Due Monday, November 13, 1995, In Class
Please Answer Each Question As Briefly As Possible
You May Work Together, But Write Up Your Answers Separately
Question 1: (Case 2 from Chapter 6)
Your room in the dormitory is particularly cold at night. Here are several things that you decide to do to make life more comfortable.
(A) You tape sheets of clear plastic across the windows of your room. Why do these plastic sheets make the room warmer?
(B) You install curtains on the windows and draw them at night. How do the drawn curtains make your room warmer?
(C) Your room has a high ceiling. You install a ceiling fan to push the air downward from the ceiling. Why does the fan make the air warmer near the floor?
(D) You put a thick, down comforter or quilt on your bed. Describe in terms of heat flow how the comforter keeps you warm in bed.
(E) You put an electric blanket on the bed. It uses electricity to make heat inside the blanket itself. How does this heat flow from the electric blanket to you?
Question 2: (Case 1 from Chapter 6)
Glass transmits the visible light radiated by the white hot sun but it absorbs the infrared light radiated by objects near room temperature. This selective absorption and transmission gives rise to the "greenhouse effect".
(A) A small patch of garden is bathed in sunlight. The sun is transferring heat to this garden by radiation but its temperature remains essentially constant. Why doesn't the garden get hotter and hotter?
(B) If you cover the garden with a glass dome, what happens to the garden's ability to get rid of heat via radiation?
(C) How does the glass dome affect the garden's temperature?
(D) Like glass, molecules such as water vapor, carbon dioxide, and methane transmit visible light while absorbing some infrared light. How does the presence of these molecules in the earth's atmosphere affect the earth's ability to get rid of heat and the earth's temperature?
(E) Why are people concerned about a large increase in the amount of carbon dioxide in the atmosphere?
Question 3: (Case 4 from Chapter 6)
Baking a potato takes a long time, even in a hot oven, because the inside of a potato warms up very slowly.
(A) What makes it hard for conduction to transfer heat to the center of the potato?
(B) What makes it hard for convection to transfer heat to the center of the potato?
(C) What makes it hard for radiation to transfer heat to the center of the potato?
(D) Why does inserting a metal skewer through the potato help it to cook quickly?
(E) Why does a baked potato stay hot inside for such a long time, even when it's sitting on your plate?
Question 4: (Case 2 from Chapter 5)
Modern Olympic Athletes are aided by a variety of scientific advances. Here are a few examples of how science enhances performance in athletics.
(A) A ski-jumper is suspended in a wind-tunnel, where a device measures the horizontal and vertical forces on him as air flows past. The jumper arranges his body to obtain the most favorable forces for a long flight. Describe the most favorable horizontal and vertical forces.
(B) In cross-country skiing, the skier slides forward first on one ski and then on the other. The ski bottoms are coated with special waxes so they exhibit large static frictional forces on snow but small dynamic frictional forces. How does this difference in frictional forces allow the skier to use a stationary ski to push herself forward on a sliding ski?
(C) Modern tracks are often constructed out of recycled rubber tires. The track surface is very resilient, deforming slightly when you push down on it and then bouncing back when you stop pushing. How does this surface improve runners' times as compared to the dirt, gravel, or cinder tracks of long ago?
(D) Bicyclists now use aerodynamically designed bicycles and clothing to minimize air drag. Bicyclists also "draft" one another, a strategy where one bicycle follows immediately behind the previous bicycle, easing the rear bicycle's passage through the air. If the front bicycle were so aerodynamic that it experienced no air drag at all, how would this affect the strategy of drafting and why?
(E) The wind is monitored during races so that a record time is labeled as "wind assisted" if the runner experienced a tail wind that was above a certain limit. Use physics to explain why a tail wind gives a runner a time advantage.
Question 5: (Case 1 from Chapter 5)
You are consulting for a screenwriter who is working on a science-fiction movie about the crew of a large, intergalactic spaceship. She wants the script to be realistic from a scientific perspective so she has asked you to check her work so far. You read through the script, which includes descriptions of special effects, and quickly find several problems with it. Here are some of the scenes that have flaws.
(A) In one scene, the spaceship must make an abrupt left turn to avoid hitting an asteroid. The crew members stand anxiously but motionlessly on the command bridge during this maneuver. In reality, what should happen to the crew members?
(B) In another scene, a guard fires a peculiar weapon at a massive alien creature. The guard does not move, but the creature is thrown backward when the weapon hits. In reality, what should happen to the guard as he fires the weapon?
(C) The main spaceship carries several small fighter spacecraft that resemble high-tech modern military fighter aircraft. In the depths of space, these fighter spacecraft dodge and turn rapidly, even though the rocket exhaust is always sent straight out of the rear of the fighter. Why can't such a spacecraft turn in space while a similar aircraft can turn near the ground?
(D) One of the small spacecraft secretly shuttles supplies from one earth-like planet to another, completing 8 round trips without replenishing its chemical rocket fuel. You inform the writer that the spacecraft will have trouble leaving a planet even once using chemical rockets, unless that spacecraft is allowed to eject stages. Why are you right?
(E) Near the end of the script, a main character falls from a cliff but is rescued only 2 meters from the ground when she lands on the wing of a passing spacecraft. You inform the writer that the impact with the wing of the spacecraft would be just as fatal as one with the ground. Why is that the case?