical very light string, as shown in the figure. What is the angular acceleration of the rod at the moment after the string is released if there is no friction in the hinge? (The moment of inertia of a rod with mass M and length L rotating around one end is 1/3 ML.) A) 3.3 rad/s B) 15 rad/s C) 11 rad/s D) 7.4 rad/s E) It cannot be calculated without knowing the mass of the rod. τ Iα α τ Mg L I 1 3 ML 3g 7.4 rad/s L

9 37. A 15-kg child is sitting on a playground teeter-totter, 1.5 m from the pivot. What is the magnitude of the minimum force, applied 0.30 m on the other side of the pivot, that is needed to make the child lift off the ground? A) 66 N B) 75 N C) 44 N D) 740 N E) 3 N τ F τ child F( 0.3 m) ( 15 kg) ( 9.8 m/s )( 1.5 m) F 740 N 38. A store's sign has a mass of 0 kg and is 3.0 m long. It is uniform, so its center of gravity is at the center of the sign. It is supported horizontally by a small loose bolt attached to the wall at one end and by a wire at the other end, as shown in the figure. What is the tension in the wire? A) 460 N B) 300 N C) 30 N D) 00 N E) 10 N τ wire τ weight 0 TLsin( 5 o ) mg( L / ) T mg sin 5 o 9.8 m/s 0 kg sin 5 o 30 N

10 39. A ball is thrown against a wall; it bounces off and returns with speed equal to its initial speed before striking the wall. Which of the following statements is true from before to after the collision between the ball and the wall? A) The kinetic energy of the ball is the same. B) The momentum of the ball is the same. C) Both the kinetic energy and the momentum of the ball are the same. D) Neither the kinetic energy nor the momentum of the ball are the same. E) The collision is inelastic. The kinetic energy of the ball is the same before and after since the speed is the same, and thus the collision is elastic. The momentum is not since the direction is different. 40. What average force will stop a hammer with a momentum of 48.0 kg m/s in a time interval of s? A) 1600 N B) 1440 N C) 1.44 N D).88 N E) Cannot be calculated without knowing the mass of the hammer. F Δp 48.0 kg m/s 1600 N Δt s

11 41. Consider a spring initially hanging vertically from the ceiling with nothing attached to it. The spring is at rest in its relaxed state. Then a massive object is attached to the spring and gently lowered until the spring holds the object and the spring and the object are at rest. Which of the statements about this process is correct? A) The elastic potential energy of the spring increased and the gravitational potential energy of the object decreased. B) The elastic potential energy of the spring decreased and the gravitational potential energy of the object increased. C) The elastic potential energy of the spring and the gravitational potential energy of the object both decreased. D) The elastic potential energy of the spring and the gravitational potential energy of the object both increased. E) Neither the elastic potential energy of the spring nor the gravitational potential energy of the object changed. The potential energy of a spring is the smallest when the spring is in the relaxed state. Gravitational potential energy increases with height. Thus, the elastic potential energy of the spring increased and the gravitational potential energy of the object decreased. 4. Two forces F1 and F both of magnitude 10 N are acting on a box (as shown in the figure) and moving it 0 meters to the right. What is the net work done by the forces F1 and F during this process? A) 00 J B) 0 J C) 400 J D) 350 J E) 100 J W F! s cos 30 o 10 N 0 m 350 N

12 Physics 111 Exam - KEY D 3 A 4 D 3 C 33 E 4 B 34 C 5 C 35 C 6 C 36 D 7 C 37 D 8 A 38 C 9 D 39 A 30 D 40 A 31 B 41 A