Some of the best puzzlers, platformers, and casual time wasters on the iPhone have one thing in common: physics.
From Angry Birds to Cut the Rope, they all adhere to a few core principals of physics - like gravity, inertia, and momentum - to give games a feeling of real-world predictability, but a true sense of incalculable chaos.
There's no hard-coded algorithm to decide which way a tower of concrete will tumble when you smash an angry pelican into it, or how a pickup truck full of cuddly toys will react to a harsh drop in Snuggle Truck.
It's just down to the way of the world.
So, if tumbling blocks, flying unicorns, sure-footed sheep, and roly-poly balls take your fancy, read on to find out our top ten physics-based games on iPhone.
Cut the Rope
While Zeptolab's casual hit Cut the Rope might dabble with anti-gravity and space-time-defying wormholes in its later levels, the game's early concepts are very much taken from the real world.
As OmNom's candy swings back and fourth on a rope, you'll have to contend with gravity, which is always trying to pull the sweet down; momentum, as you swing it back and forward; inertia, if you run out of slack on the rope; and a big green monster, who'll gobble your sweety while you're desperately trying to get that last star.
There's something gleefully amusing about seeing a mannequin tumble down some stairs. Maybe it's the flailing limbs, perhaps it's the squishy impact sounds, or possibly it's the fact that you can slap a photo of your friends' face on the dummy's mug: who knows? Who cares.
In Stair Dismount, you've got to poke a perilous step-stander, so he'll fall down a flight of stairs.
The more bones he breaks, the more gruesome impacts his head endures, and the funnier his roly-poly tumble is, the more points you'll unlock.
Don't try this at home, kids! Unless someone's videotaping it, of course, for You've Been Framed. Just kidding.
In this multi-million selling blockbuster, you're at the absolute mercy of the Box2D physics engine. You can line up the perfect shot, choose the best bird and pray to whichever deity you like, but once the fowl is in the air, it's all down to a rather crude emulation of Newton's principals.
It is, by all accounts, what makes Angry Birds so darn frustrating. When a wooden palette tumbles the wrong way or a precarious slab of concrete refuses to topple, you can't help screaming out against Sir Issac and his irrefutable laws.
But it's also what makes the game so enjoyable, as every shot ends in chaotic and unpredictable shower of destruction.
It's visceral, it's messy, and it's cathartic. I love the smell of squished pig in the morning.
Saving Private Sheep
In Saving Private Sheep, you've got to get a brave woolly soldier onto safe ground. He's currently in enemy territory, standing on a precarious tower of boxes and blocks.
You must carefully shoot out his footing, so he'll topple onto the ground, and not into the toothy maw of a salivating wolf.
This Jenga-style puzzler starts getting even trickier when you have to account for slippery ice, timing-dependent blocks, and explosive barrels of TNT.
An acute appreciation of Newton's laws will get you far here: you'll earn shinier medals if you can beat the stage quickly and with more blocks left over.
Like a first-person Angry Birds, Siege Hero requires you to fend off the opposition by wrecking their towers, castles, vehicles, and structures.
To do so, simply lob a few cannon balls at the enemy-filled building and hope that your carefully placed explosive does the deed.
Also like in Rovio's bird-flinging puzzler, you can crush the castles in Siege Hero by focusing on weak points, nailing foundations, and then letting good-old gravity take over to do the rest. The fewer bombs you lob, the better the loot you'll take home.
In Snuggle Truck, you're tasked with ferrying nine cuddly toys to the zoo over giant leaps, cavernous drops, tiny caves, and harsh terrain.
Unfortunately, each toy - from dog to unicorn - is an individual physics object that will react to the ebb and flow of the hazardous terrain in a realistic and chaotic fashion.
Bump your van's nose into a rock, for instance, and the animals will fly out and spill onto the ground in front of you. Slam your truck down onto a ramp at a wonky angle, and toys will bounce out all over the place. Accidentally turn your van upside down on a gnarly jump, and its pretty much game over.
Snuggle Truck demands a smart mix of speed and safety to ace, and, of course, a little luck, too. You'll nab a special medal for getting all of the passengers over the line safely.
Ragdoll Blaster 2
The best part of manic physics puzzler Ragdoll Blaster is the way your cotton corpses litter up the stages. The last five fired ragdolls will stick around, causing pileups, handy objects and, nuisance obstacles.
Ragdoll Blaster 2 is one of the most creative puzzlers on this list, and developer Blackflip is constantly throwing out new ideas in the game's bumper crop of 150 stages.
There are portals, massive swinging hammers, rolling balls, levels on minecarts, and levels on blocks of slippery ice.
Casey's Contraptions HD
Rube Goldberg-inspired contraptions and puzzles are ten a penny on the App Store, but they do a grand job of showing off the cause and effect of different physics-based objects.
In Casey's toy-box world of robots, remote control cars, and balloons, you'll find out exactly how a basket of books will react when slugged by a boxing glove on a spring. Or how a wall of boxes will like being walloped by a bowling ball in a bucket, swinging on a rope.
Without a smart knowledge of gravity, you'll flounder on the game's harder challenges.
If you want to see just how much physics algorithms have improved the ball-rolling genre, compare the iPhone's lush puzzler Gears to the NES's Marble Madness. While the latter is clunky and unrealistic, the former is free-flowing, with the cog-collecting sphere in Gears actually behaving like a real-life ball.
Which is handy, because the perilous drops, tough jumps, and narrow pathways in Gears require a dab hand and some dependable physics.
If you enjoy this, check out Aerox, Super Monkey Ball, and Marble Blast Ultra.
In this fruity puzzle game, a dab hand at physics principals is the difference between a juicy orange or a shriveled pile of acidic pulp.
You see, you've got to use the objects in your toolbox and the innate properties of gravity to manoeuvre an orange to safety.
Otherwise, he'll be left to the elements, and will get caught in a flash flood of acid rain. No one likes acidic oranges. They're gross. So shove your fruity pal into cover by dropping a barrel onto a seesaw, or rolling a giant gear with a tumbling crate.