## What packing material protects best? When we drop an egg wrapped in various pack…

#### What packing material protects best? When we drop an egg wrapped in various packaging materials, we know the force that gravity exerts on the egg but how do we know the force of the impact? — DL, Springboro, Ohio

I like to view problems like this one in terms of momentum: when it reaches the pavement, a falling egg has a large amount of downward momentum and it must get rid of that downward momentum gracefully enough that it doesn’t break. The whole issue in protecting the egg is in extracting that momentum gracefully.

Momentum is a conserved physical quantity, meaning that it cannot be created or destroyed. It can only be passed from one object to the other. When you let go of the packaged egg and it begins to fall, the downward momentum that gravity transfers into the egg begins to accumulate in the egg. Before you let go, your hand was removing the egg’s downward momentum as fast as gravity was adding it, but now the egg is on its own!

Because momentum is equal to an object’s mass times its velocity, the accumulating downward momentum in the egg is reflected in its increasing downward speed. With each passing second, the egg receives another dose of downward momentum from the earth. By the time the egg reaches the pavement, it’s moving downward fast and has a substantial amount of downward momentum to get rid of. Incidentally, the earth, which has given up this downward momentum, experiences an opposite response—it has acquired an equal amount of upward momentum. However, the earth has such a huge mass that there is no noticeable increase in its upward speed.

To stop, the egg must transfer all of its downward momentum into something else, such as the earth. It can transfer its momentum into the earth by exerting a force on the ground for a certain amount of time. A transfer of momentum, known as an impulse, is the product of a force times a time. To get rid of its momentum, the egg can exert a large force on the ground for a short time or a small force for a long time, or anything in between. If you let it hit the pavement unprotected, the egg will employ a large force for a short time and that will be bad for the egg. After all, the pavement will push back on the egg with an equally strong but oppositely directed force and punch a hole in the egg.

To make the transfer of momentum graceful enough to leave the egg intact, the protective package must prolong the momentum transfer. The longer it takes for the egg to get rid of its downward momentum, the smaller the forces between the egg and the slowing materials. That’s why landing on a soft surface is a good start: it prolongs the momentum transfer and thereby reduces the peak force on the egg.

But there is also the issue of distributing the slowing forces uniformly on the egg. Even a small force can break the egg if it’s exerted only on one tiny spot of the egg. So spreading out the force is important. Probably the best way of distributing the slowing force would be to float the egg in the middle of a fluid that has the same average density as the egg. But various foamy or springy materials will distribute the forces nearly as well.

In summary, (1) you want to bring the egg to a stop over as long as period of time as possible so as to prolong the transfer of momentum and reduce the slowing forces and (2) you want to involve the whole bottom surface of the egg in this transfer of momentum so that the slowing forces are exerted uniformly on the egg’s bottom surface. As for the actual impact force on the egg, you can determine this by dividing the egg’s momentum just before impact (its downward speed times its mass) by the time over which the egg gets rid of its momentum.

## Why are physicists so skeptical about peoples’ claims to have invented motors th…

#### Why are physicists so skeptical about peoples’ claims to have invented motors that provide mechanical power without consuming electric power or generators that produce electric power without consuming mechanical power from the systems that turns them? — LB (Yes, I’m asking myself this question)

While it may seem as though there is some grand conspiracy among physicists to deny validation to those inventors, nothing could be farther from the truth. Physicists generally maintain a healthy skepticism about whatever they hear and are much less susceptible to dogmatic conservativism than one might think. However, physicists think long and deep about the laws that govern the universe, especially about their simplicity and self-consistency. In particular, they learn how even the slightest disagreement between a particular law and the observed behavior of the universe indicates either a problem with that law (typically an oversimplification, but occasionally a complete misunderstanding) or a failure in the observation. The law of energy conservation is a case in point: if it actually failed to work perfect even one time, it would cease to be a meaningful law. The implications for our understanding of the universe would be enormous. Physicists have looked for over a century for a failure of energy conservation and have never found one; not a single one. (Note: relativistic energy conservation involves mass as well as energy, but that doesn’t change the present story.)

The laws of both energy conservation and thermodynamics are essentially mathematical laws—they depend relatively little on the specific details of our universe. Just about the only specific detail that’s important is time-translation symmetry: as far as we can tell, physics doesn’t change with time—physics today is the same as it was yesterday and as it will be tomorrow. That observation leads, amazingly enough, to energy conservation: energy cannot be created or destroy; it can only change forms or be transferred between objects. Together with statistical principals, we can derive thermodynamics without any further reference to the universe itself. And having developed energy conservation and the laws of thermodynamics, the game is over for free-energy motors and generators. They just can’t work. It’s not a matter of looking for one special arrangement that works among millions that don’t. There are exactly zero arrangements that work.

It’s not a matter of my bias, unless you consider my belief that 2 plus 2 equals 4 to be some sort of bias. You can look all you like for a 2 that when added to another 2 gives you a 5, but I don’t expect you to succeed.

About once every month or two, someone contacts me with a new motor that turns for free or a generator that creates power out of nowhere. The pattern always repeats: I send them the sad news that their invention will not work and they respond angrily that I am not listening, that I am biased, and that I am part of the conspiracy. Oh well. There isn’t much else I can do. I suppose I could examine each proposal individually at length to find the flaw, but I just don’t have the time. I’m a volunteer here and this is time away from my family.

Instead, I suggest that any inventor who believes he or she has a free-energy device build that device and demonstrate it openly for the physics community. Take it to an American Physical Society conference and present it there. Let everyone in the audience examine it closely. Since anyone can join the APS and any APS member can talk at any major APS conference, there is plenty of opportunity. If someone succeeds in convincing the physics community that they have a true free-energy machine, more power to them (no pun intended). But given the absence of any observed failure of time-translation symmetry, and therefore the steadfast endurance of energy conservation laws, I don’t expect any successful devices.

## My 10-year old son understands that body temperature is related to the speeds/ki…

#### My 10-year old son understands that body temperature is related to the speeds/kinetic energies of the molecules inside you, but does friction play a role as well? — MR

You’re both right about temperature being associated with kinetic energy in molecules: the more kinetic energy each molecule has, the hotter the substance (e.g. a person) is. But not all kinetic energy “counts” in establishing temperature. Only the disordered kinetic energy, the tiny chucks of kinetic energy that belong to individual particles in a material contributes to that material’s temperature. Ordered kinetic energy, such as the energy in a whole person who’s running, is not involved in temperature. Whether an ice cube is sitting still on a table or flying through the air makes no difference to its temperature. It’s still quite cold.

Friction’s role with respect to temperature is in raising that temperature. Friction is a great disorderer. If a person running down the track falls and skids along the ground, friction will turn that person’s ordered kinetic energy into disordered kinetic energy and the person will get slightly hotter. No energy was created or destroyed in the fall and skid, but lots of formerly orderly kinetic energy became disordered kinetic energy—what I often call “thermal kinetic energy.”

The overall story is naturally a bit more complicated, but the basic idea here is correct. Once energy is in the form of thermal kinetic energy, it’s stuck… like a glass vase that has been dropped and shattered into countless pieces, thermal kinetic energy can’t be entirely reconstituted into orderly kinetic energy. Once energy has been distributed to all the individual molecules and atoms, getting them all to return their chunks of thermal kinetic energy is hopeless. Friction, even at the molecular level, isn’t important at this point because the energy has already been fragmented and the most that any type of friction can do is pass that fragmented energy about between particles. So friction creates thermal kinetic energy (out of ordered energies of various types)… in effect, it makes things hot. It doesn’t keep them hot; they do that all by themselves.

## How does one “pull up their legs”? Wouldn’t you have to jump in some way or an…

#### How does one “pull up their legs”? Wouldn’t you have to jump in some way or another?

It is possible to simply pull up your legs. When you do that, you reduce the downward force your feet exert on the ground and the ground responds by pushing upward on your feet less strongly. With less upward force to support you, you begin to fall.

## Is hydroplaning a form of sliding friction?

#### Is hydroplaning a form of sliding friction?

Not exactly. Sliding friction refers to the situation in which two surfaces slide across one another while touching. In hydroplaning, the two surfaces are sliding across one another, but they aren’t touching. Instead, they’re separated by a thin layer of trapped water. While hydroplaning still converts mechanical energy into thermal energy, just as sliding friction does, the lubricating effect of the water dramatically reduces the energy conversion. That’s why you can hydroplane for such a long distance on the highway; there is almost no slowing force at all.

Dan Barker, one of my readers, informed me of a NASA study showing that there is a minimum speed at which a tire will begin to hydroplane and that that speed depends on the square root of the tire pressure. Higher tire pressure tends to expel the water layer and prevent hydroplaning, while lower tire pressure allows the water layer to remain in place when the vehicle is traveling fast enough. As Dan notes, a large truck tire is typically inflated to 100 PSI and resists hydroplaning at speed of up to about 100 mph. But a passanger car tire has a much lower pressure of about 32 PSI and can hydroplane at speeds somewhat under 60 mph. That’s why you have to be careful driving on waterlogged pavement at highway speeds and why highway builders carefully slope their surfaces to shed rain water quickly.

## How do anti-lock brake systems work?

#### How do anti-lock brake systems work?

If you brake your car too rapidly, the force of static friction between the wheels and the ground will become so large that it will exceed its limit and the wheels will begin to skid across the ground. Once skidding occurs, the stopping force becomes sliding friction instead of static friction. The sliding friction force is generally weaker than the maximum static friction force, so the stopping rate drops. But more importantly, you lose steering when the wheels skid. An anti-lock braking system senses when the wheels suddenly stop turning during braking and briefly release the brakes. The wheel can then turn again and static friction can reappear between the wheel and the ground.

## If ball bearings create no friction, why do bearings have bearing grease as an e…

#### If ball bearings create no friction, why do bearings have bearing grease as an essential ingredient?

Actually, some bearings are dry (no grease or oil) and still last a very long time. The problem is that the idea touch-and-release behavior is hard to achieve in a bearing. The balls or rollers actually slip a tiny bit as they rotate and they may rub against the sides or retainers in the bearing. This rubbing produces wear as well as wasting energy. To reduce this wear and sliding friction, most bearings are lubricated.

## If you walk up 10 steps, one by one, do you exert the same amount of energy if y…

#### If you walk up 10 steps, one by one, do you exert the same amount of energy if you walk up the same set of 10 steps two by two? How are energy and effort related, or are they?

Ideally, it doesn’t matter how many steps you take with each step—the work you do in lifting yourself up a staircase depends only on your starting height and your ending height (assuming that you don’t accelerate or decelerate in the overall process and thus change your kinetic energy, too). But there are inefficiencies in your walking process that lead you to waste energy as heat in your own body. So the energy you convert from food energy to gravitational potential energy in climbing the stairs is fixed, but the energy you use in carrying out this procedure depends on how you do it. The extra energy you use mostly ends up as thermal energy, but some may end up as sound or chemical changes in the staircase, etc.

## How can a ball create thermal energy or “get hotter”?

#### How can a ball create thermal energy or “get hotter”?

When a ball bounces, some of its molecules slide across one another rather than simply stretching or bending. This sliding leads to a form of internal sliding friction and sliding friction converts useful energy into thermal energy. The more sliding friction that occurs within the ball, the less the ball stores energy for the rebound and the worse the ball’s bounce. The missing energy becomes thermal energy in the ball and the ball’s temperature increases.

## I am a huge figure skating fan and was wondering if you could explain to me the …

#### I am a huge figure skating fan and was wondering if you could explain to me the physics of a triple axle jump? My friends and I are always asking ourselves how it’s done. — AF

While I don’t know the details of the jump, there are some basic physics issues that must be present. At a fundamental level, the skater approaches the jump in a non-spinning state, leaps into the air while acquiring a spin, spins three times in the air, lands on the ice while giving up the spin, and then leaves the jump in a non-spinning state. Most of the physics is in spin, so that’s what I’ll discuss.

To start herself spinning, something must exert a twist on the skater and that something is the ice. She uses her skates to twist the ice in one direction and, as a result, the ice twists her in the opposite direction. This effect is an example of the action/reaction principle known as Newton’s third law of motion. Because of the ice’s twist on her, she acquires angular momentum during her takeoff. Angular momentum is a form of momentum that’s associated with rotation and, like normal momentum, angular momentum is important for one special reason: it’s a conserved physical quantity, meaning that it cannot be created or destroyed; it can only be transferred between objects. The ice transfers angular momentum to the skater during her takeoff and she retains that angular momentum throughout her flight. She only gives up the angular momentum when she lands and the ice can twist her again.

During her flight, her angular momentum causes her to spin but the rate at which she spins depends on her shape. The narrower she is, the faster she spins. This effect is familiar to anyone who has watched a skater spin on the tip of one skate. If she starts spinning with her arms spread widely and then pulls them in so that she becomes very narrow, her rate of rotation increases dramatically. That’s because while she is on the tip of one skate, the ice can’t twist her and she spins with a fixed amount of angular momentum. By changing her shape to become as narrow as possible, she allows this angular momentum to make her spin very quickly. And this same rapid rotation occurs in the triple axle jump. The jumper starts the jump with arms and legs widely spread and then pulls into a narrow shape so that she spins rapidly in the air.

Finally, in landing the skater must stop herself from spinning and she does this by twisting the ice in reverse. The ice again reacts by twisting her in reverse, slowing her spin and removing her angular momentum. She skates away smoothly without much spin.