. When accelerating, can you decelerate by going in a direction that is not opposite (your velocity)? For example, going north can you decelerate by going east?
Decelerating is a very specific acceleration—always in the direction opposite your velocity. If you were heading north and accelerated toward the east, your velocity would soon point toward the northeast. It would have some northward aspect because you were initially heading north and hadn't yet accelerated toward the south. It would have some eastward aspect because you had initially been heading neither eastward nor westward and had since accelerated toward the east.
On the other hand, if you were heading north and then turned toward the east, you would have lost your northward velocity and obtained an eastward velocity. This "turning" would have involved a southward acceleration (to get rid of the northward velocity) and an eastward acceleration (to acquire an eastward velocity).
. You said that from the moment the ball leaves your hand (after you threw it upward), it accelerates downward even though you threw it upward. However you then said that the ground (gravity) pushed on your foot to make you accelerate, so why would you also not be accelerating in the opposite direction, like the ball? Why would you not accelerate in the direction in which you were pushed?
I got ahead of myself by using forces I had not yet introduced. I was using friction to push me horizontally across the floor! Here is the complete story:
When I tossed the ball upward and it was rising, gravity was pulling downward on it and it was accelerating downward. But when I obtained a force from the ground, it was not gravity that exerted that force on me; it was friction! As we will discuss in a few days, whenever you try to slide your foot across the floor toward the left, friction pushes your foot toward the right. In class, I traveled toward the right because I was being pushed by friction toward the right. I was actually accelerating in the direction I was pushed, just as you expect.
. In today's lecture, you stated that a person accelerating downward OR UPWARD does not feel the effects of gravity. How do you explain the g-forces felt by astronauts at escape velocity? - TH
In the lecture, I said that a person who is falling does not feel the effects of gravity, even when they are traveling upward. But when they are falling, they are accelerating downward at a very specific rate—the acceleration due to gravity, which is 9.8 meters/second2 at the earth's surface. When an astronaut is accelerating upward during a launch, they are not falling and they do feel weight. In fact, because they are accelerating upward, they feel particularly heavy.
. What is heat? — PM, Princeton, NJ
Heat is thermal energy that's flowing from one object to another because of a temperature difference between those two objects. Whenever an object contains thermal energy—which it always does—the atoms and molecules in that object are jittering about microscopically. Each atom or molecule isn't completely stationary; instead it is vibrating back and forth, and pushing or pulling on its neighbors. The object's thermal energy is the sum of the tiny kinetic and potential energies of those atoms and molecules as they move back and forth (kinetic energy), and push or pull on one another (potential energy). The hotter an object is, the more thermal energy each of its atoms has, on average, so this thermal energy tends to flow to a colder object when you touch the two objects together. When that thermal energy is flowing from the hotter object to the colder object, we call it "heat."
. How does a refrigerator work? - SK
A refrigerator uses a material called a "working fluid" to transfer heat from the food inside the refrigerator to the air around the refrigerator. This working fluid moves through the refrigerator's three main components—the compressor, the condenser, and the evaporator—over and over again, in a continuous cycle. I'll begin as the fluid enters the refrigerator's compressor, which is usually located on the bottom of the refrigerator where it's exposed to the room air. The working fluid enters the compressor as a low-pressure gas at roughly room temperature. The compressor squeezes the molecules of that gas closer together, increasing the gas's density and pressure. Since squeezing a gas involves physical work (a force exerted on an object as that object moves in the direction of the force), the compressor transfers energy to the working fluid and that fluid becomes hotter as a result.. The working fluid leaves the compressor as a high-pressure gas that's well above room temperature. The working fluid then enters the condenser, which is typically a snake-like pipe on the back of the refrigerator. Since the fluid is hotter than the room air, heat flows out of the fluid and into the room air. The fluid then begins to condense into a liquid and it gives up additional thermal energy as it condenses. This additional thermal energy also flows as heat into the room air.
The working fluid leaves the condenser as a high-pressure liquid at roughly room temperature. It then flows into the refrigerator, then through a narrowing in the pipe, and then into the evaporator, which is another snake-like pipe that's wrapped around the freezing compartment (in a non-frostfree refrigerator) or hidden in the back of the food compartment (in a frostfree refrigerator). When the fluid goes through the narrowing in the pipe, it's pressure drops and it enters the evaporator as a low-pressure liquid at roughly room temperature. It immediately begins to evaporate and expands into a gas. In doing so, it uses its thermal energy to separate its molecules from one another and it becomes very cold. Heat flows from the food to this cold gas. The working fluid leaves the evaporator as a low-pressure gas a little below room temperature and heads off toward the compressor to begin the cycle again. Overall, heat has been extracted from the food and delivered to the room air. The compressor consumed electric energy during this process and that energy has become thermal energy in the room air.
. If heat rises, how come snow accumulates on mountains? Why is it colder up there instead of down here? — HG, Grand Prairie, TX
On a local scale, hot air does rise through cold air. That's because when hot air and cold air are at the same temperatures, the hot air has fewer air molecules per liter than the cold air and so each liter of hot air is lighter than each liter of cold air. In short, hot air is less dense than cold air and it floats upward in cold air. But when hot air rises a long way through the atmosphere, something begins to happen to the hot air. It cools off! That's because the air pressure decreases with altitude. The air pressure that's around us on the ground is only present because the air down here must support the air overhead. The air down here must push upward on the air overhead and it does this by developing a high pressure. But as you move upward in the atmosphere, there's less air overhead and therefore less air pressure around you.
So as the hot air rises upward, the air pressure around it gradually diminishes and the hot air expands. It has to expand because whenever its pressure is higher than the surrounding pressure, its molecules experience outward forces that cause them to spread out. But this expansion process uses some of the hot air's thermal energy—the hot air must push the surrounding air out of the way as it expands. With less thermal energy in it, the hot air becomes cooler. Dry air loses about 10° C for every kilometer it rises, while moist air loses about 6° or 7° C per kilometer. This cooling effect explains why air at higher altitudes, such as the air on mountains, is colder than the air at lower altitudes, such as the air in valleys.
Furthermore, whenever cold air descends through the atmosphere, it is compressed and its temperature rises! This warming process also increases the air's water-carrying ability so that it becomes relatively dry. That effect explains the special "Katabatic" winds that blow warm and dry out of the mountains—including the Santa Ana winds near Los Angeles, the Chinook in the Rocky Mountains, the Foehn in the Alps, and the Zonda in Argentina.
. Is there any way to make a homemade fog machine, like they use in clubs? — JW, Westport, CT
While it's pretty clear that fog machines fill the air with tiny water droplets, I'm not sure how all of them work. Some probably use high-frequency sound waves to break up water into tiny droplets and then blow these droplets into the room with a fan. That technique is used in some room humidifiers and you can see a stream of fog emerging from them as they operate. An easier way to make fog is to mix water and liquid nitrogen. While liquid nitrogen is harder to find, all you have to do is put them together and they'll start making fog. The boiling nitrogen shatters the water into tiny droplets, which flow out of the mixture in a layer of cold nitrogen gas.
. How do steam generators produce electricity? — KA, North Platte, NE
In a steam generating plant, water is boiled in a confined container (a "boiler") to produce very high-pressure steam. This steam is allowed to flow through a turbine to the low-pressure region beyond the turbine. A turbine resembles a fan, but one that is turned by the gas that flows through it rather than by a motor. The steam flows through the blades of the turbine and exerts forces on those blades to keep the turbine rotating. The steam loses energy as it twists the turbine around in a circle and this energy is transferred to the rotating turbine. The low-pressure steam is recovered from the end of the turbine. It is then condensed back into liquid water with the help of a cooling tower and then returned to the boiler for reuse.
The rotating turbine is connected to the rotating portion of a generator. This rotating component is an electromagnet and, as it spins, its magnetic field passes across a set of stationary wire coils. Whenever the magnetic field through a coil of wire changes, any current flowing through that coil experiences forces that may add or subtract energy from it. In this case, the rotating magnet transfers energy to the current passing through the wire coils and "generates" electricity. The current in these stationary wires carries away energy from the generator and it is this energy that eventually arrives in your home through the power lines. Overall, the energy flows from the boiler, to the steam, to the turbine, to the generator, to the current, and to your home.
. Why do metal objects spark/arc in the microwave? Why don't the metal walls of the microwave spark? - JR
Like all electromagnetic waves, microwaves are composed of electric and magnetic fields. Since an electric field exerts forces on charged particles, a microwave pushes electrons back and forth through any metals it encounters. It is this motion of electrons back and forth through the metal walls of the microwave oven that allow that metal to reflect the microwaves and keep them inside the oven. If you leave a spoon in you cup of coffee as you heat it in the microwave, electrons will move back and forth through the spoon. This motion of charge will cause no problems so long as (1) the spoon can tolerate this flow of charge without overheating and (2) the spoon doesn't allow the charges at its ends to leap into the air as a spark. To keep the spoon from overheating, it must be a good conductor of electricity. Since most spoons are pretty thick, the modest currents flowing through them in the microwave will leave little energy inside them and they won't overheat. But a thin twist-tie or small bit of aluminum foil may well overheat and begin to burn. To keep the spoon from sparking, it should have smooth ends. Electrons are more likely to leave the end of a metal surface at a sharp point, so avoiding points is important. Most spoons are smooth enough that no sparks will occur. But a fork, a sharp piece of foil, or a twist-tie may well begin to emit electrons into the air as those electrons pile up at one end of the wire while the microwave oven is on. Like a spoon, the walls of the oven are good conductors of electricity and they have no sharp points. While electrons move back and forth in these walls, they simply reflect the microwaves without becoming very hot and without emitting any sparks. You'll note that the light bulb for the microwave is always outside the cooking chamber because it contains small bits of metal that would have trouble inside a microwave oven.
. What are the two chemical in glow sticks? — JW, Westport, CT
I believe that the glow sticks contain luminol and hydrogen peroxide, which mix when you crack the glass ampoule and begin to emit light. There are several other chemicals present in the sticks to assist and control the process, but the principal reaction is one in which the hydrogen peroxide oxidizes ("burns") the luminol molecule. The result is a product molecule that is initially in an excited state—its electrons have more energy than they need—and it emits a particle of bluish-violet light. Since our eyes aren't particularly sensitive to that bluish-violet light, it's often converted into more visible light with the help of a fluorescent dye. The green light sticks probably contain sodium fluorescein molecules, each of which can absorb a photon of bluish-violet light and reemit some of its energy as a photon of green light. Other dyes, probably rhodamines, are used to make red or orange light sticks.