Exam #3 Physics I Spring 1999 Name__________________________ Section # ______ 1 ______ 2 ______ 3 ______ 4 ______ 5 ______ 7 ______ 8 ______ 9 ______ 10 ______ 11 ______ 12 ______ 13 ______ 14 Section Value MW 8-10 (Adams) (problem) MW 8-10 (Stoler) Part A-MC 28 MW 10-12 (Sperber) MW 12-2 (Stoler) Part B 17 MW 2-4 (Hayes) Part C (1) 30 MW 4-6 (Cummings) MW 4-6 (Kendall) Part C (2) 25 TTh 8-10 (Adams) 100 TTh 10-12 (Sperber) Total TTh 12-2 (Roberge) TTh 2-4 (Cummings) TTh 4-6 (Kendall) MW 12-2 (Casabella's laptop section) Score If you cheat on this exam, you are in big trouble! GOOD LUCK!!! Preliminary Grades will be posted outside of the physics department office (frist floor of the science center) on Wednesday after 6 pm. Part A-Multiple Choice. 4 points each. Choose the best answer and write it on the line to the left of the question number. ________ 1. The mass of a hypothetical planet is 1/100 that of Earth and its radius is 1/4 that of Earth. If a person weighs 150 lb on Earth, what would he weigh on this planet? A) 150 lb B) 12 lb C) 48 lb D) 6 lb E) 24 lb ________ 2. When a hard rubber rod is given a negative charge by rubbing it with wool: A) positive charges are transferred from rod to wool B) negative charges are transferred from rod to wool C) positive charges are transferred from wool to rod D) negative charges are transferred from wool to rod E) negative charges are created and stored on the rod __________3. Points R and T are each a distance d from each of two equal and opposite charges as shown. If k = 1/4o, the work required to move a negative charge q from R to T is: A) B) C) D) E) zero kqQ/d2 kqQ/d kqQ/(2d) kQq/(2d) _______4. Positive charge +Q is uniformly distributed on the upper half a semicircular rod and negative charge -Q is uniformly distributed on the lower half. What is the direction of the electric field at point P, the center of the semicircle? A) B) C) D) E) _________5. An electron and a proton each travel with equal speeds around circular orbits in the same uniform magnetic field, as shown in the diagram. The field is into the page on the diagram. Because the electron is less massive than the proton and because the electron is negatively charged and the proton is positively charged: A) the electron travels clockwise around the smaller circle and the proton travels counterclockwise around the larger circle. B) the electron travels counterclockwise around the smaller circle and the proton travels clockwise around the larger circle C) the electron travels clockwise around the larger circle and the proton travels counterclockwise around the smaller circle D) the electron travels counterclockwise around the larger circle and the proton travels clockwise around the smaller circle E) the electron travels counterclockwise around the smaller circle and the proton travels counterclockwise around the larger circle _______6. The earth exerts a gravitational force on the Moon, keeping it in its orbit. The reaction to this force, in the sense of Newton's third law, is: A) the centripetal force on the Moon B) the nearly circular orbit of the Moon C) the gravitational force exerted on the Earth by the Moon D) the tides due to the Moon E) the apple hitting Newton on the head ________7. An electron moves from point i to point f, in the direction of a uniform electric field. During this placement: A) the work done by the field is positive and the potential energy of the electron-field system increases B) the work done by the field is negative and the potential energy of the electronfield system increases C) the work done by the field is positive and the potential energy of the electron-field system decreases D) the work done by the field is negative and the potential energy of the electron-field system decreases E) the work done by the field is positive and the potential energy of the electron-field system does not change. Part B-Short Answer 1. The torque acting on an object is given by = 2t2 + 4 What is the change in angular momentum between time t = 2 seconds and t = 4 seconds? (4 points) Hint: Recall F=dp/dt 2. An electron moves to the left with a velocity v of 106 m/s in an electric field E of 10 N/C that is directed up toward the top of the page. a) What is the direction of the electric force on this electron? (3 points) Circle only ONE Right Left Into the Page Out of the Page Up (toward the top of the page) Down (toward the bottom of the page) b) If an appropriate magnetic field is present, we can cancel out this electric force with a magnet force. What direction must the magnetic force point? (2 points) Circle only ONE Right Left Into the Page Out of the Page Up (toward the top of the page) Down (toward the bottom of the page) c) What direction must a magnetic field point to produce such a force on the electron? (4 points) Circle only ONE Right Left Into the Page Out of the Page Up (toward the top of the page) Down (toward the bottom of the page) d) What must the magnitude of the magnetic field be for electron to pass through the fields without being deflected? (4 points) Show all work to receive credit for your answer. _________ (unit) Part C-SHOW ALL YOUR WORK TO RECEIVE CREDIT FOR YOUR ANSWER!! 1. (30 points total) Consider two small balls attached to a horizontal rod that is so lightweight that we can ignore its mass. Each ball has a mass of 0.3 kg and a radius of 1 cm. The two balls and the rod are free to rotate as a unit about a vertical axis that is perpendicular to the rod. See the figure below which is a view looking down on the object. The center of the ball on the right is 1 meter from the vertical axis and the center of the ball on the left is 3 meters from the vertical axis. 3m 1m a) What is the rotational inertia I of the object (the two balls and the rod) about the vertical axis? (3 points) _________ (unit) A horizontal force of 3 N makes an angle of 60o with the rod and is applied to the center of the ball on the right. A second horizontal force of 5 N makes an angle of 40o with the rod and is applied to the center of the ball on the left. 40o 60o 5N 3N b. What is the magnitude of the torque 1 produced by the 5 N force? (2 points) _________ (unit) c. What is the direction of the torque 1 produced by the 5 N force? Circle only ONE Right Left Into the Page Out of the Page Up (toward the top of the page) Down (toward the bottom of the page) (2 points) d. What is the magnitude of the torque 2 produced by the 3 N force? (2 points) _________ (unit) e. What is the direction of the torque 2 produced by the 3 N force? Circle only ONE Right Left Into the Page Out of the Page Up (toward the top of the page) Down (toward the bottom of the page) (2 points) f. What is the magnitude of the angular acceleration of the object (the two balls and the rod)? (5 points) _________ (unit) g. The object is at rest before the two torques are applied. What direction will the object rotate? Circle only ONE (2 points) Clockwise Counterclockwise h. After some time, the angular velocity of the object is 5 rad/sec. What is the magnitude of the angular momentum L of the object? (4 points) _________ (unit) i. A 300-gram wad of clay is now thrown at a speed of 5 m/s. The clay strikes the rotating object, perpendicular to the horizontal rod connecting the balls, 2 meters to the right of the vertical axis of rotation. The clay sticks to the rod. What is the magnitude of the angular velocity of the object now? (8 points) _________ (unit) 2. (25 points total )Consider the two charges shown in the figure below. Hint: C=10-6 C y + 6 C 60o 3.00 m 5.00 m +2 C 4.00 m x a. What is the x component of the net force on the 6 C charge at y = 3m? (6 points) _________ (unit) b. What is the x component of the E field at y = 0, x = 0 m? (4 points) _________ (unit) c. What is the electric potential energy of the two charge configuration? (6 points) _________ (unit) d. What is the electric potential at y =0 , x=0 due to the two charges? (4 points) _________ (unit) -6 e. Assume that the 6 C charge has a mass of 10 kg. At t=0, this charge is released from rest and moves out to infinity. What is the speed of this particle at infinity? (5 points) _________ (unit)