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SPH3U Physics 11 Forces Test 4 Part A: Knowledge and Understanding (15 marks) Multiple Choice Identify the choice that best completes the statement or answers the question. 1. An elevator is suspended by a cable and moves upward. Which of the following free-body diagrams best represents the forces acting on the elevator? a. A Answer: A b. B c. C d. D e. E 2. Study the free-body diagram below and choose the statement that best describes the object's motion. a. The object will accelerate north. b. The object will be motionless. c. The object will accelerate northeast. d. The object will travel with uniform motion. e. The object will travel north with a constant velocity. Answer: C 3. A curling stone is sliding along the ice just after the curler has released it. Which of the following statements is true? a. The net force acting on the stone is supplied by friction. b. The applied force is just equal to the frictional force. c. The stone is moving with uniform motion. d. The frictional force is greater than the applied force which is acting. e. The net force is zero. Answer: A 4. Two identical arrows, A and B, are fired with different bows. The bow that fires arrow A exerts twice the average force as the bow that fires arrow B. Compare the accelerations of the two arrows. a. Arrow B will have twice the acceleration of arrow A. b. Arrow A will have twice the acceleration of arrow B. c. Arrow A and arrow B will have the same acceleration. d. Arrow B will have four times the acceleration of arrow A. e. Arrow A will have four times the acceleration of arrow B. Answer: B 5. The action and reaction forces of Newton's third law a. act on the same object b. act as a pair of forces that are "balanced" c. make the net force zero because they are equal in strength and opposite in direction d. act on two different objects e. cancel each other out like Fg and FN Answer: D 6. Your "weight" is properly defined as a. the amount of material of which you are composed b. the gravitational force which Earth exerts on you c. the gravitational force you exert on Earth d. the force you exert on a set of bathroom scales e. none of the above Answer: B 7. The coefficient of friction stems from the a. nature of the two surfaces in contact b. mass of the object c. strength of the applied force d. strength of the normal force e. strength of the gravitational force Answer: A 8. Non-collinear forces are forces that a. act in the same direction. b. act in opposite directions. c. act along a straight line. d. act only at an angle of 90°. e. act at any angle other than 0° or 180°. Answer: E 9. What does a force of 1.0 N equal? a. 1.0 kg/s b. 1.0 kg m2/s2 c. 1.0 kg m/s2 Answer: C d. 1.0 kg m/s e. 1.0 kg m2/s 10. A bowling ball and a bag of potato chips are dropped at the same time from a tall tower on the Moon. There is no air on the Moon. What happens? a. The bowling ball hits the ground first. b. The bag of potato chips hit the ground first. c. Both objects will hit the ground at the same time. d. Both objects will hit the ground at the same time. e. Both objects won't move at all. They will just stay where they were released. Answer: C Short Answer 11. State Newton’s second law and give an example that clearly illustrates it. (2 marks) Answer: A wagon accelerates along a sidewalk when the applied force exerted by a child exceeds the frictional force exerted by the ground that acts in the opposite direction. 12. State Newton’s third law and give an example that clearly illustrates it. (2 marks) Answer: When you stand beside a wall and exert a force on it, you are "pushed" away from the wall. 13. Draw the free body diagram for the following truck parked on a hill. (1 marks) Answer: Part B: Inquiry (27 marks) Problems 14. A stationary elevator and its contents have a combined mass of 2000 kg. The elevator is suspended by a single cable. (Assume 3 significant digits.) (a) Draw a free-body diagram of the elevator and calculate the values of all the forces that are acting on it when at rest. (5 marks) Solution (b) If the elevator is ascending at a speed of 4.0 m/s, what are the values of the forces acting at this point? (2 marks) Solution (c) If the elevator is descending at 4.0 m/s2, what are the values of all the forces acting at this point? (3 marks) Solution 15. If 6.8 N of force are exerted horizontally on a 1.1-kg object and 2.4 N of friction are impeding its slide, what is the object's acceleration? Draw a freebody diagram. (5 marks) Solution Let [fwd] be "positive" and [bkwd] be "negative". The acceleration is 4.0 m/s2 [fwd]. 16. The gravitational field strength on the surface of Mars is 3.7 N/kg. (a)What would a person weigh on Mars if this person weighs 637 N on Earth? (4 marks) Solution On Earth: On Mars: m = 65 kg The person would weigh 2.4 x 102 N. (b) What is the mass of Mars if its radius is 3.4 x 106 m? (3 marks) Solution The mass of Mars is 6.4 x 1023 kg. 17. A truck with wide rubber tires has a mass of 1200 kg. The acceleration due to gravity is 9.80 m/s2 [down] and the coefficient of kinetic friction for rubber on dry asphalt is 0.8. What is the force of kinetic friction on dry asphalt when the truck skids with all four wheels locked? (5 marks) Solution Ff = 𝜇mg = (0.8)(1200)(9.8) = 9 408 N Part C: Application (8 marks) 18. How would you demonstrate Newton’s First Law for a group of grade 8 students? What would you say to the class to explain this law? (3 marks) Answer: Show them an inertia trick such as (ex. pull table cloth from under a beaker) and explain that things like to keep doing what they are doing. 19. Aristotle (384-322 BC) said that things move because of a ‘prime mover’ but Galileo (1564-1642) challenged this belief with a thought experiment. Outline the experimental procedure that Galileo used to prove this assumption incorrect. Why would this type of experiment be much easier to carry out today than in Galileo's time? (5 marks) Answer: A smooth ball released in a smooth bowl will go to the same height. If you keep lowering the ramp on one side of the bowl the ball still returns to the same height even though it has to travel further. If you lower the ramp completely on one side of the bowl the ball will go on forever trying to reach the same height. Galileo was always plagued by not being able to measure time intervals precisely (although he recognized the potential for pendulum clocks), which is why he "slowed" the fall using ramps. Today, with access to instruments that are capable of measuring time intervals precisely, this experiment would be very easy to perform.