I got to drive the world's first car (replica), patented by Benz in 1886 Check out the series on new safety features: http://ve42.co/MB This video is sponsored by Mercedes-Benz, who invited me to come to Stuttgart to see their latest cars, crash test facilities and experience the innovations they are working on. Physics is something that is directly applicable to car safety. Cars go fast, but they also sometimes collide with obstacles, which brings them to a sudden stop, subjecting the car and occupants to very high accelerations, which can cause injury or death. So the major idea to improve car safety is to reduce these accelerations and there are a number of ways to do this: Passive safety: Seat belts: keep passengers in the vehicle, preventing them from continuing with constant velocity, flying through the windshield and suffering a worse deceleration when they make contact with the road. Crumple zones: increase the distance over which deceleration occurs, thereby reducing peak magnitude of deceleration. Air bags: increase the distance over which the head decelerates, again reducing peak magnitude of deceleration of the head. Active Safety: Anti-lock braking system: rather than 'locking' the wheels as can happen if you slam on the brakes with a traditional braking system leading to the tires skidding across the road, ABS attempts to control the amount of braking so that the tires always roll with static friction in contact with the road. This increases the backward frictional force that can be applied to the tires, again increasing the distance over which deceleration occurs, and it gives the driver an opportunity to steer to avoid the collision (hence why it's referred to as an active safety system). Special thanks to Mercedes for having me visit facilities in Stuttgart. I had a lot of fun making these videos so please do check out the series on Mercedes Benz's channel: http://ve42.co/MB Filmed by Simon Schneider Edited by Hoplite Creative and Trevor Carlee
3 meses atrás
These are the molecular machines inside your body that make cell division possible. Animation by Drew Berry at the Walter and Eliza Hall Institute of Medical Research. http://wehi.tv Special thanks to Patreon supporters: Tony Fadell, Donal Botkin, Curational, Jeff Straathof, Zach Mueller, Ron Neal, Nathan Hansen, Corvi Support Veritasium on Patreon: http://ve42.co/patreon Every day in an adult human roughly 50-70 billion of your cells die. They may be damaged, stressed, or just plain old - this is normal, in fact it’s called programmed cell death. To make up for that loss, right now, inside your body, billions of cells are dividing, creating new cells. And cell division, also called mitosis, requires an army of tiny molecular machines.DNA is a good place to start - the double helix molecule that we always talk about. This is a scientifically accurate depiction of DNA. If you unwind the two strands you can see that each has a sugar phosphate backbone connected to the sequence of nucleic acid base pairs, known by the letters A,T,G, and C. Now the strands run in opposite directions, which is important when you go to copy DNA. Copying DNA is one of the first steps in cell division. Here the two strands of DNA are being unwound and separated by the tiny blue molecular machine called helicase. It literally spins as fast as a jet engine! The strand of DNA on the right has its complimentary strand assembled continuously but the other strand is more complicated because it runs in the opposite direction. So it must be looped out with its compliment strand assembled in reverse, section by section. At the end of this process you have two identical DNA molecules, each one a few centimeters long but just a couple nanometers wide. To prevent the DNA from becoming a tangled mess, it is wrapped around proteins called a histones, forming a nucleosome. These nucleosomes are bundled together into a fiber known as chromatin, which is further looped and coiled to form a chromosome, one of the largest molecular structures in your body. You can actually see chromosomes under a microscope in dividing cells - only then do they take on their characteristic shape. The process of dividing the cell takes around an hour in mammals. This footage is from a time lapse. You can see how the chromosomes line up on the equator of the cell. When everything is right they are pulled apart into the two new daughter cells, each one containing an identical copy of DNA. As simple as it looks, this process is incredibly complicated and requires even more fascinating molecular machines to accomplish it. Let’s look at a single chromosome. One chromosome consists of two sausage-shaped chromatids - containing the identical copies of DNA made earlier. Each chromatid is attached to microtubule fibers, which guide and help align them in the correct position. The microtubules are connected to the chromatid at the kinetochore, here colored red. The kinetochore consists of hundreds of proteins working together to achieve multiple objectives - it’s one of the most sophisticated molecular mechanisms inside your body. The kinetochore is central to the successful separation of the chromatids. It creates a dynamic connection between the chromosome and the microtubules. For a reason no one’s yet been able to figure out, the microtubules are constantly being built at one end and deconstructed at the other. While the chromosome is still getting ready, the kinetochore sends out a chemical stop signal to the rest of the cell, shown here by the red molecules, basically saying this chromosome is not yet ready to divide The kinetochore also mechanically senses tension. When the tension is just right and the position and attachment are correct all the proteins get ready, shown here by turning green. At this point the stop signal broadcasting system is not switched off. Instead it is literally carried away from the kinetochore down the microtubules by a dynein motor. This is really what it looks like. It has long ‘legs’ so it can avoid obstacles and step over the kinesins, molecular motors walking the other direction.
3 meses atrás
The total solar eclipse from Madras, Oregon on August 21, 2017. As the moon passed in front of the sun turning day to night and revealing the sun's corona, apparently all I could think to say was 'Oh my goodness!' Special thanks to Patreon Supporters: Nathan Hansen, Donal Botkin, Ron Neal, Zach Mueller, Jeff Straathof, Curational, Tony Fadell Everyone says not to photograph your first solar eclipse and I think they might be right. I was focused on getting the exposure right for Bailey's beads and the diamond ring, plus making sure to get the corona and solar flares. This was a bit stressful but I'm delighted with the results. This video originally included more info but since I'm uploading from Madras where the internet is sluggish, I cut out three minutes so the upload would happen before I had to leave for my flight. Special thanks also to Dr. Teagan Wall for sharing this experience with me and Raquel Nuno for inspiring me to come to Oregon. Music from http://epidemicsound.com "Spinning Earth 2" and Kevin MacLeod http://incompetech.com "Big Mojo"
6 meses atrás
In 2018 the kg will be defined by Planck's constant, not a hunk of metal. Try a free book from Audible for 30 days http://ve42.co/audible Special thanks to the staff at NIST who made this possible: Darine Haddad, Jon Pratt, Stephan Schlamminger, and Ben Stein. Additional footage and animations by Sean Kelley, Jennifer Lauren Lee, and Frank Seifert. I have been obsessed with measurement for a long time and I'm not sure quite how it happened. The world's roundest object played a role in this. I guess I'm just fascinated by how difficult it is to pin down a quantity like a kilogram. A physical object seemed like a good idea until the mass of the international prototype kilogram wasn't as constant as expected. These methods of the Kibble balance and silicon sphere have shown better precision than 20 parts per billion, making them superior to the old method. The agreement between Avogadro approaches Special thanks to Patreon Supporters: Tony Fadell, Donal Botkin, Jeff Straathof, Zach Mueller, Ron Neal, Nathan Hansen Support Veritasium on Patreon: http://ve42.co/patreon Interferometer video by TSG Physics: https://www.youtube.com/watch?v=j-u3IEgcTiQ Music from http://epidemicsound.com "ExperiMental1" by Gunnar Johnsén Studio filming by Raquel Nuno
7 meses atrás
On a stream of water you can levitate light balls of all sizes and even disks and cylinders. The mechanism is not the Bernoulli effect... Want to make this at home? https://youtu.be/BppcHF2EdAY My friend Blake from InnoVinci emailed me with a cool idea for a video and footage of levitating balls in water streams. Initially it was tough to explain the physics of what was going on. The standard Bernoulli effect relies on the object being completely immersed in the upward-flowing fluid. But in this case the water seems to form a single stream around the object and it's deflected away and down from the stream. By Newton's third law, the force on the water by the ball is equal and opposite to the force of the water back on the ball, pushing it up into the stream. There is a stable equilibrium position because if the ball moves into the stream, it "cuts off" the water going over the ball so it drifts out. If it drifts out too far, then lots of water passes over the ball, pushing it back into the stream. Special thanks to Patreon Supporters: Tony Fadell, Donal Botkin, Jeff Straathof, Zach Mueller, Ron Neal, Nathan Hansen Support Veritasium on Patreon: http://ve42.co/patreon Filmed by Raquel Nuno Slow motion by Hollywood Special Ops http://hollywoodspecialops.com Music from Epidemic Sound http://epidemicsound.com "Colored Spirals 3" "Magnified X 3" "In Orbit 2" "ExperiMental 1"
7 meses atrás
This is what the world would look like if you could see invisible air currents, temperature gradients, and differences in pressure or composition of the air. Support Veritasium on Patreon: http://ve42.co/patreon Special thanks to Patreon Supporters: Tony Fadell, Donal Botkin, Jeff Straathof, Zach Mueller, Ron Neal, Nathan Hansen I first saw a Schlieren imaging setup around ten years ago in Melbourne. I was immediately fascinated by the way I could see the warm air coming off my hand. I hadn't expected the currents to be moving that fast or to be so visible. This was a tricky setup to get right because alignment is very important and here I'm just working with what I had lying around the house mostly (plus the mirror). For the best Schlieren photography, making sure the mirror is stable is essential. I want to improve my setup so the mirror doesn't wobble back and forth too much creating the pulsing light and dark sections of this video. The relationship between index of refraction of air and temperature, pressure, humidity and wavelength is complicated. This website will calculate it for you: http://emtoolbox.nist.gov/Wavelength/Ciddor.asp Slow motion by Hollywood Special Ops: http://www.hollywoodspecialops.com Sound Effects by A Shell in the Pit: http://www.ashellinthepit.com Filmed by Raquel Nuno Special thanks to Blake Nichols for assistance
8 meses atrás
Grant Thompson - the King of Random - teaches me how to start a fire with a Sandwich bag. And we tried to melt glass in my backyard: https://goo.gl/zb0uE0 Check out his channel: http://ve42.co/KoR The intensity of sunlight on Earth is about 1300 Watts per square meter. When you focus the sun's rays using a magnifying glass (or in this case sphere of water) you can increase the intensity roughly ten thousand fold. This increases the temperature of wood to its autoignition point starting the reaction with oxygen in the atmosphere. By protecting the hot embers and adding more energy and fuel, you can get these hot coals to start a roaring fire. Special thanks to Patreon Supporters: Tony Fadell, Donal Botkin, Jeff Straathof, Zach Mueller, Ron Neal, Nathan Hansen Support Veritasium on Patreon: http://ve42.co/patreon
8 meses atrás
Scientists have JUST published this new observation. On January 4th, 2017 they detected the merger of two black holes 3 billion light-years away. This marks the furthest detection they've been able to make and increases confidence that these events will be seen with increasing frequency as the LIGO interferometers become more sensitive to low amplitude gravitational waves (as sources of noise are eliminated). Special thanks to: Prof. Rana Adhikari Prof. David Reitze Resources by: Binary Neutron Star merger: Relastro @ ITP - Goethe University, Frankfurt https://www.youtube.com/watch?v=nOTXC4FG9gU Numerical simulation of black hole merger: S. Ossokine/A. Buonanno/T. Dietrich (MPI for Gravitational Physics)/R. Haas (NCSA)/SXS project Artist's impression of merger and chart: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet) Simulation of black hole merger: SXS Collaboration Special thanks to Patreon Supporters: Tony Fadell, Donal Botkin, Jeff Straathof, Zach Mueller, Ron Neal, Nathan Hansen Support Veritasium on Patreon: http://ve42.co/patreon Sound Recording by Raquel Nuno
8 meses atrás
How do you measure big forces accurately? By calibrating your force transducer on the world's biggest weight - 1,000,000 pounds of force. This machine ensures planes don't break apart, jets provide required thrust, and rockets make it to their destination. Thanks to the people at NIST for showing me around: Rick Seifarth and Ben Stein. Animations here are by Sean Kelley and additional footage by Jennifer Lauren Lee. Special thanks to Patreon Supporters: Tony Fadell, Donal Botkin, Jeff Straathof, Zach Mueller, Ron Neal, Nathan Hansen Support Veritasium on Patreon: http://ve42.co/patreon Before visiting NIST in Washington DC I had no idea machines like this existed. Surely there's an accurate way to measure forces without creating such a huge known force?! Nope. This appears to be the best way, with a stack of 20 x 50,000 lb masses creating a maximum force of 4.45 MN or 1,000,000 pounds of force. I also wouldn't have thought about all the corrections that need applying - for example buoyancy subtracts about 125 pounds from the weight of the stack. Plus the local gravitational field strength must be taken into account. And, the gravitational field varies below grade. All of this must be taken into account in order to limit uncertainty to just five parts per million (.0005%) Music from The Epidemic Sound http://epidemicsound.com "Serene Story 2"
8 meses atrás
In 2020, NASA will send a new rover to the Martian surface with one of its objectives to search for evidence of ancient life on the planet. I made this clip as a correspondent for Bill Nye Saves the World on Netflix. Touring the Jet Propulsion Lab (JPL) in Pasadena was an awesome experience. I didn't think we were going to get into the control room but we got lucky. Some of the greatest moments in the history of space exploration have taken place there. They have a giant vacuum chamber where they can take the rover down to the atmospheric pressure on Mars (roughly .01x Earth's atmosphere) and test all of the devices to make sure there are no electrical discharges due to the reduced pressure. I also enjoyed seeing how the rocks will be cored and stored in tubes and deposited on the Martian surface awaiting pickup by the following mission. Images courtesy of NASA. Filmed by Raquel Nuno from 3:30 onwards. Music: http://epidemicsound.com "Serene Story 2"
9 meses atrás
The US signed the metre convention and bases all customary units on SI standards. As an aside, the Utah constitution from 1895 required the metric system to be taught in schools. This requirement was repealed in 1987. ¯\_(ツ)_/¯ Huge thanks to NIST, Ben Stein and Patrick Abbott. https://www.nist.gov/ https://www.nist.gov/pml/weights-and-measures/si-units-mass Special thanks to Patreon Supporters: Tony Fadell, Donal Botkin, Jeff Straathof, Zach Mueller, Ron Neal, Nathan Hansen Back in 1875 The US signed the Metre Convention, which basically committed the country to use the metric system. In return, French scientists sent two platinum-iridium cylinders that weigh 1kg to the US in 1889 (known by their designations K4 and K20 from a set of 40 identical objects that were produced and sent around the world). So even though everything you see and buy in the US is usually reported in pounds, all weights are traceable back to the K20 kilogram (by applying a conversion factor to get to pounds). When I was in DC a few weeks ago, I visited the National Institute of Standards and Technology (NIST) and got up close with K20, which is still kept there and used to calibrate all mass standards in this country. I thought it was pretty cool. Edited by Bill Connor
9 meses atrás
In a zero-g plane I experimented with flames and slinkies with surprising results. Check out e-penser's video: http://ve42.co/EPzeroG Check out Physicsgirl's video: http://ve42.co/PGzeroG Thanks to Novespace: http://ve42.co/novespace Special thanks to Patreon Supporters: Tony Fadell, Donal Botkin, Nick Luchsinger, Jeff Straathof, Zach Mueller, Ron Neal, Nathan Hansen The zero-g plane allows for a lot of experiments to be conducted without the expense of getting equipment into orbit. Apparently 80% of microgravity research can be performed in a zero-gravity plane, which is much cheaper than going to space. The flame from the lighter seemed much lazier in zero-g because without weight there is no buoyant force and therefore no convection. This makes the process of combustion more challenging because it's difficult for oxygen to reach the fuel. Music from the Epidemic Sound: http://ve42.co/epidemic "Serene Story 2" "In Orbit 2"
9 meses atrás
I have the photic sneeze reflex so I sneeze when I look at bright light. Check out 23andMe: http://ve42.co/23andme *So technically the single nucleotide swap (C instead of T) is not actually in a gene per se but in an intergenic region on chromosome 2. It's also not clear exactly how this affects physiology or causes the sun sneeze but there is correlative evidence that every copy of this single nucleotide polymorphism (SNP) is associated with a 1.3x increase in likelihood of having the photic sneeze reflex. I have wanted to make a video about sun-sneezing for a long time. It is something I've experienced my whole life. When I go from a dark room indoors into full sunlight I invariably sneeze. I thought everyone did it. So my original question was why do people sneeze when they see bright light? That led me to consider what possible evolutionary advantages there could be to sneezing in sunlight. The obvious advantage to me is that sunlight kills pathogens of which there may be many in your snot or mucus. So sneezing in sunshine is a much better idea than sneezing inside a dark, damp cave where you may be living. For more info, check out: Web-Based, Participant-Driven Studies Yield Novel Genetic Associations for Common Traits http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1000993 Filmed by Raquel Nuno
9 meses atrás
The solution to 4 rotation-related riddles, including the mystery cylinder, bike pedal pulling puzzle, track problem, and train part going backwards. Thank you to everyone who responded, liked, shared, or made a video response. Please fill out this short survey for research: http://ve42.co/Rresearch Special thanks to: Mathematician George Hart: http://georgehart.com/ For allowing me to use excerpts from his pedal pulling puzzle solution: http://ve42.co/ppp Petr Lebedev for combing through thousands of comments and providing the stats I gave in this video. Video responses I used in this video (or watched): everWonder? https://www.youtube.com/watch?v=5Ub2Cuclh1M A Random Nerdy Channel https://www.youtube.com/watch?v=I9RB9TrZGps The Physics DoJo https://www.youtube.com/watch?v=Pns0LCGLu9k Oblivious Jim https://www.youtube.com/watch?v=12WZIMEPi1A Armchair Explorers https://www.youtube.com/watch?v=S1yX_LTqtms MrEngineeringGuy https://www.youtube.com/watch?v=DRF82Rx9_YI Professor Cubers https://www.youtube.com/watch?v=qOd5orH-jfM Scoop Science https://www.youtube.com/watch?v=uzLO6GqmfhI A few notes on the puzzle: 1. A half-full container of honey does pretty well in reproducing the behaviour of the mystery cylinder. I wonder if the motion is a little smoother or more periodic with the ping-pong balls because they move as organized objects - also the delays between motion seemed to be longer with them than without ping pong balls. 2. For the average speed track problem, every time I said velocity I meant speed. Sorry to the pedants out there who are perhaps looking for some trick answer due to displacement being zero when you run around a track. 3. Although a lot of people identified it was something about a train's wheels that move backwards, fewer identified that specifically it was the part of the flange below the rail. Some simply said the bottom half of the wheel. 4. The bicycle question is perhaps the most complex of these riddles. If you tried it with a bike you likely found that it went backwards. But what happens if you sit on the bike and only push backwards on the bottom pedal. The answer might surprise you so give it a shot!
10 meses atrás
Can you solve these four rotation-related riddles? Support Veritasium on Patreon: http://ve42.co/patreon Test yourself playlist: http://ve42.co/testurself Huge thanks to Patreon supporters: Jeff Straathof, Zach Mueller, Ron Neal, Nathan Hansen I came across these four physics puzzles over the years in discussions with Neil deGrasse Tyson (riddle 4: which part(s) of a moving train are going backwards with respect to the ground?), Simon Pampena (riddle 2: run around a track twice, the first time slowly, the second time much faster so that the average for the two laps is twice the speed of the first lap). Someone tweeted me a video of the mystery cylinder rolling down the ramp in riddle 1 (sorry I'm not sure who it was). Riddle three about a bicycle going forward or backward when it's bottom peddle is pulled back was brought to me by a number of people and I appreciate all of their help! Filmed by Raquel Nuno. Thanks to everyone at the Palais de la Decouverte! I've had this footage for five years and am only finally releasing it now. I wanted to talk about the way grass grows on a spinning turntable but I couldn't locate the footage...
10 meses atrás
Bayes' theorem explained with examples and implications for life. Check out Audible: http://ve42.co/audible Support Veritasium on Patreon: http://ve42.co/patreon I didn't say it explicitly in the video, but in my view the Bayesian trap is interpreting events that happen repeatedly as events that happen inevitably. They may be inevitable OR they may simply be the outcome of a series of steps, which likely depend on our behaviour. Yet our expectation of a certain outcome often leads us to behave just as we always have which only ensures that outcome. To escape the Bayesian trap, we must be willing to experiment. Special thanks to Patreon supporters: Tony Fadell, Jeff Straathof, Donal Botkin, Zach Mueller, Ron Neal, Nathan Hansen, Saeed Alghamdi Useful references: The Signal and the Noise, Nate Silver The Theory That Would Not Die: How Bayes’ Rule Cracked the Enigma Code, Hunted Down Russian Submarines, and Emerged Triumphant from Two Centuries of Controversy, by Sharon Bertsch McGrayne Bayes' theorem or rule (there are many different versions of the same concept) has fascinated me for a long time due to its uses both in mathematics and statistics, and to solve real world problems. Bayesian inference has been used to crack the Enigma Code and to filter spam email. Bayes has also been used to locate the wreckage from plane crashes deep beneath the sea. Music from http://epidemicsound.com "Flourishing Views 3"
10 meses atrás
The definitive answer about the direction water swirls in two hemispheres Sync the videos yourself: http://toiletswirl.com For the record Destin and I repeated the experiment 3-4 times each in each hemisphere and got the same results every time. The idea that water going down a drain or flushed down a toilet swirls in opposite directions in the Northern and Southern Hemispheres has a long history. But few have ever done the experiment. Destin from Smarter Every Day and I performed identical experiments in the Northern and Southern hemispheres. What we found is the direction of water swirl in a toilet, sink, or bathtub is determined by other sources of angular momentum. However if the body of water is big enough, e.g. a kiddy pool, and left still for long enough (at least 24 hours), then the Coriolis effect is observable with water swirling counterclockwise in the Northern hemisphere and clockwise in the Southern hemisphere. Veritasium on Instagram: http://instagram.com/veritasium Patreon Support Link: http://www.patreon.com/veritasium Twitter: http://twitter.com/veritasium http://www.facebook.com/veritasium Smarter Every Day Instagram: http://instagram.com/smartereveryday Patreon Support Link: http://www.patreon.com/smartereveryday Twitter: http://twitter.com/smartereveryday www.facebook.com/SmarterEveryDay Gordon McGladdery did all of the sound design for the video. We used two songs from other artists (licensed of course). Derek split the first one up so it fades from video to video, and Gordon split the instruments up on the second one. There are violins on one video and percussion on the other for example. It's really neat. The neat earth animation at the beginning and the synchronizing timer was made by http://eisenfeuer.com/. He also made still images of the earth from the top and the bottom. Thanks to Vanessa for filming in Sydney: http://youtube.com/braincraftvideo MORE INFO: There was a study performed at MIT years ago (http://web.mit.edu/hml/ncfmf/09VOR.pdf) that explained the physics involved. We repeated some of these demonstrations, but on opposite sides of the globe…and in a way that can be easily understood. This site is a great resource on the Coriolis effect and ways people have gotten it wrong: http://www.ems.psu.edu/~fraser/Bad/Ba...
11 meses atrás
How the brain works, how we learn, and why we sometimes make stupid mistakes. Submit ideas: http://ve42.co/GotIdeas Apply to work with me: http://ve42.co/JoinUs Thanks to Patreon supporters: Nathan Hansen, Donal Botkin, Tony Fadell, Zach Mueller, Ron Neal Support Veritasium on Patreon: http://bit.ly/VePatreon This video was inspired by the book Thinking Fast and Slow by Daniel Kahneman Harpist: Lara Somogyi http://ve42.co/Lara Animator: Jesse Agar http://ve42.co/ThisPlace Filmed by Raquel Nuno Music by Kevin MacLeod, http://incompetech.com "Sneaky Adventure" "Harlequin"
11 meses atrás
NEW CHANNEL! http://youtube.com/sciencium For a long time we thought the Moon was completely dry, but it turns out there are actually three sources of lunar water. Thanks to Google Making and Science for supporting the new channel! http://youtube.com/makingscience Thanks to Patreon supporters: Nathan Hansen, Donal Botkin, Tony Fadell, Zach Mueller, Ron Neal Support Veritasium on Patreon: http://bit.ly/VePatreon References: Great history of water on the moon: https://arxiv.org/pdf/1205.5597.pdf Filmed by Raquel Nuno Music from http://epidemicsound.com "Serene Story 2"
1 ano e 2 semanas atrás
Spinning magnets near copper sheets create levitation! Try Audible free for 30 days: http://bit.ly/AudibleVe Special thanks to Hyperloop One for showing me around. Thanks to Patreon supporters: Nathan Hansen, Donal Botkin, Tony Fadell, Saeed Alghamdi, Zach Mueller, Ron Neal, Perry cl, Bryan Baker Support Veritasium on Patreon: http://bit.ly/VePatreon Filmed by Raquel Nuno, Edited by Trevor Carlee Obviously this "quadcopter" is a demonstration device, showing how moving magnets over a conducting surface can generate levitation. It has not been optimized to minimize losses or be an efficient mode of transport. I still think it's pretty cool. I'm used to seeing light things levitated by induced currents but not a 100+ lb machine. For more on Hyperloop One: https://hyperloop-one.com/
1 ano e 3 semanas atrás