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Critical Thinking Questions in Physics Part 2
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by Hasan Fakhruddin
The Indiana Academy for Science, Mathematics, and Humanities BSU
Muncie, Indiana
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Heat and Thermodynamics Fluids Waves
and Optics Modern Physics
22. Coffee and Cream. Two identical cups P and Q have equal amounts of hot coffee at the same temperature. Cold creamer is now added to the cup P. A few minutes later, the same amount of cold creamer at the same temperature is added to the cup Q. Compare the new temperatures TP and TQ of the coffee in the two cups.
(A) TP = TQ
(B) TP > TQ
(C) TP < TQ
Answer: (B) TP > TQ
This is a question on heat transfer and Newton's Law of Cooling. A body at a higher temperature loses heat to the surrounding area at a higher rate. Since the cup Q was left at a higher temperature for a longer time interval, it has lost more heat.
23. Thermal Property of Empty Space? A metal rod AB is bent into the shape shown.
If the rod is heated uniformly, the space between the ends will
(A) Increase
(B) Decrease
(C) Stay the same
Answer: (A) Increase
In thermal expansion of an object of any shape, every particle moves away from every other particle. If the points A and B move closer, it will be contrary to expansion.
The straight sections ending in A and B do expand and cause the points A and B to come closer. However, the expansion of the lower straight segment moves the points A and B apart. The length of the lower segment is greater than the total length of the two upper segments. Hence the net effect is that the points A and B move farther apart.
24. The Amounts of Gas in Two Halves. A container is divided into two halves by a partition with a hole in it. The two halves contain the same gas but at different temperatures. Which half, if any, contains a greater amount of gas?
(A) The half at higher temperature
(B) The half at lower temperature
(C) The halves contain equal amounts of the gas.
Answer: (B) The half at lower temperature
The answer can be quickly arrived at by using the well-known equation PV = nRT. The hole in the partition causes the pressure to be the same in both halves. Hence the number of moles 
.
25. Liquid Pressure. A bottle is filled completely with water, as shown in the diagram below. Which of the points shown have the same pressure?
(A) P1 and P2
(B) P2 and P3
(C) P1 and P3
(D) P1, P2, and P3
(E) None of these
Answer: (B) P2 and P3
The pressure due to a liquid at a point is proportional to the depth of the point below the open surface of the liquid. This is true even if the open surface is not vertically above the point, as is the case with the point P3.
One might think that P1 and P3 have the same pressure, as both of them are 10 cm below the surface of the liquid. This not true because the water surface directly above P3 is not an open surface.
26. Beakers With and Without a Wooden Block. Two identical beakers carry water to the same height, but one of them has a wooden block floating in it. Which beaker, if either, weighs more?
(A) A
(B) B
(C) None
Answer: (C)
The amount of water in the beaker B is less than that in A due to the water displaced by the floating block. However, applying the Archimedes principle, the weight of the floating block is equal to the weight of the water it has displaced. Hence the answer!
27. Beakers With and Without a Completely Immersed Wooden Block. Two identical beakers carry water to the same height, but one of them has a block of wood immersed completely and attached by a string at the bottom. Which beaker, if either, weighs more?
(A) A
(B) B
Answer: (A)
In the beaker B, the displaced volume of water has been replaced by the wooden block of lesser density. Hence it weighs less.
28. Beakers With and Without an Iron Block. Two identical beakers carry water to the same height, but one of them has an iron block immersed in it. Which beaker, if either, weighs more?
(A) A
(B) B
(C) None
Answer: (B)
In the beaker B, the volume of displaced water is occupied by the iron block of greater density. Hence it weighs more.
29. Beakers With and Without a Suspended Iron Block. Two identical beakers carry water to the same height, but one of them has an iron block suspended in it by a string. Which beaker, if either, weighs more?
(A) A
(B) B
(C) None
Answer: (C) None
The beaker B is missing water displaced by the partly immersed iron block. The buoyancy force on the block equals the weight of the water displaced. This force also acts on the bottom of the beaker as a reaction force and compensates exactly for the reduction in the weight due to missing water.
30. Beakers With and Without a Completely Immersed Suspended Iron Block. Two identical beakers carry water to the same height, but one of them has a completely immersed iron block suspended in it by a string. Which beaker, if either, weighs more?
(A) A
(B) B
(C) None
Answer: (C) None
In the beaker B, the iron block experiences an upward buoyancy force equal to the weight of water displaced. The buoyancy force is applied by the water upward on the block and as a reaction downward on the bottom of the beaker, thus compensating for the weight of the displaced water.
31. The Level of the Lake. A boat in a lake drops an anchor into the lake. The level of the lake will
(A) Remain the same
(B) Rise
(C) Fall
Answer: (C) Fall
When in the boat, the anchor displaces water equal to its own weight. Since the density of the anchor is greater than the density of water, the volume of water displaced is greater than the volume of the anchor. When the anchor is dropped in water, the anchor displaces volume equal to its own volume. Hence the volume of water displaced when the anchor is dropped in the lake is less than when the anchor was in the boat. Thus the level of the lake falls.
32. Buoyancy Force on a Boat. A 2-ton boat is floating in a lake. The buoyancy force on the boat must be
(A) 2 tons
(B) More than 2 tons
(C) Less than 2 tons
(D) Dependent on the density of water in the lake
Answer: (A) 2 tons
When in the boat, the anchor displaces water equal to its own weight. Since the density of the anchor is greater than the density of water the volume of water displaced is greater than the volume of the anchor. When the anchor is dropped in water the anchor displaces volume equal to its own volume. Hence the volume of water displaced when the anchor is dropped in the lake is less than when the anchor was in the boat. Thus the level of the lake falls.
33. Look Again! When we look at ourselves in a plane mirror, we see left-right reversal but not up-down reversal. Why?
Answer: There is depth inversion and no left-right inversion in a plane mirror. Right appears right, left appears left, up appears up, and down appears down. However, to a person looking into the mirror, there is an illusion of left-right reversal and no up-down reversal. This is due to the fact that for another person to face the observer, the other person always turns around a vertical axis, causing a real left-right reversal.
34. Red Is Red. The wavelength of red light is close to blue in water, yet the red exit signs appear red to a swimmer from inside water. Why?
Answer: When light enters a medium from another medium, its wavelength and speed change, and frequency stays the same. The wavelength in the second medium is not affected by the introduction of a third medium between the two mediums. Thus red light entering the eye directly from air has the same wavelength inside the eye as the red light first entering water then into the eye. Thus red light appears red to a swimmer under water.
35. Danger Signal. Our eyes are most sensitive to greenish-yellow light, yet the danger signals are red. Why?
Answer: Red light can penetrate through atmosphere containing dust, clouds, and fog much more efficiently than any other color. Light toward the blue end of the visible spectrum is scattered to a much greater extent by the atmosphere. That's why the sky appears blue, and sunset and sunrise skies are reddish.
36. It Just Does Not Happen in a Vacuum. Why can't the electron-positron pair-production take place in a vacuum?
Answer: Pair-production is the creation of an electron-positron pair by a gamma ray photon. In this process, the photon disappears and its energy is converted into the rest mass of the electron-positron pair and the kinetic energy they carry. A portion of the energy of the photon is turned into the rest mass of the electron-positron pair. Thus the linear momentum of the pair is less than the momentum of the photon; this is violation of the law of conservation of linear momentum. Hence the pair-production always takes place in the vicinity of a heavy nucleus. The recoil of the nucleus ascertains conservation of linear momentum.
Hasan Fakhruddin taught AP Physics B and C for 11 years at the Indiana Academy for Science, Mathematics, and Humanities. He has been an AP Physics B and C Reader for six years and is a member of the AP Physics Development Committee. He has published a number of articles in The Physics Teacher. He received the 1998 John Vossen Memorial Award, sponsored by the American Vacuum Society, for developing the project titled "Measurement of the Refractive Index of Air Using a Vacuum Chamber.
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