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Physics 101 Final Exam
8 Dec 2003 Prof L. Weinstein

There are 34 questions. Unless otherwise noted, please give a short explanation for all of your non-numerical answers. Show your work for all numerical answers.

$G = 6.67\cdot10^{-11}$ N$\cdot$m$^2$/kg$^2$

$k = 9\cdot 10^9$ N$\cdot$m$^2$/C$^2$

Earth's mass $M_e = 6\cdot 10^{24}$ kg

Earth's radius $R_e = 6.4\cdot 10^6$ m

Moon's mass $M_m = 7\cdot 10^{22}$ kg

Moon's radius $R_m = 1.7\cdot 10^6$ m

Earth-Moon distance $d_{E-m} = 3.8\cdot 10^8$ m

Earth-Sun distance $d_{E-S} = 1.5\cdot10^{11}$ m

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1. If two electrons initially at rest are placed close to each other they will exert a force on each other. Ignore all other forces. When the electrons are released and are free to move, they will move
   1. further away from each other
   2. not at all (they will not move)
   3. closer to each other
   4. Need more information
   
   
   
   
   
   
   

2. In the previous problem, when the electrons are released and start moving, the force on each will
   1. increase as they move
   2. decrease as they move
   3. remain constant as they move
   4. Nonsense, they won't move
   5. Need more information
   
   
   
   
   
   
   

3. (no explanation needed) You electrically polarize a neutral metal sphere. This means that
   1. the sphere is now electrically charged
   2. some positive charges are on one side of the sphere and an equal number of negative charges are on the other side
   3. it is now an insulator
   4. it is magnetic
   5. Need more information

4. (No explanation needed) I have two 100-g copper rods. I stretch one so that it is 10 cm long and about 1 cm in diameter. I stretch the other one so that it is 100 cm long and about 0.3 cm diameter. Which one has more resistance?
   1. the 10 cm long rod
   2. the 100 cm long rod
   3. both are the same
   4. Need more information

5. A 32-inch television set has about a 5000 V difference between the back of the set and the screen. The distance between the back of the TV set and the screen is 0.6 m. If 3 mC of electrons travel from the back of the set to the screen, how much kinetic energy do they gain?

   
   
   
   
   
   
   

6. A 6-Volt battery is connected to a 3-$\Omega$ light bulb. What is the current flowing through the circuit?

   
   
   
   
   
   
   

7. How much electrical power does the light bulb in the previous problem use?

   
   
   
   
   
   
   

8. If you double the voltage of the battery in the previous problem, what happens to the current in the circuit?
   1. it quarters
   2. it halves
   3. it does not change
   4. it doubles
   5. it quadruples
   6. Need more information
   
   
   
   
   
   
   

9. A pair of light bulbs connected in parallel to a battery will draw
   1. more current than a single bulb would draw
   2. less current than a single bulb would draw
   3. the same current that a single bulb would draw
   4. Need more information
   
   
   
   
   
   
   

10. As you increase the number of identical light bulbs in this circuit from 1 to 2 to 3 as shown, the total power used by the circuit (if it is plugged into a 120-V AC wall outlet)

    = 1.5in

    1. increases
    2. decreases
    3. remains the same
    4. Need more information

11. (no explanation needed) A moving electric charge will always interact with (ie: have a force exerted on it by)
    1. a constant electric field
    2. a constant magnetic field
    3. both
    4. neither
    5. Need more information

12. (No explanation needed) Like kinds of magnetic poles repel. Unlike kinds of magnetic poles
    1. attract
    2. repel
    3. either attract or repel
    4. Need more information

13. A beam of electrons is travelling toward the East. If I want to deflect the beam so it travels toward the north-east, then I should apply a force pointing

    = 1.5in

    1. North
    2. East
    3. Up
    4. West
    5. other
    6. Need more information

14. A beam of electrons is travelling from East to West. If I want to apply a southward force, then I should apply a magnetic field pointing

    = 1.5in

    1. North or South
    2. East or West
    3. Up or down (ie: into or out of the page)
    4. nonsense, a magnetic field won't deflect the electron beam
    5. Need more information
    
    
    
    

15. My particle detector needs about 12,000 V but only about 20 W to operate. I have a transformer with 50 turns in the primary coil. I connect the primary coil to a 120-V AC outlet and the secondary coil to my detector. How many turns does the transformer need to have in its secondary coil to supply 12,000 V to my detector?

    
    
    
    
    
    
    

16. I make an electromagnet by winding an insulated wire around a hollow plastic tube. If I put 3 A of current through the wire, then the electromagnet can pick up two paperclips. Now I place an iron rod inside the plastic tube. How many paperclips can the electromagnet pick up now?

    = 1.5in

    1. less than 2
    2. 2
    3. more than 2
    4. Need more information

17. You do an experiment in outer space far away from the pesky effects of gravity and friction. A 0.2-kg bar magnet is launched horizontally from left to right at a speed of 4 m/s through a 0.5 m long stationary aluminum pipe with a mass of 1 kg. What is the speed of the magnet after it exits the pipe?

    = 1.in

    1. less than 4 m/s
    2. 4 m/s
    3. more than 4 m/s
    4. Need more information
    
    
    
    

18. In the previous problem, in what direction is the pipe moving after the magnet exits the pipe?
    1. to the left (opposite the direction of the magnet)
    2. to the right (in the same direction as the magnet)
    3. nonsense, the pipe is not moving
    4. Need more information
    
    
    
    
    
    
    

19. (no explanation needed) I use a bar magnet to pick up a small iron rod. This temporarily magnetizes the rod. Which diagram indicates the magnetic orientation of the rod?

    = 1.in

    1. N on top, S on bottom
    2. S on top, N on bottom
    3. N on top, N on bottom
    4. S on top, S on bottom
    5. it is not magnetized at all
    6. Need more information

20. A large wheel is free to rotate around its hub. Three ropes are attached to three points on the wheel. You can pull on any one rope with a certain force for a very short time (same force and time on each rope). Which rope should you pull on in order to exert the maximum torque on the wheel? (You cannot change the direction of the rope.)

    = 1.in

    1. rope A
    2. rope B
    3. rope C
    4. rope A or C
    5. Need more information

21. The wheel shown below has a mass of 30 kg. I add a 5-kg block to the wheel. Where should I add the block in order to maximize the rotational inertia of the wheel plus block?

    = 1.in

    1. point A
    2. point B
    3. point C
    4. points A or C
    5. Need more information

22. A ball slides down a curved frictionless slope as shown in the figure. At which point does the ball have the greatest acceleration?

    = 1.in

    1. point A
    2. point B
    3. point C
    4. all the same
    5. Need more information

23. A ball slides rolls down a curved frictionless slope as shown in the previous problem. At which point does the ball have the greatest speed?
    1. point A
    2. point B
    3. point C
    4. all the same
    5. Need more information
    
    
    
    
    
    
    

24. Dave's pickup truck has rear-wheel drive. In order to improve traction in the snow, Dave put three fifty-pound sandbags in the back of the pickup truck over the rear wheels. The sandbags are not tied down. If the pickup truck stops VERY suddenly, where will the sandbags end up?
    1. They will move backwards (relative to the pickup) and fall off the rear of the pickup truck
    2. They will stay where they are in the pickup truck
    3. They will move forwards (relative to the pickup) and slam into the cabin (front) of the pickup truck
    4. Need more information
    
    
    
    
    
    
    

25. An 5-kg object is sliding in a straight line along the surface of an icy pond at a constant 13 m/s. What is the net force on the object?

    
    
    
    
    
    
    

26. A proton (mass = 1 amu) travelling at $2\cdot10^7$ m/s hits a neutron of the same mass. They stick together to form a deuterium nucleus.
    1. What is their total momentum before the collision? (Hint: use `amu' for the mass units instead of kg.)
       
       
       
       
       
       

    2. What is the speed of the deuterium nucleus (ie: the proton and neutron stuck together) after the collision?
       
       
       
       
       
       

27. Suppose that there is another planet named `Dirt' that is the same size as the Earth but with only half the mass. The gravitational acceleration $g_{Dirt}$ on the surface of Dirt will be
    1. 1/4 that of Earth
    2. 1/2 that of Earth
    3. the same as Earth
    4. twice that of Earth
    5. four times that of Earth
    6. Need more information
    
    
    
    
    
    
    

28. A satellite in orbit 200 km above the Earth needs a speed of 8 km/s to stay in orbit. A satellite in orbit around Dirt will need a speed of
    1. less than 8 km/s
    2. 8 km/s
    3. more than 8 km/s
    4. Need more information
    
    
    
    
    
    
    

29. A satellite is in orbit around the Earth at a height of 200 km (6600 km from the center of the Earth). It has a tangential speed of 8 km/s. How much time does it take to complete one orbit around the Earth?

    
    
    
    
    
    
    

30. A gallon of gasoline contains about $10^8$ J of chemical energy. If we could convert that chemical energy to kinetic energy with 100% efficiency, what speed would your 2000 kg car be going (ignore friction and air resistance)? Give your answer in m/s and then connvert it to mph using 1 m/s $\approx 2$ mph. Is this a lot?

    
    
    
    
    
    
    
    
    

31. You raise a textbook of mass $m$ to certain height, giving it a certain amount of potential energy. If you now raise a second textbook of twice the mass to the same height, its potential energy will be
    1. 1/4 as much
    2. 1/2 as much
    3. the same
    4. twice as much
    5. four times as much
    6. Need more information
    
    
    
    
    
    
    

32. Two charged objects that are free to move are separated by a distance of 10 m. Object $A$ has a mass $m_A = 2$ kg and a charge $q_A = +4$ mC. Object $B$ has a mass $m_B = 3$ kg and a charge $q_B = +8$ mC. What is the electrical force acting on object $A$?

    
    
    
    
    
    
    

33. In the previous problem, what is the electrical force acting on object $B$?

    
    
    
    
    
    
    

34. In the previous problem, what is the acceleration of object $B$?