Author: Lou Bloomfield

What is the difference between an elastic collision and an inelastic one? How do…

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What is the difference between an elastic collision and an inelastic one? How does an inelastic collision work and why?

When two objects collide with one another, they usually bounce. What distinguishes an elastic collision from an inelastic collision is the extent to which that bounce retains the objects’ total kinetic energy—the sum of their energies of motion. In an elastic collision, all of the kinetic energy that the two objects had before the collision is returned to them after the bounce, although it may be distributed differently between them. In an inelastic collision, at least some of their overall kinetic energy is transformed into another form during the bounce and the two objects have less total kinetic energy after the bounce than they had before it.

Just where the missing energy goes during an inelastic collision depends on the objects. When large objects collide, most of this missing energy usually becomes heat and sound. In fact, the only objects that ever experience perfectly elastic collisions are atoms and molecules—the air molecules in front of you collide countless times each second and often do so in perfectly elastic collisions. When the collisions aren’t elastic, the missing energy often becomes rotational energy or occasionally vibrational energy in the molecules. Actually, some of the collisions between air molecules are superelastic, meaning that the air molecules leave the collision with more total kinetic energy than they had before it. This extra energy came from stored energy in the molecules—typically from their rotational or vibrational energies. Such superelastic collisions can also occur in large objects, such as when a pin collides with a toy balloon.

Returning to inelastic collisions, one of the best examples is a head-on automobile accident. In that case, the collision is often highly inelastic—most of the two cars’ total kinetic energy is transformed into another form and they barely bounce at all. Much of this missing kinetic energy goes into deforming and heating the metal in the front of the car. That’s why well-designed cars have so called “crumple zones” that are meant to absorb energy during a collision. The last place you want this energy to go is into the occupants of the car. In fact, the occupants will do best if they transfer most of their kinetic energies into their airbags.

What is reverse osmosis and how it is used in the process of purifying seawater …

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What is reverse osmosis and how it is used in the process of purifying seawater for drinking water? — CS

In the form used for water desalination, reverse osmosis involves a special membrane that allows water molecules to pass through it while blocking the movement of salt ions. When water molecules are free to move between two volumes of water, they move in whichever direction reduces their chemical potential energy. The concept of a chemical potential is part of statistical physics—the area of physics that deals with vast collections of particles—and it depends partly on energy and partly on probability. Factors that contribute to a water molecule’s chemical potential are the purity of the water and the water’s pressure. Increasing the salt content of the water lowers a water molecule’s chemical potential while increasing the water’s pressure raises its chemical potential.

Because salty water has a lower chemical potential for water molecules than pure water, water molecules tend to move from purer water to saltier water. This type of flow is known as osmosis. To slow or stop osmosis, you must raise the chemical potential on the saltier side by applying pressure. The more you squeeze the saltier side, the higher the chemical potential there gets and the slower water molecules move from the purer side to the saltier side. If you squeeze hard enough, you can actually make the water molecules move backwards—toward the purer side! This flow of water molecules from the saltier water toward the purer water with the application of extreme pressure is known as reverse osmosis.

In commercial desalination, high-pressure seawater is pushed into jellyroll structures containing the semi-permeable membranes. The pressure of the salty water is so high that the water molecules flow through the membrane from the salty water side to the pure water side. This pure water is collected for drinking.

Does gravity have a speed at which it acts upon another body?

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Does gravity have a speed at which it acts upon another body? — CP, Billings, Montana

Yes, the speed of light. The gravitational interaction between two objects can be viewed as the exchange of particles called “gravitons,” just as the electromagnetic interaction between two objects can be viewed as the exchange of particles called “photons.” Gravitons and photons are both massless particles and therefore travel at a special speed: the “speed of light.” Since light is easier to work with than gravity, people discovered this special speed in the context of light first. If gravity had been easier to work with, they might have named it “the speed of gravity” instead. Sometime in the not too distant future, gravity-wave detectors such as the LIGO project will begin to observe gravity waves traveling through space from nearby cosmic events, particularly star collapses. These gravity waves will reach us at essentially the same time as light waves from those events since the gravity and light travel at the same speed.

I understand that the speed of electricity varies with the conductor, but is sup…

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I understand that the speed of electricity varies with the conductor, but is supposedly 2/3 the speed of light. I had thought the speed would equal the speed of light. Why isn’t it? — AP

Although electricity involves the movement of electrically charged particles through conducting materials, it can also be viewed in terms of electromagnetic waves. For example, programs that reach your home through a cable TV line are actually being carried by electromagnetic waves that travel in the cylindrical space between coaxial cable’s central wire and the tubular metal shield around it. These waves would travel at the speed of light, except that whenever charged particles in the wires interact with the passing waves, they introduce delays. The charged particles in the wires don’t respond as quickly as empty space does to changes in electric or magnetic fields, so they delay these changes and therefore slow down the waves. The materials that insulate the wires also influence the speed of the electricity by responding slowly to the changing fields. The fastest wires are ones with carefully chosen shapes and almost empty space for insulation. In general, the less the charges in the wire respond to the passing electromagnetic waves, the faster those waves can move.

How come if I stand on the balcony of my third story apartment and drop a hose t…

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How come if I stand on the balcony of my third story apartment and drop a hose to the swimming pool down below, I can’t suck any water up through the hose into my mouth?

While it may seem that you are somehow attracting the water to your mouth when you suck, you are really just making it possible for air pressure to push the water up toward you. By removing much of the air from within the hose, you are lowering the air pressure in the hose. There is then a pressure imbalance at the bottom end of the hose: the pressure outside the hose is higher than the pressure inside it. It’s this pressure imbalance that pushes water into the hose and upward toward your mouth.

But air pressure can’t push the water upward forever. As the column of water in the hose rises, its weight increases. Atmospheric pressure can only lift the column of water so high before the upward force on the water is balanced by the water’s downward weight. Even if you remove all of the air inside the hose, atmospheric pressure can only support a column of water about 30 feet tall inside the hose. If you’re higher than that on your balcony, the water won’t reach you no matter how hard you try. The only way to send the water higher is to put a pump at the bottom end of the hose. This pump can push upward harder than atmospheric pressure can and it can support a taller column of water. That’s why deep home wells have submersible pumps at their bottoms—they must pump the water upward because it’s impossible to suck it upward more than 30 feet from above.

How does a cassette tape recorder work?

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How does a cassette tape recorder work? — TW, Ottawa, Ontario

Like any tape recorder, a cassette recorder uses the magnetization of the tape’s surface to represent sound. The tape is actually a thin plastic film that’s coated with microscopic cigar-shaped permanent magnets. These particles are aligned with the tape’s length and can be magnetized in either of two directions—they can have their north magnetic poles pointing in the direction of tape motion or away from that direction. In a blank tape, the particles are magnetized randomly so that there are as many of them magnetized in one direction as the other. In this balanced arrangement, the tape is effectively non-magnetic. But in a recorded tape, the balance is upset and the tape has patches of strong magnetization. These magnetized patches represent sound.

When you are recording sound on the tape, the microphone measures the air pressure changes associated with the sound and produces a fluctuating electric current that represents those changes. This current is amplified and used to operate an electromagnet in the recording head. The electromagnet magnetizes the tape—it flips the magnetization of some of those tiny magnetic particles so that the tape becomes effectively magnetized in one direction or the other. The larger the pressure change at the microphone, the more current flows through the electromagnet and the deeper the magnetization penetrates into the tape’s surface. After recording, the tape is covered with tiny patches of magnetization, of various depths and directions. These magnetized patches retain the sound information indefinitely.

During playback, the tape moves past the playback head. As the magnetic fields from magnetized regions of the tape sweep past the playback head, they cause a fluctuating electric current to flow in that head. The process involved is called electromagnetic induction; a moving or changing magnetic field produces an electric field, which in turn pushes an electric current through a wire. The current from the playback head is amplified and used to operate speakers, which reproduce the original sound.

The rest of the cassette recorder is just transport mechanism—wheels and motors that move the tape smoothly and steadily past the recording or playback heads (which are often the same object). There is also an erase head that demagnetizes the tape prior to recording. It’s an electromagnet that flips its magnetic field back and forth very rapidly so that it leaves the tiny magnetic particles that pass near it with randomly oriented magnetizations.

Why are swept wings preferred for transonic/supersonic flight, but not for lower…

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Why are swept wings preferred for transonic/supersonic flight, but not for lower speeds? — CL

While the designers of low speed planes focus primarily on lift and drag, designers of high speed planes must also consider shock waves—pressure disturbances that fan out in cones from regions where the plane’s surface encounters supersonic airflow. The faster a plane goes, the easier it is for the plane’s wings to generate enough lift to support it, but the more likelihood there is that some portions of the airflow around the plane will exceed the speed of sound and produce shock waves. Since a transonic or supersonic plane needs only relatively small wings to support itself, the designers concentrate on shock wave control. Sweeping the wings back allows them to avoid some of their own shock waves, increasing their energy efficiencies and avoiding shock wave-induced surface damage to the wings. Slower planes can’t use swept wings easily because they don’t generate enough lift at low speeds.

How does one find out the speed of a quark? Is it 7000 times the speed of light?…

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How does one find out the speed of a quark? Is it 7000 times the speed of light? — D

It seems that quarks are forever trapped inside the particles they comprise—no one has ever seen an isolated quark. But inside one of those particles, the quarks move at tremendous speeds. Their high speeds are a consequence of quantum mechanics and the uncertainty principle—whenever a particle (such as a quark) is confined to a small region of space (i.e. its location is relatively well defined), then its momentum must be extremely uncertain and its speed can be enormous. In fact, a substantial portion of the mass/energy of quark-based particles such as protons and neutrons comes from the kinetic energy of the fast-moving quarks inside them.

But despite these high speeds, the quarks never exceed the speed of light. As a massive particle such as a quark approaches the speed of light, its momentum and kinetic energy grow without bounds. For that reason, even if you gave all the energy in the world to a single quark, its speed would still remain just a hair less than the speed of light.

What happens to a permanent magnet’s magnetic field if its temperature is lowere…

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What happens to a permanent magnet’s magnetic field if its temperature is lowered? What happens to a magnetic field at absolute zero?

Thermal energy is actually bad for permanent magnets, reducing or even destroying their magnetizations. That’s because thermal energy is related to randomness and permanent magnetization is related to order. Not surprisingly, cooling a permanent magnet improves its ordering and makes its magnetization stronger (or at least less likely to become weaker with time). At absolute zero, a permanent magnet’s magnetic field will be in great shape—assuming that the magnet itself doesn’t suffer any mechanical damage during the cooling process.

I recently read in a sales brochure for a major international energy services co…

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I recently read in a sales brochure for a major international energy services company that the speed of light had been exceeded in 1995. Is this true? If so, could you explain how this was accomplished? — TS

For very fundamental reasons, the speed of light in vacuum cannot be exceeded. Calling it the “speed of light” is something of a misnomer—it is the fundamental speed at which all massless particles travel. Since light was the first massless particle to be studied in detail, it was the first particle seen to travel at this special speed.

While nothing can travel faster than this special speed, it’s easy to go slower. In fact, light itself travels more slowly than this when it passes through a material. Whenever light encounters matter, its interactions with the charged particles in that matter delay its movement. For example, light travels only about 2/3 of its vacuum speed while traveling in glass. Because of this slowing of light, it is possible for massive objects to exceed the speed at which light travels through a material. For example, if you send very, very energetic charged particles (such as those from a research accelerator) into matter, those particles may move faster than light can move in that matter. When this happens, the charged particles emit electromagnetic shock waves known as Cherenkov radiation—there is light emitted from each particle as it moves.

I suppose that the brochure could have been talking about this light/matter interaction. But since that effect has been observed for decades, there is nothing special about 1995. More likely, the brochure is talking about nonsense.