Inertia Is a Cruel Mistress

"Kitty: Kurt, can't you just 'port us to the ground [from this airplane in mid-flight]?

Kurt: Right! Picture this: Bumpity, bumpity, bumpity, SPLAT! Too high up and way too fast!"

- X-Men Evolution, "Grim Reminder"

Pretty much Exactly What It Says On the Tin; once the character enters teleportation, or starts a set of movements, he can't change destination or direction of movement at all.

The most oft-cited weakness given to an enemy with Teleportation or Super Speed if the plot requires a not-so-powered character to beat them. All an underpowered character has to do is figure this out, and then intercept them. Heaven forbid the opponent has the Required Secondary Powers needed to see it coming (ergo, Super Reflexes) and maneuver out of the way, is a Nigh Invulnerable Lightning Bruiser who can just barrel through the obstacle, has a Tele Frag effect that will just destroy the offending obstacle, or worse, breaks the Magic a Is Magic A by not being subject to this problem...

A key aspect of Deadly Dodging, Wronski Feint, and defeating most Bullfight Bosses. A key part of considerations of Jump Physics in videogames. Compare Too Fast to Stop. Not to be confused with Gravity Is a Harsh Mistress.

Anime & Manga

 * Mentioned as the core weakness of "instant movement" in Mahou Sensei Negima! In the tournament arc, it became a key strategic fact in the battle between Negi and Takamichi, who explained it while tripping his opponent up. It showed up again in the fight between Negi and Rakan. Negi's solution?  so he had the boosted reflexes to handle it.
 * In Slayers, the minion Rahanimu was magically modified so he could fly in a straight line at immense speed. His only battle tactic was to charge straight at Lina, and Lina's only defense tactic was to duck. This went on for quite some time, in what Lina describes as a pathetic excuse for a battle. Later, when Zelgadis gets in a fight with Rahanimu, Lina warns him about how dangerous Rahanimu is. Zel simply holds his sword up in front of him, and Rahanimu is promptly sliced in half.
 * A contractor in the second season of Darker Than Black had the ability to move at Flash Step-like speeds, but because of this trope, had to move in straight lines and at one point crashed face-first into a tree. While this was handy for dodging bullets, he ran up against the little snag that The results were not pretty.
 * Oddly enough, air resistance didn't seem to bother him at all.
 * A magic student in Mx 0 specializes in warp magic. However, the process takes a very long time and in the meantime, the two halves are in separate locations. Hilarity Ensues.
 * When Claire partially awakened her legs in Claymore, the resulting speed boost was so much that the only way for her to stop or change direction was to stab her sword into the ground.
 * Used offensively by Gaap in Umineko no Naku Koro Ni where she.
 * Invoked in ~Jojo's Bizarre Adventure~, where a Stand that has the ability to move between reflective surfaces is intercepted that way. Leaving him with but a single point of destination becomes to strategy to defeat him.
 * In Naruto, when Killer Bee uses his tailed beast's power he can move very fast, but his movements are very linear, which allowed Sasuke to put him in a genjutsu with some help from Karin (which he easily broke out of) and
 * This short film takes Newton's first law to its inevitable, over-the-top conclusion.
 * "A body in motion tends to stay in motion unless acted upon by an outside force", like a terrified middle school kid.

Comics

 * Two examples from Marvel Comics, using their signature speedster Quicksilver.
 * Quicksilver was once defeated by Spider-Man, against whom Quicksilver was executing a whirlwind vacuum attack. Spidey simply stuck out his arm in Quicksilver's path. And yeah, it was a strain, but ol' Webhead is tougher than Quicksilver.
 * Quicksilver defeated a Sentinel which was imitating Quicksilver's speed, by deliberately running into a wall, knowing his pursuer would have no chance to avoid impact. Quicksilver was hurt, but the Sentinel was destroyed.
 * In Ultimate X-Men, Ultimate Arcade's debut brought him into conflict with Nightcrawler. He figured out where Kurt was going to 'port to next and was ready with a knife to the gut.
 * Canonically, Nightcrawler is supposed to be subject to this -- he keeps the same inertia when he teleports. On the other hand, he once avoided going splat by inverting the bamf -- so that he came out of it falling up.
 * It is mentioned in X-Men Evolution when Nightcrawler explains why he can't teleport off an airplane in flight.
 * He does this in his debut in the movies, using his momentum to confuse the people he's fighting as well as maintain a full sprint through obstacles.
 * He does however teleport off and back to a flying plane, and doesn't seems to have too much trouble with inertia.
 * Why would he have trouble with inertia when he and Rogue are both traveling at or near the plane's speed already?
 * For the same reason parachutists fall behind a plane when they jump out. Sudden deceleration caused by being exceedingly unaerodynamic. He'd be travelling noticably faster than rogue when he caught her, and she'd be travelling slower than the plane when she returned. His speed wouldn't change much, but it wouldn't have been static.
 * Hawkeye is used to being able to stop speedsters by predicting where they're gonna wind up next, because of his experience with the Squadron Supreme's Whizzer, but when he tried it on The Flash in JLA / Avengers, it failed. Pretty much.
 * It seems to work better on bad guys; Green Arrow II, Connor Hawke, managed to down a Hardlight copy of the Flash in Morrison's run by aiming where he was going to be next as opposed where he's supposed to be. C'est la vie.
 * The difference here is that the hard light copy of the Flash was being controlled by someone who doesn't have super speed and Connor noted that the hard light Flash was moving in repetitive patterns as a result, which made him easier to predict.
 * And the Flash lampshaded it at the time: Hawkeye was using a boomerang arrow, and the Flash has some experience with fighting boomerang guys. Meanwhile, the guy controlling the hardlight copy of the Flash... not so much.
 * In DC's Identity Crisis, Deathstroke detonates a series of explosives and immediately sticks out his katana. Just as planned, The Flash runs through the only safe path and impales himself. Given how the Flash's power actually functions and the sheer level of speed and reflexes it gives him, there is absolutely no way in hell this should have worked, since from his perspective he should have been able to see Deathstroke soooo veerryy sloowwllyy sticking out the sword and just dodged the thing.
 * In a spinoff comic based on the movie Jumper, a Paladin kills a teleporting Jumper by placing his sword where he is about to appear.
 * Inverted in PS 238, when teleporting villain uses his own inertia to his advantage to do quite a bit of nasty things. The heroes also employ this trope (and Lampshade it).
 * One of the smarter things he does is set up a mattress in an abandoned building. He memorizes its location, and teleports to it whenever he needs to break falls.

Films -- Animation

 * This is invoked in The Incredibles as Mr. Incredible is fighting Syndrome's robot. He decides to jump over the top of the robot to avoid one blow. The robot does the math on the arc of his jump, intercepts him, and then continues to lay down the smackdown.

Films -- Live Action

 * In X-Men Origins: Wolverine Victor takes advantage of this to kill
 * In the third X-Men film, Kitty uses this against Juggernaut. When she realizes that the kid she's been sent to rescue has the ability to suppress powers..
 * Used as a weapon in Jumper: teleport onto a busy street, grab a speeding car or bus, and teleport back....
 * Riddick uses a variant of this trope- rather than hitting the Big Bad with a stopping force while he teleports towards Riddick,.
 * In the dojo fight in The Matrix, when Neo does a wallrun and backflips over Morpheus, Morpheus traces his path and kicks him when he lands.

Literature

 * The Drizzt Do'Urden novel Starless Night allows the title character to defeat Dantrag Baenre, who had been using Bracers of Blinding Speed to fight much more quickly: because of a lack of control, Dantrag was incapable of fighting using anything other than stock combos. Drizzt was trained the same way and knows all the corresponding counterattacks.
 * When Drizzt kills Dantrag, he loots the bracers off the corpse, but finds that he can't get around the disadvantage. So he puts them on his ankles, killing two birds with one stone.
 * Heinlein's Number of the Beast featured a teleportation device like this. Each jump went precisely X distance in a single direction, so, of course, an accurate map and compass are necessary to get anywhere that's not exactly X away.
 * Averted by the fact that since the characters were all math wizzes it didn't take them long to figure out how to take multiple jumps (that are instantaneous) to get anywhere.
 * In the Discworld series, teleportation to the other end of the Disc requires extensive calculation, lest the subject keep their absolute velocity and end up a thin smear on a wall 500 feet away.
 * Inertia is a major issue to consider when using inertialess star-drives in EE Doc Smith's Lensman series. Since ships retain the inertia they had when the drive was switched on, when it's switched off, they could arrive at their destination, switch off their drive and promptly fire off in a completely unexpected direction. If people are transferred from ship to ship whilst 'free', matching their velocities and inertias once they again go inert takes serious equipment. Or, as with the inertialess planet "nutcracker" weapon, free objects rendered inert can fire off in completely expected directions...
 * In the Ciaphas Cain novels, Vail has a device that teleports her away when she's attacked. The problem is that she'll still be going in the same direction she was when she teleported. In the first book, she's making a diving attack on the villain when his counterattack teleports her into the hallway... whereupon she slams into a table.
 * Larry Niven wrote a series of short stories around a teleportation system that preserved inertia, as well as potential energy. Attempting to teleport from one spot on the equator to a spot on the other side of the world would result in the teleportee coming out of the receiver at about 2000 miles an hour, relative to it. Teleporting to a higher altitude could freeze you, or to a lower altitude boil you.
 * He did another set of stories where light-speed teleportation required a receive station and preserved momentum but not potential energy. Solution to interstellar spaceflight? Build a Railgun launch system, and put the receiver at one end and the transmitter at the other. The payload slowly got pushed up to nearly lightspeed, and then they pushed the transmitter out of the way.
 * He then showed his work in an essay about the subject, titled "The Theory and Practice of Teleportation". It was intended in part as humor, in part as mental exercise for scifi buffs, and in part so nobody could call bullshit on him when he gave it as a speech to MIT. =)
 * The hyperdrive in his Known Space series has the same issue. Several times, ships end up having to dump relativistic velocities after returning to their home system, though Protector introduces in a gadget which at least makes that possible for reaction-drive ships.
 * In Foundation's Edge, it's mentioned that it's possible to figure out where a ship jumped to based on its initial velocity before the jump. Following someone through 32 consecutive hyperspace jumps in a matter of seconds, on the other hand...
 * In Piers Anthony's Isle of View, the son of a family of flying centaurs can temporarily remove some of a person or object's weight but cannot affect the inertia, and at one point he cautions a character not to bounce off the walls, since she'll hit just as hard as if she still had all her weight.

Live Action TV

 * Used in an episode of Heroes. Speedster Daphne attempts to run through a doorway blocked by Knox, but he simply clotheslines her.
 * Stargates on Stargate SG-1 work like this as well (most of the time). Objects such as bullets and the UAV can be fired through an outbound gate (albeit blindly) and emerge with the same speed.
 * Subverted in the episode "Enemies".
 * A variant occurs in Stargate Atlantis: Sheppard is forced by two Travelers to command an abandoned Ancient warship. When told to start moving the ship, he does so after helpfully informing them that they shouldn't have ordered that without activating the inertial compensators first. Cue the Travelers' Oh Crap reaction before getting knocked out cold by the wall behind him.
 * In Hercules: The Legendary Journeys, Hercules uses this to defeat the Lightning Bruiser Morrigan during their rematch. Similar to the Deathstroke/Flash example above, this shouldn't have worked. Earlier, her Super Speed was similar to Time Stands Still, where everything is in slow-motion and she travels along at a regular pace.
 * In NCIS there was an episode when Gibbs had the perp in a Med-Evac plane with the perp holding a gun to the pilot's head screaming to get the plane to go. Gibbs orders the pilot to get clearance and slyly wraps his arm around a strap on the side of the hull. The pilot notices, gets clearance and goes from 0 horizontal Gs to several, knocking the perp off his feet and throwing him towards Gibbs, who quickly disarms him.

Tabletop Games

 * Traveller Classic used a Larry Niven style "conservation of momentum while teleporting" system when psionic characters used the Teleport power. It was also mentioned in Adventure 13 Secret of the Ancients: the Ancients had teleport networks accelerate masses to near light speed and used them as weapons to destroy planets.

Video Games

 * The portals in Portal work this way, as explained with the page quote.
 * Another Armored Core example: In many of the games, Overed Boosting has the disadvantage of not being able to stop or turn on a dime. PA Jousters and Sword-based Mechs often rely on their Overed Boost to close the gap, but can easily miss and render themselves vulnerable while they try to correct their course to come in for a second strike. The most common way to defeat a PA Jouster or Swordy-mech is to either quick dash to the side multiple times in rapid succession, or just shoot them with superior firepower. The former is tricky, while the latter requires heavy artillery like chain guns or grenade cannons.
 * Early video game characters (Arthur from Ghouls 'N Ghosts and early Belmonts from Castlevania being notable examples) offered suffered from this... Arthur still does.
 * Halo: "As for tracking us all the way from Reach, well, at light speed my maneuvering options were limited."
 * This is the problem that the Pokémon Linoone has -- it can run really fast, but only in a straight line (and can be Too Fast to Stop).
 * In Unreal Tournament: the translocator (portable teleporter) retains inertia. Many a death results from someone trying to save themselves from a fall off the edge of the map by flinging the translocator beacon, only to still be going terminal velocity when they teleport back up onto the ground.
 * This often leads to the morbid scenario where someone is firing translocator beacons upward while falling downward, only to realize after several successive teleports that they're falling faster than the launcher can shoot the beacon above them- they can't even make it back to solid ground, not that they'd live through the experience even if they could.
 * In Blaz Blue, most characters with dash moves are still vulnerable during the dash. Therefore you can strike them out of it with proper timing. Need hilarious examples?
 * Pretty common in fighting games in general. Particularly hilarious when an insanely powerful super move gets stopped dead with a light jab.
 * The jump mechanics in Prinny: Can I Really Be the Hero? do a version of this. Generally, you either jump straight up or with a set horizontal speed and can't adjust your direction in mid-jump, though there are ways around this (after doing a Double Jump or using a Ground Pound to bounce off an enemy, you can change which direction you're moving.)
 * A boss in the coliseum themed To C raid dungeon in World of Warcraft demonstrates this, it's special attack is slam the ground, throwing everyone against the nearest wall, then picking a random target and charging at them full speed, if the person doesn't move out of the way in time, he's instantly killed and the boss enrages, pretty much killing everyone unless the tank's extremely well geared, however if everyone gets out of the way the boss slams into the wall and is stunned for a time.
 * You ran 2917m before hitting a wall and tumbling to your death.
 * In Star Fox 64, this turns out to be the undoing of the boss of Macbeth if you shoot all of the junction switches. The boss, who has been taunting you the whole level by speeding up his train, ends up crashing his train into a fuel bunker and meets an explosive doom.
 * Dissidia Final Fantasy has its version of Kuja glide instead of walk. This means that, unlike most characters, he doesn't stop on a dime (he floats just high enough that he can't brace with his feet), but rather slides for a second when the player releases the analog stick. While it's impossible to die by falling in this game, it makes aiming something of a tricky process - a real pain given that all of Kuja's attacks are projectiles, and only his HP attacks stop his movement.
 * While the 2D Sonic the Hedgehog games for Sega Genesis don't feature much teleportation, an interesting effect can be found in the debug mode, which allows you to turn into and place objects. If you're moving and turn into an object, you'll still have the same momentum when you turn back.

Web Comics

 * Slightly subverted in Errant Story when a speedster-monk is tricked into using a Slow Fall spell with painful results.
 * In Everyday Heroes, Mr. Mighty use Dot Dash's momentum against her in a move called "The Backdoor Bounce". For experienced heroes only. Do not attempt at home.
 * Antihero for Hire demonstrates in the picture for this page.

Western Animation

 * In Wakfu, Yugo's dimension doors work this way, keeping the momentum of anything going through. He can, however, orient the receiving portal in any direction he wish, and is very adept at using this to his advantage. For example, he often uses them to cushion a fall by having himself or a friend go through a pair of portals (or several pairs if the fall is very high) sending the subject up, thus slowing him down enough to land safely.

Real Life

 * Several Weak but Skilled martial arts rely on using the opponent's momentum against him, with Taijiquan being one of the most famous examples.