The lens of a slide projector has a focal length of 10.0 cm. It is used to project the image of a 35 mm slide 
on a square screen located 6.0 m away. Of the following screen widths, which is the smallest that would allow the image to fit?
A. 5.1 m
B. 4.4 m
C. 2.2 m
D. 1.1 m
E. 0.88 m
C. 2.2 m
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A plot of coffee has been allowed to cool to 17°C. If the electrical coffee maker is turned back on, the hot plate on which the pot rests is brought up to 70°C immediately and held at that temperature by a thermostat. Consider the pot to be a vertical cylinder 130 mm in diameter and the depth of coffee in the pot to be 100 mm. Neglect heat losses from the sides and top of the pot. How long will it take before the coffee is drinkable (50°C)? How much did it cost to heat the coffee if electricity costs $0.05 per kilowatt-hour?
GIVEN
• Coffee pot, idealized as a vertical cylinder, on a hot plate
• Initial temperature of the pot and coffee (Ts,i) = 17°C
• Hot plate temperature (Thp) = 70°C (constant)
• Pot diameter (D) = 130 mm = 0.13 m
• Depth of coffee (?) = 100 mm = 0.1 m
FIND
(a) Time for the coffee to reach 50°C (b) Cost to heat the coffee if electricity costs $0.05/kWh
ASSUMPTIONS
• Heat losses from the sides and the top are negligible
• All energy from the hot plate goes into the coffee
• Internal resistance of the coffee is negligible
• Thermal resistance of the bottom of the pot is negligible
• Coffee has the thermal properties of water
• Variation of the thermal properties of the coffee with temperature can be neglected
SKETCH
PROPERTIES AND CONSTANTS
The relevant thermal properties will be evaluated using the average coffee temperature of (17°C + 50°C)/2 = 33.5°C.
for water At 33.5°C
Density (?) = 994.6 kg/m3
Specific Heat (c) = 4175 J/(kg K) At the mean temperature of (33.5°C + 70°C)/2 = 51.8°C
Thermal expansion coefficient (?c) = 0.00047 1/K Thermal conductivity (kc) = 0.648 W/(m K) Kinematic viscosity (?c) = 0.549 × 10–6 m2/s Prandtl number (Prc) = 3.5
The oxygen in Earth's atmosphere
a. was manufactured inside stars. b. was added to the atmosphere by plant life. c. has grown more abundant since the origin of Earth. d. all of these e. none of these
A binary star system in the constellation Orion has an angular separation between the two stars of 1.2 × 10?5 radians. If ? = 5 × 10?7 m, what is the smallest-aperture (diameter) telescope that may be used to resolve the two stars?
A. 10 cm B. 5 cm C. 50 cm D. 1 m E. 4 cm
Consider a collection of charges in a given region and suppose all other
charges are distant and have a negligible effect. Further, the electric potential is taken to be zero at infinity. If the electric potential at a given point in the region is zero, which of the following statements must be true? (a) The electric field is zero at that point. (b) The electric potential energy is a minimum at that point. (c) There is no net charge in the region. (d) Some charges in the region are positive, and some are negative. (e) The charge shave the same sign and are symmetrically arranged around the given point.
1.(a) only 2.(d) only 3.(b), (c), and (d) 4.(a) and (e)