Physics Final Exam Review Answers

Physics Semester 2 Final Exam Review Answers A student attaches a string to a 3 kg block resting on a frictionless surface, and then pulls steadily (with a constant force) on the block as shown below. Assume no effect of air resistance on the block. 3 kg Point P 1. Write a statement that describes the motion of the block along the table WHILE the student is pulling steadily. The block is speeding up 2. What amount of force would be required to cause the block to move from rest to a have an acceleration of 2 m/s/s? 6N 3. After the student pulls the block to Point P on the table, the string breaks. The motion of the block AFTER the string breaks could best be described as: The block moves at a constant speed 4. Which exerts more force; the table on the block or the block on the table? They exert the same force on each other N3LPF 5. What is the weight of the block? (assume g = 10 N/kg) weight = 30 N 6. Name the force(s) acting in the horizontal direction on the block AFTER the string breaks? There are no forces in the horizontal direction! Eventually the block reaches the edge of the table. 7. How is the block speeding up after it has left the table? The block accelerates downward but goes at a constant speed to the right. 8. Suppose that the student sets up the 2 kg block again at rest attached to the string, but this time steadily pulls with twice as much force on the block in the same direction. What could be said of the block’s motion while it was being pulled? It would have twice the acceleration 9. Suppose that the same student sets up a block, which is 4 kg, at rest attached to the string, and pulled with the same original force on the block in the same direction. What could be said of the block’s motion while it was being pulled? It would have half the acceleration Physics Semester 2 Final Exam Review Answers Page 2 To test Newton's Laws, two physics students of equal mass sit in low friction chairs (as shown at right.) One student (A) is holding a 16-pound bowling ball. The other student (B) puts her feet on A’s knees and pushes off (see picture at the right). 10. Write a statement that best compares the forces of the students. They exert the same force on each other N3LPF 11. Write a statement that best compares the accelerations of the students. Student A has less acceleration because he has more mass 12. Why is it more difficult to get student A to move? More mass means more inertia and more resistance to change in motion 13. If student A took his bowling ball to the moon how would its mass and weight change? The mass (the amount of matter in the ball) would remain the same – same ball but it would weigh less because the moon pull objects down less; less gravitational field strength. A B 14. The gravitational field strength near the surface of the earth is 10 N/kg. What does this mean? For every 1 kilogram increase in mass the Earth pulls down an object with 10 N more force. 15. The free fall acceleration of an object near the surface of the earth is 10 m/s/s. What does this mean? For every increase in 1 second of time that an object falls, its velocity increases by 10 m/s. 16. Why is it that the free fall acceleration is the same for all objects? The earth pulls more on more massive objects, but since they are more massive they have more inertia (more resistance to change in their motion) and are therefore harder to move. These two concepts “balance out” causing all objects to fall with the same acceleration. A bowling ball is dropped from the edge of a building 20 meters above the ground. 17. What happens to its speed as it falls? It speeds up the whole time that it falls. 18. How much time does it take for the ball to reach the ground? 2 seconds 19. How fast is the ball moving just as it hits the ground? 20 m/s The same bowling ball is now thrown horizontally from the same height with a speed of 10 m/s. 20. How does the time in the air for the dropped ball compare to the thrown ball? The time is the same; the horizontal motion doesn’t affect the vertical motion. 21. How far from the edge of the building does the ball land? 20 meters 22. Draw the ball’s path to the ground. 23. This time a bowling ball and a golf ball are both dropped from the building at the same time. Which hits the ground first? The hit at the same time. 24. Which is the best reason for your answer in the previous question? Although the bowling ball experiences a larger force it also has a larger mass: acceleration = Force/mass 20 meters Physics Semester 2 Final Exam Review Answers Page 3 Imagine that a steel ball is swung in a horizontal circle at a constant speed. E 25. In what direction does the velocity of the ball point? The velocity always points tangent to the circular path A Top View 28. In what direction does the unbalanced force on the ball point?? The unbalanced force always points toward the center (same as the acceleration) 29. What is the nickname we give to the unbalanced force on the ball? What provides that unbalanced force? unbalanced force = centripetal force and is provided (in this case) by the string 30. What provides the outward force on the ball? It’s a trick! There are no outward forces 31. If you wanted to increase the speed of the ball what would you have to do to the force on the string? Pull harder on the string; increase the inward force 32. Look back at the original picture above. At the position shown the string breaks. Which path will the ball take? D 33. Describe the path of the ball after the string breaks. The ball will go in the direction of D because an object in motion will stay motion in a straight line C B 26. In what direction does the acceleration of the ball point? The acceleration always points toward the center 27. Are the forces balanced or unbalanced? How do you know? The forces are unbalanced – you know this because the velocity is changing directions so the ball is acceleration and acceleration means unbalanced forces D Physics Semester 2 Final Exam Review Answers Page 4 Pictured below is a new roller coaster. A physics student riding and the coaster car have a combined mass of 200 kg. The car is at rest at point A and there is no braking at point D. Assume no friction between the coaster car and the track. A C 80 m 40 m B 5m 34. Rank the total energy from least to greatest. The total amount of energy stays the same the whole time 35. Rank the gravitational energy from least to greatest. DBCA 36. Rank the kinetic energy from least to greatest. ACBD 37. Rank the speed of the cart from least to greatest. ACBD 38. What happens to the energy as the cart rolls from A to B? The gravitational energy is converted to kinetic energy 39. Determine the student’s gravitational energy at point A. 160000 J 40. Determine the student’s kinetic energy at point A. 0 J (it is stopped) 41. At point B, her height is only 5 m. Determine her gravitational energy and kinetic energies at point B. Gravitational Energy = 10000 J Kinetic Energy = 150000 J 42. At point D the coaster is on the ground and still moving. How fast is it going at Point D? 40 m/s On a real rollercoaster friction (both from the track and the air) are very present. 43. Draw pie charts for a real rollercoaster as it moves from points A to B to C to D? G  K + G +T  G + K +T  K + T D Physics Semester 2 Final Exam Review Answers Page 5 A crane lifts a steel girder of mass 1000 kg to a height of 45 meters. 44. Did the crane do work on the girder? How do you know? Yes – the crane lifts the girder giving it gravitational energy. Work is the transfer of energy and the motor transfers electrical energy to gravitational energy 1000 kg 45. How much work was done by the crane’s motor? 450000 J 46. If the motor pulls the girder up in 30 seconds what is the power output of the motor? 15000 Watts 45 m 47. Why is it that objects float? Objects float if the upward force by the water is equal to the downward force by the earth. 48. How is the density of an object related to its ability to float in a fluid? If the density of the object is less than the density of the surrounding fluid the object will float. 3 A sample of mineral oil has a mass of 8 g and a volume of 10 ml. A piece of Styrofoam density of 0.60 g/cm . 49. Calculate the density of the mineral oil. 3 Density of mineral oil = 0.80 g/ml = 0.80 g/cm 50. Styrofoam will float in mineral oil. What percent of the Styrofoam remains under the oil? 75% under the surface An aluminum foil boat of mass is constructed in the form of an open topped cube with individual side lengths of 4 cm. You place 5 nickels, each with a mass of 5 g, into the boat so that it sinks halfway into the water. 51. Calculate the volume of water displaced by the boat? 3 32 cm 52. What mass, in grams, can the above volume of displaced water support? 32 g 53. What is the total mass of the boat? 7g Physics Semester 2 Final Exam Review Answers Page 6 A cardboard boat is to be constructed so that the bottom of the boat has the dimensions shown at the right and straight sides. Rower #1 weighs 140 pounds, the boat weighs 10 pounds and the gear weighs 2 pounds. The water line is 12 inches up from the bottom of the boat. 54. How much weight can one cubic foot displaced of water support? 62.4 pounds 3 ft 55. Calculate the volume of water displaced by the boat. 3 10 ft 2 ft 56. What is the weight of rower #2? 160 pounds 57. As the moon orbits the earth, how many forces are on the moon (ignore any forces by the sun)? There is one force, the earth pulling on the moon. 58. In what direction does the unbalanced force on the moon point? Inward toward the earth, that is why is goes in a circle. 59. How would the force of gravity between the moon and the earth change if you moved the moon three times as far away? The force would decrease by a factor of 9. 60. Which force is bigger the moon on the earth on the earth on the moon? The apply equal forces on each other; N3LFP B At the right is a picture of a comet moving in an elliptical orbit around a star. Assume that the comet moves from A to B to C. 61. Rank the force on the planet from greatest to least? A, C, B C 62. Ranks the speed of the planet from least to greatest? B, C, A 63. Draw in a force arrow at each of the points A, B and C. A 64. Draw in a velocity arrow for each of the points A, B and C. 65. In terms of the motion of the earth, the sun and the moon, define a day, a month and a year. A day is the time it takes for the earth to rotate once around on its axis. A month is approximately the time it takes for the moon to revolve around the earth once. A year is the time it takes for the earth to revolve around the sun once.