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Chapter 6 Clickers Work and Energy Prepared by Dedra Demaree, Georgetown University © 2014 Pearson Education, Inc. Which of the following statements is true about external force and displacement? a) If an external force is applied in the direction of an object's displacement, the object gains more potential to do something. b) If an external force is applied against the direction of an object's displacement, the object's potential to do something decreases. c) If an external force is applied perpendicular to an object's displacement, the object's potential to do something is unchanged. d) All of the above are correct. © 2014 Pearson Education, Inc. Which of the following statements is true about external force and displacement? a) If an external force is applied in the direction of an object's displacement, the object gains more potential to do something. b) If an external force is applied against the direction of an object's displacement, the object's potential to do something decreases. c) If an external force is applied perpendicular to an object's displacement, the object's potential to do something is unchanged. d) All of the above are correct. © 2014 Pearson Education, Inc. The two bar charts represent a falling acorn. Which statement below is NOT correct? Representation A: [make a bar chart with 0 initial energy, -Ug and +K final energy] Representation B: [make a bar chart with 0 initial energy, +work done, and +K final energy] a) In representation A, Earth was in the system; in representation B, it was not. b) In representation A, the origin for the y-axis was at the initial position of the acorn. c) In representation B, the person who drew the bar chart forgot to include the potential energy of gravity. d) More than one representation can be correct for this situation. © 2014 Pearson Education, Inc. The two bar charts represent a falling acorn. Which statement below is NOT correct? Representation A: [make a bar chart with 0 initial energy, -Ug and +K final energy] Representation B: [make a bar chart with 0 initial energy, +work done, and +K final energy] a) In representation A, Earth was in the system; in representation B, it was not. b) In representation A, the origin for the y-axis was at the initial position of the acorn. c) In representation B, the person who drew the bar chart forgot to include the potential energy of gravity. d) More than one representation can be correct for this situation. © 2014 Pearson Education, Inc. What can we NOT conclude from the following graphed data? A graph of the stretch of springs 1 and 2 when the same force is exerted on each spring. a) b) c) d) Force = mx + 0, where m is the slope. Spring 2 stretches farther than spring 1 when the same amount of force is applied. If 2 N of force is exerted on spring 1, it is stretched by 0.1 m. All of the above are correct. © 2014 Pearson Education, Inc. What can we NOT conclude from the following graphed data? A graph of the stretch of springs 1 and 2 when the same force is exerted on each spring. a) b) c) d) Force = mx + 0, where m is the slope. Spring 2 stretches farther than spring 1 when the same amount of force is applied. If 2 N of force is exerted on spring 1, it is stretched by 0.1 m. All of the above are correct. © 2014 Pearson Education, Inc. A pole vaulter crosses a bar high above a cushion below. Which equation is NOT correct for the initial situation when the vaulter is at the highest point and the final situation when the vaulter has sunk a distance x into the cushion (modeled as a spring)? a) b) c) d) mgh = -mgx + (1/2)kx2 mg(h + x) = (1/2)kx2 0 = -mg(h + x) + (1/2)kx2 All of the above are correct. © 2014 Pearson Education, Inc. A pole vaulter crosses a bar high above a cushion below. Which equation is NOT correct for the initial situation when the vaulter is at the highest point and the final situation when the vaulter has sunk a distance x into the cushion (modeled as a spring)? a) b) c) d) mgh = -mgx + (1/2)kx2 mg(h + x) = (1/2)kx2 0 = -mg(h + x) + (1/2)kx2 All of the above are correct. © 2014 Pearson Education, Inc. You load an arrow into a bow and pull the bowstring back 0.50 m. The bow has a spring constant k = 2000 N/m. What is the potential energy of the bowstring? a) b) c) d) e) 125 J 250 J 500 J 1000 J Impossible to determine without knowing the mass © 2014 Pearson Education, Inc. You load an arrow into a bow and pull the bowstring back 0.50 m. The bow has a spring constant k = 2000 N/m. What is the potential energy of the bowstring? a) b) c) d) e) 125 J 250 J 500 J 1000 J Impossible to determine without knowing the mass © 2014 Pearson Education, Inc. Which of the following statements is NOT correct? a) If you include both the work done by friction and the internal energy evidenced by the surface's heating, you are overcounting. b) It is best to include the surface in your system so there is no work done by friction. c) Our model for friction leads to energy (evidenced by heat) that we cannot incorporate through work done by friction. d) All of the above are correct. © 2014 Pearson Education, Inc. Which of the following statements is NOT correct? a) If you include both the work done by friction and the internal energy evidenced by the surface's heating, you are overcounting. b) It is best to include the surface in your system so there is no work done by friction. c) Our model for friction leads to energy (evidenced by heat) that we cannot incorporate through work done by friction. d) All of the above are correct. © 2014 Pearson Education, Inc. Which of the following statements is NOT correct about work-energy bar charts? a) A work-energy bar chart helps to organize the energies involved in a process. b) A work-energy bar chart can be translated directly into an equation. c) A work-energy bar chart represents the constancy of energy. d) All of the above are correct. © 2014 Pearson Education, Inc. Which of the following statements is NOT correct about work-energy bar charts? a) A work-energy bar chart helps to organize the energies involved in a process. b) A work-energy bar chart can be translated directly into an equation. c) A work-energy bar chart represents the constancy of energy. d) All of the above are correct. © 2014 Pearson Education, Inc. For the example of an elevator slowing to a stop, to find the constant tension force that the cable exerts on the elevator, which of the following statements is NOT correct? a) This problem can be solved by finding the work that the rope tension force exerts on the elevator. b) This problem can be solved using Newton's second law and kinematics. c) Both of the above are correct. © 2014 Pearson Education, Inc. For the example of an elevator slowing to a stop, to find the constant tension force that the cable exerts on the elevator, which of the following statements is NOT correct? a) This problem can be solved by finding the work that the rope tension force exerts on the elevator. b) This problem can be solved using Newton's second law and kinematics. c) Both of the above are correct. © 2014 Pearson Education, Inc. For the example of an elevator slowing to a stop, to find the constant tension force that the cable exerts on the elevator, which of the following equations is NOT helpful? a) b) vf2 = vi2 + 2a(yf-yi) c) Both of the above are correct. © 2014 Pearson Education, Inc. For the example of an elevator slowing to a stop, to find the constant tension force that the cable exerts on the elevator, which of the following equations is NOT helpful? a) b) vf2 = vi2 + 2a(yf-yi) c) Both of the above are correct. © 2014 Pearson Education, Inc. To launch a 100-kg human so that he leaves a cannon moving at a speed of 4 m/s, you need a spring with an appropriate spring constant. This spring will be compressed 2.0 m from its natural length to launch the person. Which spring constant do you need? a) b) c) d) 200 N/m 400 N/m 800 N/m 1600 N/m © 2014 Pearson Education, Inc. To launch a 100-kg human so that he leaves a cannon moving at a speed of 4 m/s, you need a spring with an appropriate spring constant. This spring will be compressed 2.0 m from its natural length to launch the person. Which spring constant do you need? a) b) c) d) 200 N/m 400 N/m 800 N/m 1600 N/m © 2014 Pearson Education, Inc. Which of the following is always true about collisions? a) Energy is conserved in a system that includes only the colliding objects. b) Linear momentum is conserved in a system that includes only one of the colliding objects. c) Mass is conserved for a system that includes the colliding objects. d) All of the above are correct. © 2014 Pearson Education, Inc. Which of the following is always true about collisions? a) Energy is conserved in a system that includes only the colliding objects. b) Linear momentum is conserved in a system that includes only one of the colliding objects. c) Mass is conserved for a system that includes the colliding objects. d) All of the above are correct. © 2014 Pearson Education, Inc. Xueli is doing a dead lift. She lifts a 10-kg barbell from the floor to just below her waist (a vertical distance of 0.50 m) in 1 s. What is the power in this process? (Assume g to be 10 m/s2.) a) b) c) d) 10 W 50 W 100 W Impossible to determine from the given information © 2014 Pearson Education, Inc. Xueli is doing a dead lift. She lifts a 10-kg barbell from the floor to just below her waist (a vertical distance of 0.50 m) in 1 s. What is the power in this process? (Assume g to be 10 m/s2.) a) b) c) d) 10 W 50 W 100 W Impossible to determine from the given information © 2014 Pearson Education, Inc. Jim (mass = 100 kg) rollerblades on a smooth horizontal floor at a constant speed of 2.0 m/s for a distance of 5.0 m in 5.0 s. What is the power in this process? (Assume that g is10 m/s2.) a) b) c) d) 0W 40 W 1000 W Impossible to determine with the given information © 2014 Pearson Education, Inc. Jim (mass = 100 kg) rollerblades on a smooth horizontal floor at a constant speed of 2.0 m/s for a distance of 5.0 m in 5.0 s. What is the power in this process? (Assume that g is10 m/s2.) a) b) c) d) 0W 40 W 1000 W Impossible to determine with the given information © 2014 Pearson Education, Inc. Unit analysis on the following equation shows that "G" must have which units? a) b) c) d) Nm/kg Nm2/kg Nm/kg2 Nm2/kg2 © 2014 Pearson Education, Inc. Unit analysis on the following equation shows that "G" must have which units? a) b) c) d) Nm/kg Nm2/kg Nm/kg2 Nm2/kg2 © 2014 Pearson Education, Inc.