Author: Lou Bloomfield

Is it possible to heat up the surface of a stealth aircraft by exposing it to st…

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Is it possible to heat up the surface of a stealth aircraft by exposing it to strong microwaves? Also, I heard that local forces in the recent Balkans conflict used cellular phone technology to down the U.S. stealth aircraft. Is that possible? – JG

Stealth aircraft are designed to absorb most of the microwave radiation that hits them and to reflect whatever they don’t absorb away from the microwave source. That way, any radar system that tries to see the aircraft by way of its microwave reflection is unlikely to detect anything returning from the aircraft. In effect, the stealth aircraft is “black” to microwaves and to the extent that it has any glossiness to its surfaces, those surfaces are tipped at angles that don’t let radar units see that glossiness. Since most radar units emit bright bursts of microwaves and look for reflections, stealth aircraft are hard to detect with conventional radar. Just as you can’t see a black bat against the night sky by shining a flashlight at it, you can’t see a stealth aircraft against the night sky by shining microwaves at it.

Like any black object, the stealth aircraft will heat up when exposed to intense electromagnetic waves. But trying to cook a stealth aircraft with microwaves isn’t worth the trouble. If someone can figure out where it is enough to focus intense microwaves on it, they can surely find something better with which to damage it.

As for detecting the stealth aircraft with the help of cell phones, that brings up the issue of what is invisibility. Like a black bat against the night sky, it’s hard to see a stealth aircraft simply by shining microwaves at it. Those microwaves don’t come back to you so you see no difference between the dark sky and the dark plane. But if you put the stealth aircraft against the equivalent of a white background, it will become painfully easy to see. Cell phones provide the microwave equivalent of a white background. If you look for microwave emission near the ground from high in the sky, you’ll see microwaves coming at you from every cell phone and telephone tower. If you now fly a microwave absorbing aircraft across that microwave-rich background, you’ll see the dark image as it blocks out all these microwave sources. Whether or not this effect was used in the Balkans, I can’t say. But it does point out that invisibility is never perfect and that excellent camouflage in one situation may be terrible in another.

I have noticed that the more I stir the milk into my coffee, the hotter it gets,…

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I have noticed that the more I stir the milk into my coffee, the hotter it gets, even though the milk is cold. How does it work?

Stirring the coffee involves a transfer of energy from you to the coffee. That’s because you are doing physical work on the coffee by pushing it around as it moves in the direction of your push. What began as chemical energy in your body becomes thermal energy in the coffee. That said, the amount of thermal energy you can transfer to the coffee with any reasonable amount of stirring is pretty small and you’d lose patience with the process long before you achieved any noticeable rise in coffee temperature. I think that the effect you notice is more one of mixing than of heating. Until you mix the milk into the coffee, you may have hot and cold spots in your cup and you may notice the cold spots most strongly.

I teach a class on safety helmets (hard hats) and had a question about one of th…

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I teach a class on safety helmets (hard hats) and had a question about one of their specifications. The manufacturer rates their crown impact energy level at 40 foot-pounds. Would this be equivalent to taking an object that weighs 20 pounds and dropping it 2 feet onto a hard hat? – AH

Assuming that the wearer doesn’t let the helmet move and that the object that hits the helmet is rigid, my answer is approximately yes. If a 20-pound rigid object hits the hat from a height of 2 feet, that object will transfer just over 40 foot-pounds of energy to the helmet in the process of coming to a complete stop. The “just over” has to do with the object’s continued downward motion as it dents the hat and the resulting release of additional gravitational potential energy. Also, the need for a rigid dropped object lies in a softer object’s ability to absorb part of the impact energy itself; a dropped 20-pound sack of flour will cause less damage than a dropped 20-pound anvil.

However, the true meaning of the “40 foot-pound” specification is that the safety helmet is capable of absorbing 40 foot-pounds of energy during an impact on its crown. This energy is transferred to the helmet by doing work on it: by pushing its crown downward as the crown dents downward. The product of the downward force on the crown times the distance the crown moves downward gives the total work done on the helmet and this product must not exceed 40 foot-pounds or the helmet may fail to protect the wearer. Since the denting force typically changes as the helmet dents, this varying force must be accounted for in calculating the total work done on the helmet. While I’m not particularly familiar with safety helmets, I know that bicycle helmets don’t promise to be useable after absorbing their rated energies. Bicycle helmets contain energy-absorbing foam that crushes permanently during severe impacts so that they can’t be used again. Some safety helmets may behave similarly.

Finally, an object dropped from a certain height acquires an energy of motion (kinetic energy) equal to its weight times the height from which it was dropped. As long as that dropped object isn’t too heavy and the helmet it hits dents without moving overall, the object’s entire kinetic energy will be transferred to the helmet. That means that a 20-pound object dropped from 2 feet on the helmet will deposit 40 found-pounds of energy in the helmet. But if the wearer lets the helmet move downward overall, some of the falling object’s energy will go into the wearer rather than the helmet and the helmet will tolerate the impact easily. On the other hand, if the dropped object is too heavy, the extra gravitational potential energy released as it dents the helmet downward will increase the energy transferred to the helmet. Thus a 4000-pound object dropped just 1/100th of a foot will transfer much more than 40 foot-pounds of energy to the helmet.

I understand now why the sky is blue, but why are sunsets red and orange? – AB, …

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I understand now why the sky is blue, but why are sunsets red and orange? – AB, Oak Ridge, Tennessee

As I discussed previously, the sky is blue because tiny particles in the atmosphere (dust, clumps of air molecules, microscopic water droplets) are better at deflecting shorter wavelength blue light than they are at deflecting longer wavelength red light. As sunlight passes through the atmosphere, enough blue light is deflected (or more technically Rayleigh scattered) by these particles to give the atmosphere an overall blue glow. The sun itself is slightly reddened by this process because a fraction of its blue light is deflected away before it reaches our eyes.

But at sunrise and sunset, sunlight enters our atmosphere at a shallow angle and travels a long distance before reaching our eyes. During this long passage, most of the blue light is deflected away and virtually all that we see coming to us from the sun is its red and orange wavelengths. The missing blue light illuminates the skies far to our east during sunrise and to our west during sunset. When the loss of blue light is extreme enough, as it is after a volcanic eruption, so little blue light may reach your location at times that even the sky itself appears deep red. The particles in air aren’t good at deflecting red wavelengths, but if that’s all the light there is they will give the sky a dim, red glow.

Why is it easy to stay on a bike while moving, but impossible once it stops? – A…

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Why is it easy to stay on a bike while moving, but impossible once it stops? – AS, Switzerland

A bicycle is my favorite example of a dynamically stable object. Although the bicycle is unstable at rest (statically unstable), it is wonderfully stable when moving forward (dynamically stable). To understand this distinction, let’s start with the bicycle motionless and then start moving forward.

At rest, the bicycle is unstable because it has no base of support. A base of support is the polygon formed by an object’s contact points with the ground. For example, a table has a square or rectangular base of support defined by its four legs as they touch the floor. As long as an object’s center of gravity (the effective location of its weight) is above this base of support, the object is statically stable. That stability has to do with the object’s increasing potential (stored) energy as it tips-tipping a statically stable object raises its center of gravity and gravitational potential energy, so that it naturally accelerates back toward its upright position. Since a bicycle has only two contact points with the ground, the base of support is a line segment and the bicycle can’t have static stability.

But when the bicycle is heading forward, it automatically steers its wheels underneath its center of gravity. Just as you can balance a broom on you hand if you keep moving your hand under the broom’s center of gravity, a bicycle can balance if it keeps moving its wheels under its center of gravity. This automatic steering has to do with two effects: gyroscopic precession and bending of the bicycle about its steering axis.

In the gyroscopic precession steering, the spinning wheel behaves as a gyroscope. It has angular momentum, a conserved quantity of motion associated with spinning, and this angular momentum points toward the left (a convention that you can understand by pointing the curved fingers of your right hand around in the direction of the tire’s motion; your thumb will then point to the left). When the bicycle begins to lean to one side, for example to the left, the ground begins to twist the front wheel. Since the ground pushes upward on the bottom of that wheel, it tends to twist the wheel counter-clockwise according to the rider. This twist or torque points toward the rear of the bicycle (again, when the fingers of your right hand arc around counterclockwise, your thumb will point toward the rear). When a rearward torque is exerted on an object with a leftward angular momentum, that angular momentum drifts toward the left-rear. In this case, the bicycle wheel steers toward the left. While I know that this argument is difficult to follow, since angular effects like precession challenge even first-year physics graduate students, but the basic result is simple: the forward moving bicycle steers in the direction that it leans and naturally drives under its own center of gravity. You can see this effect by rolling a coin forward on a hard surface: it will automatically balance itself by driving under its center of gravity.

In the bending effect, the leaning bicycle flexes about its steering axis. If you tip a stationary bicycle to the left, you see this effect: the bicycle will steer toward the left. That steering is the result of the bicycle’s natural tendency to lower its gravitational potential energy by any means possible. Bending is one such means. Again, the bicycle steers so as to drive under its own center of gravity.

These two automatic steering effects work together to make a forward moving bicycle surprisingly stable. Children’s bicycles are designed to be especially stable in motion (for obvious reasons) and one consequence is that children quickly discover that they can ride without hands. Adult bicycles are made less stable because excessive stability makes it hard to steer the bicycle.

I have heard that we “know” the universe is expanding because everything is mo…

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I have heard that we “know” the universe is expanding because everything is moving away from everything else. My question is: if this situation is like ink dots on a balloon, then we should be able to point to the direction of the universe’s center. Which way is that center? – BS

The “ink dots on a balloon” idea provides the answer to your question. In that simple analogy, the ink dots represent stars and galaxies and the balloon’s surface represents the universe. Inflating the balloon is then equivalent to having the universe expand. As the balloon inflates, the stars and galaxies drift apart so that an ant walking on the surface of the balloon would have to travel farther to go from one “star” to another. A similar situation exists in our real universe: everything is drifting farther apart.

The ant lives on the surface of the balloon, a two-dimensional world. The ant is unaware of the third dimension that you and I can see when we look at the balloon. The only directions that the ant can move in are along the balloon’s surface. The ant can’t point toward the center of the balloon because that’s not along the surface that the ant perceives. To the ant, the balloon has no center. It lives in a continuous, homogeneous world, which has the weird property that if you walk far enough in any direction, you return to where you started.

Similarly, we see our universe as a three-dimensional world. If there are spatial dimensions beyond three, we are unaware of them. The only directions that we can move in are along the three dimensions of the universe that we perceive. The overall structure of the universe is still not fully understood, but let’s suppose that the universe is a simple closed structure like the surface of a higher-dimensional balloon. In that case, we wouldn’t be able to point to a center either because that center would exist in a dimension that we don’t perceive. To us, the universe would be a continuous, homogeneous structure with that same weird property: if you traveled far enough in one direction, you’d return to where you started.

I am being assured by very reputable scientists (Professors of Physics in Americ…

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I am being assured by very reputable scientists (Professors of Physics in American and European universities) that centrifugal force is a fictitious force, even though the action of a centrifuge is defined as depending upon it. I would be very grateful if you could help me explain this apparent contradiction and perhaps outline the physical cause that underlies the separating action of a centrifuge, since it can hardly be a nonexistent force. – RGT, Portsmouth, UK

While “centrifugal force” is something we all seem to experience, it truly is a fictitious force. By a fictitious force, I mean that it is a side effect of acceleration and not a cause of acceleration.

There is no true outward force acting on an object that’s revolving around a center. Instead, that object’s own inertia is trying to make it travel in a straight-line path that would cause it to drift farther and farther away from the center. The one true force acting on the revolving object is an inward one-a centripetal force. The object is trying to go straight and the centripetal force is pulling it inward and bending the object’s path into a circle.

To get a feel for the experiences associated with this sort of motion, let’s first imagine that you are the revolving object and that you’re swinging around in a circle at the end of a rope. In that case, your inertia is trying to send you in a straight-line path and the rope is pulling you inward and deflecting your motion so that you go in a circle. If you are holding the rope with your hands, you’ll feel the tension in the rope as the rope pulls on you. (Note that, in accordance with Newton’s third law of motion, you pull back on the rope just as hard as it pulls on you.) The rope’s force makes you accelerate inward and you feel all the mass in your body resisting this inward acceleration. As the rope’s force is conveyed throughout your body via your muscles and bones, you feel your body resisting this inward acceleration. There’s no actual outward force on you; it’s just your inertia fighting the inward acceleration. You’d feel the same experience if you were being yanked forward by a rope-there would be no real backward force acting on you yet you’d feel your inertia fighting the forward acceleration.

Now let’s imagine that you are exerting the inward force on an object and that that object is a heavy bucket of water that’s swinging around in a circle. The water’s inertia is trying to make it travel in a straight line and you’re pulling inward on it to bend its path into a circle. The force you exert on the bucket is quite real and it causes the bucket to accelerate inward, rather than traveling straight ahead. Since you’re exerting an inward force on the bucket, the bucket must exert an inward force on you (Newton’s third law again). It pulls outward on your arm. But there isn’t anything pulling outward on the bucket, no mysterious “centrifugal force.” Instead, the bucket accelerates in response to an unbalance force on it: you pull it inward and nothing pulls it outward, so it accelerates inward. In the process, the bucket exerts only one force on its surroundings: an outward force on your arm.

As for the operation of a centrifuge, it works by swinging its contents around in a circle and using their inertias to make them separate. The various items in the centrifuge have different densities and other characteristics that affect their paths as they revolve around the center of the centrifuge. Inertia tends to make each item go straight while the centrifuge makes them bend inward. The forces causing this inward bending have to be conveyed from the centrifuge through its contents and there’s a tendency for the denser items in the centrifuge to travel straighter than the less dense items. As a result, the denser items are found near the outside of the circular path while the less dense ones are found near the center of that path.

When you are defrosting and the magnetron is turning on and off, when it is off,…

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When you are defrosting and the magnetron is turning on and off, when it is off, are the microwaves still bouncing around or is the food just sitting there warming itself up? – LEA, PA

During the defrost cycle, the microwave oven periodically turns off its magnetron so that heat can diffuse through the food naturally, from hot spots to cold spots. These quiet periods allow frozen parts of the food to melt the same way an ice cube would melt if you threw it into hot water. While the magnetron is off, it isn’t emitting any microwaves and the food is just sitting there spreading its thermal energy around.

I understand how a transformer changes voltage, but how does it regulate the amp…

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I understand how a transformer changes voltage, but how does it regulate the amperage? – DE

A transformer’s current regulation involves a beautiful natural feedback process. To begin with, a transformer consists of two coils of wire that share a common magnetic core. When an alternating current flows through the primary coil (the one bringing power to the transformer), that current produces an alternating magnetic field around both coils and this alternating magnetic field is accompanied by an alternating electric field (recall that changing magnetic fields produce electric fields). This electric field pushes forward on any current passing through the secondary coil (the one taking power out of the transformer) and pushes backward on the current passing through the primary coil. The net result is that power is drawn out of the primary coil current and put into the secondary coil current.

But you are wondering what controls the currents flowing in the two coils. The circuit it is connected to determines the current in the secondary coil. If that circuit is open, then no current will flow. If it is connected to a light bulb, then the light bulb will determine the current. What is remarkable about a transformer is that once the load on the secondary coil establishes the secondary current, the primary current is also determined.

Remember that the current flowing in the secondary coil is itself magnetic and because it is an alternating current, it is accompanied by its own electric field. The more current that is allowed to flow through the secondary coil, the stronger its electric field becomes. The secondary coil’s electric field opposes the primary coil’s electric field, in accordance with a famous rule of electromagnetism known as Lenz’s law. The primary coil’s electric field was pushing backward on current passing through the primary coil, so the secondary coil’s electric field must be pushing forward on that current. Since the backward push is being partially negated, more current flows through the primary coil.

The current in the primary coil increases until the two electric fields, one from the primary current and one from the secondary current, work together so that they extract all of the primary current’s electrostatic energy during its trip through the coil. This natural feedback process ensures that when more current is allowed to flow through the transformer’s secondary coil, more current will flow through the primary coil to match.

How do people measure g-forces? I have read articles about roller coasters that …

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How do people measure g-forces? I have read articles about roller coasters that report specific numbers, such as 3 g’s. How are these numbers obtained? – T

Whenever you accelerate, you experience a gravity-like sensation in the direction opposite that acceleration. Thus when you accelerate to the left, you feel as though gravity were pulling you not only downward, but also to the right. The rightward “pull” isn’t a true force; it’s just the result of your own inertia trying to prevent you from accelerating. The amount of that rightward “pull” depends on how quickly you accelerate to the left. If you accelerate to the left at 9.8 meters/second2, an acceleration equal in amount to what you would experience if you were falling freely in the earth’s gravity, the rightward gravity-like sensation you feel is just as strong as the downward gravity sensation you would feel when you are standing still. You are experiencing a rightward “fictitious force” of 1 g. The g-force you experience whenever you accelerate is equal in amount to your acceleration divided by the acceleration due to gravity (9.8 meters/second2) and points in the direction opposite your acceleration. Often the true downward force of gravity is added to this figure, so that you start with 1 g in the downward direction when you’re not accelerating and continue from there. If you are on a roller coaster that is accelerating you upward at 19.6 meters/second2, then your total experience is 3 g’s in the downward direction (1 g from gravity itself and 2 g’s from the upward acceleration). And if you are accelerating downward at 9.8 meters/second2, then your total experience is 0 g’s (1 g downward for gravity and 1 g upward from the downward acceleration). In this last case, you feel weightless-the weightlessness of a freely falling object such as an astronaut, skydiver, or high jumper.

Note added: A reader pointed out that I never actually answered the question. He’s right! So here is the answer: they use accelerometers. An accelerometer is essentially a test mass on a force sensor. When there is no acceleration, the test mass only needs to be supported against the pull of gravity (i.e., the test mass’s weight), so the force sensor reports that it is pushing up on the test mass with a force equal to the test mass’s weight. But once the accelerometer begins to accelerate, the test mass needs an additional force in order to accelerate with the accelerometer. The force sensor detects this additional force and reports it. If you carry an accelerometer with you on a roller coaster, it will report the force it exerts on the test mass at each moment during the trip. A recording device can thus follow the “g-forces” throughout the ride.

As far as how accelerometers work, modern ones are generally based on tiny mechanical systems known as MEMS (Micro-Electro-Mechanical Systems). Their test masses are associated with microscopic spring systems and the complete accelerometer sensor resides on a single chip.