We've collected all the in-depth information you've ever wanted on how Required Secondary Powers should physically affect various superpowers.
Superpowers In General - Most portrayed uses of super-powers have No Conservation of Energy; whether strength, speed, energy blasts, etc., supers tend to use far more energy than a human metabolism can provide. To use any physical power at anything above a moderate level (think Buffy, not Hulk), there needs to be not only access to some source of energy (Another Dimension being the common Hand Wave, as is being a Big Eater - provided that they really, really consume big, like drinking several times their weight in pure nectar every day), but also super-metabolism (when they're going to use a lot of energy in a short time), super-stamina (it's all no use if you get fatigued easily, quickly) and some means of channeling it through the body's fragile systems safely. Otherwise your characters will be limited to only-slightly-greater-abilities-than-the-best-humans superheroics.
The Classicsa.k.a. Powers You Can't Be A Superman Expy Without
Also, for those that can go high into the atmosphere, they also have a reduced requirement for oxygen. As there is less atmospheric pressure higher in the atmosphere (one's lungs operate by manipulating internal air pressure to move air into and out of the lungs), flying characters would either need to stay closer to the ground, be able to generate a higher partial pressure of oxygen from their surroundings (via air-related powers or molecular control) or possess less need to breathe (and thus could stay high up for long periods of time). Presumably, this would also lead them to hold their breath longer at sea level, but even if they can breathe if they have winged-based flight a lower atmosphere pressure would mean less lift so they would need very big wings in order to reach high altitudes.
Thirdly, there is an issue of temperature. The adiabatic lapse rate is (in layman's terms) the rate at which the temperature decreases as you go higher above the earth's surface. A flier must have some way of preserving body heat at altitude or they would freeze in short order. Additionally, the speed of flight will cause the rate of freezing to increase, as Space Is Cold will tell you. You'll also be a greater conductor of electricity the colder you get, so that requires some resistance to that.
Some of the same problems associated with speed also apply to fast fliers. Flight almost invariably involves a combination of Super Strength and the ability to ignore aerodynamics, as fliers often lift off while carrying someone or something. One also has to wonder how they don't destroy half the atmosphere flying that fast. The shockwave and heat would be devastating - Tunguska style.
In addition, it has been pointed out that many fliers must also have some regenerative powers. Otherwise, the constant effects of the wind and airborne dust particles on exposed skin would cause premature aging. Storm for example explicitly has physical resistance to the elements and temperature extremes. Otherwise she'd have skin resembling leather and more wrinkles than women decades older due to constant exposure to fast moving wind, rain, and snow. Fliers with invulnerability get a pass, but that's it.
On a related note, there must be something special about fliers' hair, otherwise most of them would be sporting horribly tangled locks after each flight. Even more impressive is the way it never seems to be blown into their eyes (though many female fliers wear a headband for this exact reason).
Body Manipulationa.k.a. You Look Different. Did You Get A Haircut?
Also, to use an eye normally one must at least have a lens bending the light (thus being visible at least as glass-like objects), pigments in sensitive cells absorbing the light (as opposed to passing right through just like, say, x-rays) and something preventing the retina from blanket exposure by light not focused with the lens, that is, blocking all the rays not coming in via the pupil, including those from behind the retina. Oh, wait, doesn't that add up to almost a complete eyeball?
All of which really raises the question, what counts as an invisible person's body? Just his own cells with his own DNA? Then what about the bacterial colonies in the intestinal tract? What about the mitochondria in each cell, which have their own DNA? What about the lifeless liquids such as stomach acid and fluid in the eyes? What about dead cells, like hair and fingernails? Or the entire outer layers of skin, for that matter?
Some Invisible characters can extend the light-warping field over whatever they touch, eliminating some problems but adding others— conscious selectivity is required, otherwise if they bump into something, that item might blink in and out of sight, making them fairly easy to track, especially crossing a crowded room. The issue of how large an object their field can cover also comes up in many continuities.
Invisible Streaker is usually the result of lacking these Required Secondary Powers. A standard aversion of these required secondary powers is psychic invisibility, which is a mental or hypnotic effect that causes everybody to ignore you. The upside is that this automatically also foils hearing and smell. The downside is that it tends not to work on cameras: while some works would have the invisibility field work on anyone even if they're separated by a camera screen, others require a person to physically see the target with their own eyes for the invisibility to work. Some stories exacerbates this power to the point of forcing people to retcon their perception to ignore the psychically invisible character. For example, if he takes your sandwich, you change your memory so that you never had it, and possibly wonder why you bought a Coke but not a sandwich, considering you are hungry. Pretty much all works though would agree that film, or memory cards, are immune since they don't have a mind to mess with. Keep in mind it does not keep you from triggering the laser tripwire that fills the room with toxic gas or anything else that is automated, unless the automation takes the form of a sentient AI.
Michio Kaku of Sci-Fi Science proposed the best practical path to invisibility is a form-fitting suit that bends surrounding light. This suit has small eyeholes which doesn't have this light-bending attribute, allowing the user to see. His eyes would still be somewhat visible, though they may not be that noticeable, depending on the environment.
Those who can shift into other existing characters would need an eidetic memory to convincingly imitate every detail of a person they've seen once. For objects, change that to an encyclopedic knowledge to remember the details of a multitude of objects. This is more commonly averted, as they are just as frequently exposed as fakes because of such a flaw in the disguise, or they are copying someone they are fairly familiar withnote , or they are even not copying anyone and just using their imagination.
There's also the issues of surviving while transforming and in the transformed state (altering one's body structure even a little is usually fatal in the real world, let alone having your flesh turned into another substance or, in most cases, turning into something with no internal organs) and keeping the ability to shape shift rather than transforming into something cursed with permanent Shapeshifter Mode Lock... This one could fill a page on its own.
There's also the question of how a shape shifter continues to think (or control his or her own biology) when the brain itself is altered in shape or chemistry.
And of course, if the shape shifter can also change his mass...
Also, some shape shifters have the power to vanish their clothes into the ether, and retrieve same when they become human again. (Most common when the work is aimed at younger audiences.) See also Technically Naked Shapeshifter; contrast Shapeshifting Excludes Clothing.
- It would imply an ability to change one's mass so that one does not blow away or sink into the earth (due to the shift in surface area and pressure).
- Because of the Square-Cube Law, one would need Super Strength to be able to support the increased weight, as well as the increase in pressure on the body. For example, if one were to go from 5 feet to 50 feet, the result would be a thousandfold (10^3) increase in mass and therefore weight, but only a hundredfold (10^2) increase in surface area, thereby increasing the amount of pressure on one's body (or force per unit surface area) to ten times normal. Go big enough and you'd likely barely be able to stand, let alone fight.
- One would also need the power to vary one's internal body temperature to avoid freezing to death at small sizes, or dying of heatstroke at large sizes (get ten times bigger and the increase in mass will cause a thousandfold increase in heat generation, but the increase in surface area will only increase the rate at which you dissipate heat by one hundredfold). The overt ability to survive a superhuman range of temperatures would also solve this problem.
- The ability to adjust the performance of your vital organs to accommodate your body would help as well. For instance, a heart in a fifty-foot-tall body requires quite a bit more strength to generate the hydraulic force needed to get blood to the brain, and red blood cells that grow to ten times their ordinary size had better be able to carry far more oxygen than they do at regular size.
- Depending on the magnitude of the abilities, it could require some means of anaerobic respiration (since you can't breathe oxygen if you're smaller than an oxygen molecule).
Defense, Immunity, and Healinga.k.a. Powers That Come In Handy When Opponents Actually Fight Back
There is also the matter of the energy that is distributed over the field. Something that pushes against a force field is exerting pressure, whose energy has to go somewhere. While physical barriers absorb such pressure by spreading its energy across their structure, or by transforming it into potential energy via elastic buffers, a projected, free-standing force field has no anchoring, and often no elasticity, so it must be dealing with distributing the force inflicted upon it in some way, otherwise the field will just get pushed like anything that's thrown in the air - and potentially pushed towards you. Otherwise, if this energy were simply "done away with," there would be no reason for artificial or magical fields to buckle under assault at all. This often leads to a character suffering a Psychic Nosebleed as a way of showing his or her effort against such force, implying that the force field is braced against their brain. Ouch.
More importantly, they'd burn up a lot of energy accelerating the healing process that fast, and so would need super-stamina and more efficient internal energy use than normal (or an alternate energy source) or their own power could kill them by draining their body's resources too fast. And to regenerate takes mass, so either they have limits to what they can regrow at one time, or access to some other source of mass, like Another Dimension — again, see Shapeshifter Baggage. Also, there's something called the Hayflick limit which limits how many times a cell be able to divide themselves, so with that in mind, regenerators can only regenerate so much, after which they won't be able to do it anymore - oh, and did we mention that going out of this limit means that your organs can no longer replace their dying cells, thus you basically threw your cells' (and thus your organs', and thus yours) lifespan away? So regenerators must have some way to ignore this limit.
Also, they'd need a way to fight infection and other foreign substances. Having your guts spilled all over the floor would open the door to all sorts of germs and viruses, not to mention dust and such. There's a reason hospitals strive for perfect sterility. Conversely, they get infected like anyone else, but recover instantly due to the healing factor producing antibodies at an accelerated rate. That in turn would need another secondary power to prevent cytokine storms destroying the body. Wolverine's healing factor was used to develop antibodies to a deadly virus on at least one occasion. Most likely they would also have some kind of super-genome or very good immune system, considering that so much rapid healing and DNA replication would increase the possibility for cancer-causing genetic defects massively. This would explain Deadpool's massive scarring, since his skin is both spreading the cancer and healing the damage caused by the cancer. And, cancer aside, certain kinds of attacks could reasonably be assumed to cause genetic damage to the tissues the regenerating tissue is growing from.
In addition to that, it is possible that such a healing factor would have to display some sort of Morphic Resonance (or at least be controllable). Having broken bones heal almost instantly sounds much less impressive when you realize that this gives almost no time to set the bone before it heals crooked. On top of that, if you got shot you would have to take the bullet out yourself to prevent eventual lead poisoning, then wait for it to re-heal. Ouch. On the other hand, the healing could be "smart" enough to set bones correctly and start from the inside out so that the bullet is pushed out.
Let's not forget when things that are wrong with your body heal as they were. Deadpool gives us a pretty bad look at what happens when your healing factor heals cancerous cells or brain abnormalities as if they were normal healthy cells.
Finally, for a human to function normally, you'll need more than just your own cells replaced — there's lots of microorganisms that are important to survival. Just had your guts torn out and planning to regrow new ones? Well, unless you're also spontaneously re-creating bacteria there too, you'll have problems digesting future meals.
In extreme cases, you may be able to regenerate From a Single Cell. For that, your body has to (somehow) remember your old shape as you regenerate back. You're lucky if you end up as a younger version of yourself or even as a baby, but that would be detrimental if you were in the middle of a fight (or disaster) and nobody's helping you. Also, your cells have to be able to work like stem cells, i.e differentiating itself to multiple different kind of cells. Otherwise, you're SOL if your organs get torn out, your limbs are chopped off, or your flesh gets scorched off your bones.Finally, there's also the problem of foreign microorganisms mentioned above.
Furthermore, someone with Intangibility (if they're grounded) is still anchored relative to the Earth. Remembering that Earth whizzes around the Sun at a frankly irresponsible velocity, which itself whizzes around the galaxy at even more irresponsible velocities, anyone who can walk-through-anything should find themselves suspended in space a few seconds after their powers manifest, watching the Earth and possibly the entire solar system along with it slowly but surely arcing away from them...
Either that, or you're not fully intangible to every material - soil being an exception, for example, would allow you to continue walking on the ground (relatively) normally. If the intangible man becomes intangible by transforming into a sentient mass of energy, most types of energy are able to move through the atmosphere without shooting off into space, albeit not consciously. Being made of air is another possiblity(see Nigh-Invulnerability below); people can't touch air, but things that are airtight will trap air (and thus you) in. This may or may not be an offshoot of Shapeshifting into intangible molecules/other things that somehow can still keep their shape as your body. See Shapeshifting above for the problems this presents on its own.
Invulnerable characters need to be able to turn their ability "on" or "off" if needed for certain circumstances such as getting surgeries or injections. A good subversion is to set up the power so that just because one has bullet-proof skin, it doesn't mean that their internal organs aren't squishy. For example, at one point Luke Cage, who has bullet-proof skin, gets shot point-blank in the head with a shotgun, but while he survives it, the impact causes swelling in his brain, and the only way to ease the swelling is to insert a needle through his eye socket to drain the fluid.
Elemental Powersa.k.a. Put The Periodic Table Away, We Don't Mean THOSE Elements
For fire users, a few of them are often described as manipulating (if not producing/expelling) volatile gases that get ignited upon contact with oxygen. If you can somehow produce volatile gases from your body then your body obviously must've been either modified to be able to do so, or is inhuman to begin with. If you can both expel and manipulate then good for you; some characters can only manipulate existing fire but not producing it (they may have to carry lighters or similar things around), some others can only produce fire without controlling it (and if you aren't immune to fire yourself, then too bad).
Ice users may encounter the similar problem to water users: Ice won't just appear from nowhere without any water (or any freeze-able substance) around. Some users may be able to freeze the moisture in the air, but, like the winter's snow for example, the air has to be particularly humid. Some may be limited to just breathing cold air out of their mouth.
By the same token, psychic powers may theoretically be turning 'mental energy' into other types of energy. Not that that makes any more sense...
- Electricity has a tendency to take not only the path of least resistance, but to a lesser degree all other possible paths as well. Lightning rods, in particular, attracts lightning because the more pointy something is, the more proton juts out at its tip, meaning that electrons that caused the lightning will be highly attracted to the rod; that is to say, to hit your target with lightning, it must be the most likely one (by being the closest to you, or having some significantly sized pointy part) for the moving electron to hit. Hitting your target without inundating everything around it with current would require a great deal of setup, meticulous planning, prior knowledge of electrodynamics, and the resulting hours of linear algebra to ensure that all other available paths are sufficiently resistive enough not to cause collateral damage. Here's hoping you're really Good with Numbers. Or, in a few cases, you have a weapon that creates "lightning rods" on people. Otherwise, all that electrical charge will be released to (potentially) all directions... unless the character (who may otherwise be able to control their lightning) intentionally goes for it.
- Alternatively, ability to create the path of least resistance and to block all others my be a useful secondary ability. Remember, electricity is just the flow of electrons. A character who could psionically control electrons may be able to manipulate electrons in the air to move towards their designated target, and nothing else.
- It takes a stupefying amount of energy to break down air and channel a lightning bolt. Before anything happens, find out how many meters are between you and what you want to hit. Multiply that by 3,000,000. That's how much of a voltage difference you need to generate without leaking electrical pressure into your surroundings. Again, sustaining the bolt means sustaining that voltage difference in spite of massive amounts of charge leaving your end (i.e the voltage at your end steadily drains). This means that you have to have large amounts of energy to generate such massive voltage; see the problem with No Conservation of Energy above. Like before, manipulating the electrons themselves means you don't need too much energy.
- Natural lightning is a quick flash, a resolution of charge differences a fraction of a split second in duration. To strike something with a sustained arc means you have to complete and hold a circuit that is pumping vast amounts of current through your target, the air leading to it, the ground leading to it, and you. Naturally, one hopes both you and the ground are able to withstand this.
- Lightning - particularly a sustained arc - generates an amount of radiant heat and blinding light that is difficult to grasp, let alone defend against. It's a good idea to be fireproof and immune to blinding; better still, to be able to extend those protections to any bystanders so they won't be baked alive or permanently blinded by your display of awesomeness. Also, the massive thunderclap in the end is strong enough to generate a powerful 120-decibel (about the same volume as a jackhammer or a rock concert) shockwave capable of shattering glass and producing hearing damage. It's a good idea to make sure your lightning powers avoid anything like this, or invest in some ear protection for yourself at least. For comparison, the arc of electricity you see at the end of a taser isn't as shiny or loud as a real lightning, but it's also not as strong.
- It is possible, depending on the context, that the lightning-user channels it remotely — not through themselves (i.e making it more of a general manipulation of electricity). Many of these problems can be resolved if the electrical circuit is confined entirely to the target's body. But then you lose the cool lightning bolt, and all you get is a bad guy spasming violently like a deranged marionette until his insides boil out through his skin.note
Enhancementsa.k.a. Anything You Can Do, I Can Do Better
Heroes with this power also seem to automatically gain the skills needed to properly interpret the new sensory input. In reality people tend to struggle when they suddenly gain a new sense, and it can take years to adjust to it.
And they face a problem the other direction, of having too little friction on your footing. Cars turn easily at sixty miles per hour on (properly angled) asphalt, but they'd have a much harder time at 600 miles per hour on tile. There's a limit to how much lateral force a foot pushing back against a flat surface can create, determined by the coefficient of static friction between the sole of the foot and the ground surface. For typical shoe soles on, say, asphalt or concrete, this coefficient is between 0.8 and 1.0, which means a runner can't produce a forward force greater than 0.8-1.0 times his own weight without his foot slipping. The drag force from plowing through air at super-speed can be colossal, and if the forward force is less than this drag the speedster will slow down. The speed at which an object's drag force equals its own weight is a well-known quantity encountered by skydivers all the time: it's called the terminal velocity. The terminal velocity for a randomly-oriented human body is about 120 miles per hour; for a streamlined head-first human body it's closer to 150 miles per hour. If the speedster wants to run any faster than this, he either needs foot soles with coefficients of friction higher than 1.0, or a means of generating "downforce" to press him onto the pavement harder than his own weight.
Feet coming down at superspeed would presumably have super impact, and push them off the ground and at least a few feet in the air. Even once that's explained, someone going at superspeed over the slightest hill, or up stairs, or just random dips in the ground, would lose contact with the ground, they would keep going in the 'same direction' as before, into the air - think of skateboard ramps. note The faster ones would be launched into orbit, especially while doing the 'run around the world' trick. They have to have something that holds them in contact with the ground better than gravity if they don't want to end up hurtling comically though the air every other step like a Wuxia character.
Also, they have to have a very high stamina, as running from New York to Los Angeles all at once should be impossible no matter how fast they do it.
And if their speed is too high for a given viscosity, there's no time for collisions to be resolved quietly, so normally soft substances (like water or plastics) are going to behave like hard ones: blades of grass will become actual blades, then there's glass puddles, buckshot raindrops, shrapnel snow, fragile clothes, that sort of thing. Furthermore, speedsters seem to never get specks of dust caught in their eyes... Having Nigh-Invulnerability, like the case of air friction above, will protect them from these, but again, not all speedsters have it. Speedsters who pick up or put down anything while speeding also have to have some power that cushions the impact. Otherwise, attempting to grab a person when you're running at a hundred miles an hour wouldn't pick them up, it would snap them in half.
If these characters approach the speed of light, they would experience Time Dilation.
They would also need some type of super-concentration. Given that they must have some form of Super Reflexes so they can react normally to things at super-speed, they should perceive normal speed to be horrifyingly slow. Imagine trying to hold a conversation with people who take several hours to speak a single word. Similarly, if a speedster does something like typing millions of password combinations into a computer, he must somehow avoid getting bored after the first few hundred. It is also possible that the refresh rate of a monitor (normally 60 times per second) would be too slow for themnote , or for extreme examples, even the response time of the processor to something like a keyboard interrupt. Alternatively, they may have nervous systems that can multitask at different speeds, allowing them to perform (for example) simple manual tasks at superspeed while only being consciously aware of a few seconds passing.
Characters with super speed must also be able to withstand massive G-forces caused by rapid acceleration/deceleration, since most of them are shown to be able to start/stop instantly (rarely applies to those who lack this talent, but they may end up going Too Fast to Stop as a result and find that Inertia Is a Cruel Mistress). This must make the character somehow able to anchor to the ground while stopping too, since friction with the ground can only go so far - but often they just instantly stop. More than likely this uses some form of Inertial Dampening rather than actual friction, as it normally fails to tear up the roadway or set it on fire.
Protection for your internal organs from the effects of inertia is also a must, otherwise they'll just liquify and ooze out of your pores at extremely high speeds or fly out of your body when stopping on a dime.
Normal humans moving at very high speeds without a protective barrier would forcibly ram-scoop air into their lungs. It'd be like driving without a windshield...on a supersonic jet. So their "aura" must either slow the air, or redirect it safely around them (while still allowing in enough to breathe). There was a real life incident where the navigator in a US Navy KA-6 Tanker ended up hanging out the cockpit of the aircraft. His seat had become detached from the floor due to metal fatigue and ended up pretty much floating on the ejection guide rails. His helmet and oxygen mask were taken off by the wind. Through his apparently being one of the luckiest guys in the world, he survived, and later compared trying to breathe during the ordeal to trying to drink from a firehose. This on a subsonic aircraft with the pilot deliberately flying as slowly as he could (unlike most vehicles, aircraft stall when they fly below a certain speed due to the lack of lift).
Also traveling at very high speed would generate shock waves (i.e either sonic booms, or violent displacement of air) that even if they won't affect the speedster would affect everyone around.
And in the cases of extremely high speed, there is also the risk of the radiation (principally Gamma Rays) produced by the Doppler Effect of traveling a such high speeds.
Also, characters that are super-strong but don't explicitly have Nigh-Invulnerability have some level of enhanced resistance in their bones. If their super-powerful muscles were anchored by normal bones, the bones would repeatedly be broken apart by the muscles' exertion (or just constantly pulled off the anchor points, resulting in rather horrific sprains). This is not to even say anything about things like punching through stone.
To use super strength effectively also requires the ability to strengthen objects by touching them (unless the object is made of some advanced material, of course). Otherwise, holding up (for example) an airplane with one hand would simply result in a hand-shaped hole in the still-plummeting airplane. The so-called "tactile telekinesis" power is often the explanation for this - you touch something on your hand and your mind power (not your muscle) does the heavy lifting job, while also making the heavy burden ignore effects of gravity for as long as it's active.
Time and Spacea.k.a. Good Reasons To Call The Doctor
One explanation for this is a Time Bubble, where the character stays in bubble of sped up time, thus avoiding many of the problems that speedsters would face (and the bubble must move with you or you're stuck in one spot). Of course, while this solves some of the problems, it merely displaces others. For example, you'd still be blind unless you had a light source in the bubble with you. If you did have a light source, you could see fine inside the bubble, but any photons which hit the edge of the bubble would get "stuck," so you couldn't see anything outside. And when you stopped using your power, all of those "stuck" photons would start moving again at once, resulting in a blinding flash. note .
If objects can move in and out of the Time Bubble while it's active, trouble may arise from the change of energy and velocity: as such, an object going at normal speed entering a field of slower time may conserve its relative speed, and from the point of view of the slower time go extremely fast. Light going in and out of such a Time Bubble would also probably be blueshifted or redshifted, respectively while going into slower or faster time fields. The result would cause strange visuals from one side to another, from merely shifted colors to a bizarre world where only heat is colored due to infrareds becoming visible, or even nothing at all due to a radiation the atmosphere is opaque to. Redshift would be relatively less dangerous, although the dangers of heating up from infrareds or microwave rays, while blueshifting beyond near ultraviolets would produce ionizing radiation, capable of damaging DNA and cause cancers.
Another scenario is that rather than truly stopping time, the character simply moves really fast so everything else appears to be slower or stopped, like in the movie Clockstoppers, or one Choose Your Own Adventure story where you acquire a device to speed yourself up. Since this still leaves the problem of not being in sync with one's surroundings, for the purposes of this trope it can either be treated the same way as time "stopping", or as super-speed with all its attendant problems. Of course, there are problems with this too: the friction would probably be so great that you, and everything you touch, would either get a large hole in it or catch fire.
Over-application of this power should logically lead the user to be significantly older than they should be (other people stop aging whenever time stops). This may or may not be addressed, although it really should if several in-story years go by with the character continuing to use it.
- Motion could theoretically not take any energy at all as long as the gravity is the same in both places. The energy you spend walking, for example, ends up being absorbed by friction, but teleportation doesn't involve friction.
- The iconic "BAMF" effect of Nightcrawler's teleports in X-Men is a subversion of this - it's the sound effect of the air being violently displaced whenever he reappears.
- The makers of Jumper actually subverted this by doing something similar to a wormhole (the "jumpscar"), which also causes a spherical explosion upon arrival. The lead is jumping into a hospital, and the entire room is flattened into a circular pit with the lead in the center. Presumably there is also an implosion upon leaving, as seen in the bank robbery scene, and the money in the room starts flying after he goes back for seconds. There's also the scene in the end where he jumps with an entire apartment, causing a veritable tornado upon departure, which should have probably flattened the entire block, considering an implosion of that size would be enough to heat the air to several thousand degrees and explode violently, tactical nuke-style. For comparison, just look at the effect a pistol shrimp has when it snaps its claw at ~60MPH underwater. To quote the Cracked staff: "That's like getting punched by Mike Tyson in his prime if his outstretched arm was attached to a meteor as it entered the atmosphere.")
Arguably, there's also the problem of traveling only through time and ending up floating in space because surprise, surprise, planets, star systems, and even galaxies happen to move around.note Most time travel stories will completely ignore this problem, so the auto-compensation is a Required Secondary Power. Some time machines even teleport to a different place on Earth relative to their starting point, which just happens to be the perfect spot to set the plot rolling, with no specific input from the travelers to go there.
The short version is that, if you kept inertia, you would move through space- but you'd wind up nowhere near your planet anyway. The long version is that the time traveler would, unless the time control device had some sort of "Non-relativistic inertial dampener" built in (which would arguably be worse) or had the prerequisite auto-corrector, maintain their current heading and velocity through space at the time they leaped. Unfortunately, this would not take into account changes to the planet's velocity (as often happens, altered subtly by potential events like the gravity of other celestial bodies, meteors or very large explosions) between departure and arrival. However, this is further complicated by one main factor- your velocity being more or less perpendicular to the planet's actual travel path due to planetary spin. Your chances are... bad if you try to jump to the future. You're definitely not making it if you attempt to travel backwards through time under these conditions. Adjusting for changing terrain between leaving and arrival would also be required.
Note that these are only problems if the traveler jumps immediately from the departure time to the arrival time without existing in the time in between. If instead, the traveler could change the way time affects him, (such as speeding up time to go to the future or reversing time to go the past), any forces that would affect the rotation of the planet would affect him as well. However, this also has many of the same problems as Time Freezing. There is also the issue of traveling too long. If you're gone for six years, you can't come home on the day you left and expect that nobody notices that you're six years older.
Time travelers would need to be immune to the effects of diseases from different time periods. An modern man or woman's immune system would not likely be suited to fighting plagues from 2000 years into the past or future. Similarly, a time traveler risks becoming a Walking Wasteland by bringing germs from his or her own time period to others. Related: if traveling to the past, a person should also be concerned with eating meals that haven't gone through a modern sanitation process.
Time travelers also need some sort of resistance to the effects of jet-lag (though this is more prominent in time/space travel). Jumping between times and timezones (for example, from 12 in the afternoon to 12 midnight) would provide similar effects as traveling from Bangalore to San Francisco. Hardly any time travelers have a visible reaction to this, even if their time-travel isn't through the use of powers, but through machinery. Not so much a "required" secondary power, per se, but still notable nonetheless.
If a time traveler wants to have any sort of social interaction with the people from the time period, they'll need to do more than just pack period-appropriate attire. Sure, you might know how to dress to blend in to 1940s Germany or Walk Like an Egyptian, but can you speak the language? Understand local customs? Do you have currency that you can spend in that time period/locale? Are you familiar with virtually-universal skills of the period, such as horse riding? For more recent eras, you'd need some form of official ID to function in society. If you want to blend in, you'd need a professor-level knowledge of the time period.
... boy, sure starting to feel Made of Plasticine now, aren't you?