Exam 2--PHYS 101-Fall 2009

Transcription

1 ame: Class: Date: Exam 2PHYS 101Fall 2009 Multiple Choice Identify the choice that best completes the statement or answers the question. 1. Consider this figure of a roller coaster: At which of these points is the speed of the car the greatest? a. A c. C b. B d. D 2. You drop a ball from a certain height. Which of these statements describe the changes in gravitational potential and kinetic energy as the ball falls to the ground? (KE=kinetic energy, PE=gravitational potential energy) a. KE is decreasing and PE is increasing c. both PE and KE are decreasing b. KE is increasing and PE is decreasing d. both PE and KE are increasing 3. Two stones, one twice the mass of the other, are dropped from a cliff. Just before hitting the ground, what is the kinetic energy of the heavy stone compared to the light one? a. quarter as much d. twice as much b. half as much e. four times as much c. the same 4. Which of these statements is true? a. energy is a scalar quantity; momentum is a vector quantity b. energy is a vector quantity; momentum is a scalar quantity c. both energy and momentum are scalar quantities d. both energy and momentum are vector quantities 5. If the momentum of an object is tripled, its kinetic energy will change by what factor? (Assume the mass of the object is unchanged.) a. zero b. onethird c. nine d. three 1

2 ame: 6. A system of particles has a momentum of zero. Which of these statements is true about the kinetic energy of the system? a. the kinetic energy is zero c. the kinetic energy is negative b. the kinetic energy is positive d. there is not enough information 7. A bouncy rubber ball and bean bag, each with the same mass, fall to the floor from the same height. Which of these statements about the force imparted to the floor is true? a. the rubber ball imparts a greater force c. the ball and bean bag impart the same force b. the bean bag imparts a greater force d. there is not enough information 8. In elastic collisions a. only energy is conserved c. only momentum is conserved b. both energy and momentum are d. none of these conserved 9. Kaisa sits on the outer rim of a merrygoround, and Chad sits midway between the rim and center. The merrygoround makes 1 revolution every 2 seconds. Chad s angular velocity is... a. same as Kaisa s d. twice Kaisa s b. onehalf of Kaisa s e. onequarter of Kaisa s c. four times Kaisa s 10. An object at rest begins to rotate with a constant angular acceleration. If this object rotates through an angle q in the time t, through what angle did it rotate in the time 1/2 t? a. b. c. 1 θ 2 d. 4θ 3 θ 4 e. 2θ 1 4 θ 2

3 ame: 11. Which of these are the basic SI units for torque? a. kg m s b. m c. kg m2 s 2 d. kg m s Two spheres have the same mass. One sphere is made of aluminum, while the other sphere is made of gold. Both are solid. Because gold is more dense, the radius of the gold sphere is smaller than the aluminum sphere. Which has the larger moment of inertia? a. aluminum sphere c. both the same b. gold sphere d. not enough information 13. When you pay the electric company for so many kilowatthours, what exactly are you paying for? a. voltage d. none of these b. energy e. power c. current 14. An open cart rolls along a frictionless track while it is raining. As it rolls, what happens to the speed of the cart? a. nothing c. speeds up b. not enough information given d. slows down 15. A small car and large truck collide headon and stick together. Which has the larger change in momentum? a. the less massive car c. neither, they have the same change in momentum b. the more massive truck d. not enough information given 3

4 ame: Problem 16. (10 pts) Derive the moment of inertia for a thin rod of mass M and length L that rotates about it s center as shown in this figure: (show all steps) 4

5 ame: 17. (10 pts) A force F = 23i 15j + 20k N is applied to a body as it moves in a straight line from r 1 = 16i +3j10k to r 2 = 5i + 10j+ 14k m. How much work is done by the force? 5

6 ame: 18. (15 pts) A roller coaster cart with mass 1000 kg slides along the frictionless track shown in this figure, starting at rest from point A. Find (a) its speed at D and (b) its speed at E. (c) Now, consider that you wish to stop the cart at point E. A spring, with a spring constant of N/m, is placed at point E to absorb the energy of the oncoming cart. By how much will the spring compress when the cart comes to rest? 6

7 ame: 19. (20 pts) A bug, fitted with a metal collar, slides back and forth in a bowl 11 cm deep, starting from rest at the top, as shown in this figure. The bug has a mass of 10 g.the bowl is frictionless, but, at the bottom, a magnetic force acts on the bug over a 1.5 cm patch. This force is dependent on the position of the bug as defined by this function: F=ax 3 / 2 where a=1.25 N/m 3/2. Assume the force acts symmetrically (that is, the same force is experienced regardless of the bug s direction). a) What is the bug s total energy at the top of the bowl? b) How much energy does the bug lose when it crosses through the magnetic field? c) What is the bug s speed at the bottom on its first pass, just after it goes across the magnetic patch? d)how many times does the bug cross the 1.5 cm patch? 7

8 ame: 20. (15 pts) At the MIT Magnet Laboratory, energy is stored in huge solid flywheels of mass 3.3 X 10 4 kg and radius 1.4 m. The flywheels ride on shafts 21 cm in diameter. A frictional force of 28 kn acts tangentially on the shaft. a) How long will it take the flywheel to coast to a stop from its normal rotation rate of 420 rpm? b) What angular displacement (in radians) does the flywheel turn through in this time? 8

9 Exam 2PHYS 101Fall 2009 Answer Section MULTIPLE CHOICE 1. ANS: D 2. ANS: B 3. ANS: D 4. ANS: A 5. ANS: C DIF: 1 TOP: 6.2 Conservation of Momentum 6. ANS: D 7. ANS: A 8. ANS: B 9. ANS: A 10. ANS: C 11. ANS: C 12. ANS: A 13. ANS: B 14. ANS: D 15. ANS: C PROBLEM 16. ANS: 17. ANS: 18. ANS: 19. ANS: 20. ANS: 1