Physics In Film

Movies and TV shows, when done right, are great ways to entertain and tell stories. They can be fascinating avenues for experiencing some phenomena we may never actually witness in real life. They can also be ridiculous or laughably awful when scientific liberties are taken a bit too far. Join the CR4 team here in the Physics in Film blog as we explore the good, the bad, and the ugly of the science and engineering we see on the screen.

Back to the Future Tech - How Do We Compare?

Posted October 23, 2015 7:00 AM by cheme_wordsmithy

So two days ago was October 21, 2015 - the monumental day when Marty McFly time traveled into the future in the movie Back To The Future Part II. Besides being a great sequel in an epic trilogy, the movie was memorable and unique in its creative vision on technologies of the future. In a belated celebration of "Back to the Future Day", I wanted to give a shout out to some of the gadgets shown in the film, and see how the real world measures up.

1. Hoverboards

The Movie - Perhaps the most memorable moment in the movie is the chase scene with Marty riding on a hoverboard, a skateboard esq. device that hovers in air above the ground, similar to the landspeeder in Star Wars.

Real Life - If you read the last post on this blog you'll know that hoverboards do exist today in various forms. Some get their lift from air via rotors (like helicopters), others from electromagnets (like hi-speed trains). See this link for details. Unfortunately, all of them have severe limitations to their functionality, so we definitely aren't close to having a free-floating board that feels and acts like a skateboard.

Winner = Movie

2. Flat Screen TVs and Video Chat

The Movie - At his house, Marty's son projects a bunch of TV channels up on the wall simultaneously, and later Marty takes a video call from his friend Needles on the same screen.

Real Life - We have innumerable sizes of flat screen TVs, and projectors that can act as TVs to throw images up on the wall. With the right setup (split screen processor and multiple cable boxes, or similar), one can certainly project multiple channels on one TV display. And Skype, Google Video, and other services have given us video chat capabilities for many years now.

Winner = Real Life

3. Hydrators

The Movie - At the house, Marty's Mom Loraine puts in a tiny pizza shaped food into a device known as a hydrator, and in three seconds pulls out a fully sized and cooked pizza - YUM! Presumably, the hydrator works by hydrating and heating dehydrated foods for quick meal preparation.

Real Life - The closest conventional tech we have for fast cooking/heating today is the microwave, and the only realistic way to "hydrate" food is to boil it in water. But wouldn't it be nice...

Winner = Movie

4. Holographic Images

The Movie - When Marty first begins walking around the city of the future, a giant holographic image of a shark pretends to eat him alive, projecting from the "HoloMax" theater. This is similar to the projections seen in the Star Wars films.

Real Life - Holographic images may be a staple of science fiction, but they are moving towards reality. One approach using graphene materials and complex photophysics has brought about small holographic displays about one centimeter in length. But researchers say there is no limit to the size as the technology improves. Considering the amount of money spent on entertainment, I fully expect to see some form of movie-like holographic images in my lifetime.

Winner = Movie

5. Auto-fit & Auto-dry clothing

The Movie - When Marty puts on the jacket and shoes of the Future given to him by Doc Brown, the shoes and jacket both "autofit" to his size. In addition, after the chase scene when Marty gets out of the water, the jacket dries itself.

Real Life - NIKE, whose name was on the "power lace" shoe that Marty wore, has actually made an auto-lace shoe called the MAG based on the design from the movie. And while we don't have self-drying or auto-fitting jackets, there are advances being made in fabrics that repel water (and thus stay dry), and in smart clothing that can charge your phone or monitor your heart rate.

Winner = Tie

Some honorable mentions (for the sake of brevity):

The Pac Fax (street fax machine). Winner = Real Life (cell phones > fax)

News TV drone. Winner = Tie (Hobby drones anyone?)

Fingerprint scanner for house lock and ID. Winner = Tie

So it looks like in some ways we haven't quite lived up to movie-makers expectations of what the future would look like, though I think we made a fair showing. Considering some things in the movie however (like the inside-out clothing trend), I would say I'm quite happy we with where we are. Here's to science fiction movies, and the innovation they inspire!


CNET - Back to the Future Tech Reviewed

2 comments; last comment on 10/26/2015
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Do We Finally Get Hoverboards in 2015?

Posted July 29, 2015 2:14 PM by HUSH

Since skateboarding doesn't require enough agility and balance, next week Lexus will officially unveil a hoverboard. Ever since Marty McFly used a hoverboard to skitch behind a Jeep and start his getaway from futuristic greasers, hoverboards have remained trapped in popular culture consciousness.

At the time of the film's release, director Robert Zemeckis alleged that hoverboards were real, but that parents groups had them banned because they were unsafe. Perhaps this is what fuels the demand for hoverboards that lasts even until today. Hoverboards have crossed over into other films and TV shows, and most importantly, into inventors' imaginations.

There have been several attempts to make hoverboards come to life. Some of the more notable ones include the Mythbusters attempt back in 2004. In 2009, The Gadget Show host Jason Bradbury built his second hoverboard. Powered by two leafblower motors and a jet engine, Bradbury's hoverboard was technically a hovercraft, but it was cool nonetheless. Finally, in May 2015, we got our closest iteration to the hoverboard yet when Canadian inventor Catalin Alexandru Duru flew a hoverboard more than 900 feet.

Yet all of these appear to be outdone by the Lexus offering. In the short teaser clip, the Lexus hoverboard, named SLIDE, appears to be floating over concrete. According to experts SLIDE works via magnetic levitation (maglev). While there have been many attempts at maglev hoverboards before, they were either unable to support loads, or were proof-of-concepts for other maglev technologies.

A maglev hoverboard means that they'll only be operational over ferrous surfaces; the concrete in the video likely has crushed or embedded magnets within. The board itself utilizes a superconductor to eliminate oscillating magnetic fields. This is the same technology used on maglev bullet trains. However, the SLIDE will only work over designated surfaces. In Lexus's case they built a special skatepark. The superconductor requires liquid nitrogen fuel to remain at -321° F. Once the fuel is spent the board stops hovering.

Will LEXUS manufacture hoverboards? Probably not. They're an automaker, and even the SLIDE is part of an ad campaign for a concept car (that won't float). But the SLIDE has been an internet sensation, so they're going to have to deliver something now that millions are awaiting its debut.

The technology has been scaled down enough where hoverboards are a reality; the only question is who is going to be the one to make them accessible. They'll likely end up more like Segways--novel transportation options for tourists--than they will like depicted in Back to the Future Part 2, where toy OEM Mattel is supplying versions for kids.

So, want a hoverboard, just like in the movie? Buy one of the ones offered by Kid Logic: miniature models of that hoverboard really float (there's a DeLorean too) and will be on sale in the coming months.

4 comments; last comment on 07/31/2015
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Medieval Physics - Siege Engines

Posted September 03, 2014 12:00 AM by cheme_wordsmithy

I don't think a blog series titled Medieval Physics would be complete without a discussion of siege engines. Siege engines were the machines that made possible attacks on castles and highly defensible fortifications. It is the simple physics behind these machines that make them what they are. Let's take a look at a few examples.

The Battering Ram

When an invading force had access to the castle gate, the battering ram was the siege weapon of choice for breaching the fortress. Battering rams allowed attackers to hit doors and gates with massive weights repeatedly, in order to break or force them open.

Battering rams typically incorporated a frame which allowed for the suspension of a large wooden log. Operators inside the structure would stand on either side of the suspended log and swing it back and forth into the target structure. Some more rudimentary rams with wheels were intended to be pushed by the operators (at speed) into the door or gate. The ram housing was usually covered to protect its occupants from arrows, stones, and other methods of bombardment from above.

The basic principle of the ram is Newton's first law, which states that an object in motion (the ram) will stay in motion unless a force (the door/gate) is acted upon it. The door must repeatedly take the force of the heavy log, weakening the door each time. When the door is too weak to stop the inertia of the ram, it will break open. Newton's second law, represented by F=ma, is demonstrated in that a larger (heavier) battery ram traveling at a higher speed will hit the door with more force.

The Onager

Onagers (also sometimes called Mangonels) are catapults that were used to bombard fortresses in Ancient Rome and the Middle Ages. They hurled large stones (sometimes laced with an combustible material and set aflame) across great distances. They could also be used as a defensive weapon against siege towers and the like.

Onagers worked using the principle of torsion. To fire, an operator would force down the firing arm held in tension by twisted ropes or a similar spring mechanism (see left) wound on a windlass (a winch). At full extension, the bucket or sling of the arm would be loaded with the projectile. When released from tension, the arm would swing forward, hitting a padded stop as the projectile was released. Firing distance and power were based solely on the construction of the onager, with variables including size and length of the arm and tension of the spring mechanism.

The Trebuchet

The trebuchet was the most fearsome of catapult technology used in ancient and medieval history. Trebuchets utilized the principle of counterweights to propel extremely large objects hundreds to thousands of feet at high speeds. The effect was devastating to enemy defenses and forces.

The mechanics of the trebuchet are different than the onager but equally straightforward - just imagine a see-saw (a really really big see-saw) with a menacing object sitting in a sling at one end. When a heavyweight (or force) is applied at the other end, the arm with the object will swing up at speed. As it hits a stopping point, the menacing object will be released from the sling, remaining in motion until impact with its target. The momentum transferred into the sling adds to the force of the release. Positioning the fulcrum closer to the side applying the force allows the object to be projected farther, but requires more force application and puts more strain on the lever.

If you want a very good visual example of the mechanics of these machines, check out the battle of Minas Tirith in the fantasy film The Lord of the Rings: The Return of the King.

In fact, that movie battle includes the use of not just of trebuchets, but of onagers, ladders, siege towers, and a battering ram. In the Middle Ages, some attackers would also use fiery weapons to melt the cement holding the stone construction together, or they would mine underneath the structure to destroy its foundation… the possibilities were many. Looking at the engineering and physics behind these things gives me a better understanding and appreciation of the scale and complexity of Medieval technology.


9 comments; last comment on 09/09/2014
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Medieval Physics - Jousting

Posted August 26, 2014 12:00 AM by cheme_wordsmithy

In a previous blog entry, I gave a short summary of the physics and history of the bow and arrow (thanks to all you CR4 users who shared your insight and knowledge on the subject). Today I'd like to delve a little bit into the "Sport of Kings": jousting.

The most vivid picture I have of jousting is from the movie A Knight's Tale, in which a squire (Heath Ledger) takes up the sport in order to "change his stars" and become a knight. Jousting was (and still is) a sport involving a horse, a rider (knight), and a lance (the knight's weapon). In jousting competitions, two knights would draw their lances, ride at each other, and attempt to knock the other off their horse. In earlier (more brutal) versions of jousting, hand-to-hand combat often followed, the objective being to incapacitate the opponent in order to win his horse and equipment.

The sport of jousting came about as a result of the popularity of the lance in medieval warfare. The combat lance was the primary weapon used by mounted soldiers, especially those leading a charge. It is a long spear-like weapon, ranging from 9 to 15 feet in length. It was made of wood, with a metal (typically iron or steel) tip on the end. Its shape and length made it unsuitable for throwing or repeated thrusting, but was instead an impact weapon meant to utilize the force of the horse and rider. The circular handguard on the lance (called a vamplate) was meant to keep the rider's hand from sliding upon impact.

In this spirit of combat, the joust was a show of a knight's ability to utilize his lance to transfer his and his horse's momentum into his opponent, all while deflecting the blow of his opponent to remain on his horse. The knight who could break more lances on his opponent (i.e. score more points) and/or dehorse him was the victor. To reduce the chance of death or severe injury, lances used in the jousting sport were made with coronals (crown shaped metal caps with three blunted prongs) instead of metal blades. The prongs allowed the lance to more easily catch the knight's shield or armor.

So how much force was a jouster actually able to generate through his lance? One might think that a simple force equation of mass times acceleration (F=ma) would suffice. Sure, it is true that a larger knight with heavier armor and a bigger and faster horse with be able to generate more momentum and force. Unfortunately, this generic force equation is an oversimplification that incorporates some faulty assumptions, the first being that the lance, knight, and horse are all one mass. This is not true, as the lance is held by the rider's hand/arm which is connected to the rider's torso, which is connected to the rider's lower body, which is on top of the horse. All of these are separate pieces which have the potential to move and give upon impact and reduce the force from and to the riders. Thus it is the skill of the knight that determines how effectively he can incorporate all of these parts together to generate more force.

Another misplaced assumption is that all the force in the equation translates to the tip of lance. In reality, other factors will reduce the force of the blow, such as the lance's point of attack (i.e. if the opponent is not directly in line with the rider's acceleration, the lance will be hitting at an angle). Also, any deflection of the lance off the opponent's armor will reduce the force of the blow. Because the upper and lower and lower body can move separately (the torso can rotate left and right), a knight that deflects will experience torque when hit more than a force that pushes him backward off the saddle.

Ultimately, the true limiting factor on force is the strength of the lance. Because the lance is made of wood, the lance will often break and shatter from a good hit. Thus the strength of the lance is a limit on the maximum force that can be transferred to the victim's body from the hit.

There are other factors to consider as well, such as return force. Assuming the lance is well gripped or attached to the knight, the knight must be ready to resist the return force generated when hitting his opponent, else he be dehorsed himself. Also, since both knights will be armed in the contest, there is added complexity of who hits first and what movement occurs in between hits.

Obviously this is not an exhaustive analysis, and I don't have any numbers on what forces generated by jousters would actually be (any takers?). Safe to say, if it's enough force to cause severe injury and knock a heavily armored man off his horse, it's quite a bit of force! Because of this, jousting today has moved away from direct physical combat to a skill sport of hitting targets and catching rings. In 1962, this evolved form of jousting was made the official sport of the state of Maryland, and it is still alive and well today.

Other sources

8 comments; last comment on 08/29/2014
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Medieval Physics - Bows & Arrows

Posted August 12, 2014 7:00 AM by cheme_wordsmithy
Pathfinder Tags: olympics
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Of the many interesting events that take place in the summer, one of my favorites are Renaissance fairs. Seeing the participants in character and costume is an entertaining tribute to the days of knights and castles. However, the combat and jousting presentations rarely do justice to their crafts. As such, I thought it would be neat to dive into some of the science behind medieval sport and combat. To start, let's take a closer look at a practice that predates recorded history: archery.

Archers (users of the bow and arrow), were a common element among most warring nations in Medieval times. Before the age of gunpowder, bows were the only weapons available to foot soldiers that could significantly extend their range of combat. Some bows (specifically longbows) had the ability to launch a deadly arrow over great distances and with great force. Between the 14th and 16th centuries, the English longbowman (seen left) was a force to be reckoned with due to the range and power of his weapon.

While there is a lot of diversity among bows and arrows, all essentially consist of three parts:

1. the bow (or bowstave) - a flexible arc shaped piece with a handle grip near the center, a notch to hold the arrow, and attachments for the string at either end. They can be made from wood, bone, metal, plastic, or carbon composite

2. the arrow - a straight projectile made of similar materials to the bow, with a pointed metal blade on the front, and fletchings (plastic fins or feathers) on the back.

3. the bowstring - a strong thread of hide, intestine, or artificial materials attached to both ends of the bow to hold it taut.

A bow and arrow works like a spring. When the archer pulls the string back (called drawing), the string puts compression and tension forces on the bow, bending it and storing elastic potential energy. When the string is released, the tension between the bow and the bowstring causes the string to move forward rapidly as the elastically deformed bow returns to its original position. The result (hopefully) is a large transfer of energy from the string to the attached arrow, turning it into a high speed projectile.

When released, there is a certain amount of force put on the back end of the arrow from the bow and the archer's fingers (or other release mechanism). This force causes the arrow's backend to wobble. The fletchings on the arrow catch the wind in order to correct this movement and help the arrow fly true. Larger fletchings will correct wobble faster, but will create slightly more wind resistance. Configuration of the fletching into different patterns (helical or offset vs. straight) can add spin to the arrow to further help dampen oscillation.

There are a number of other variances in arrow construction that can be chosen based on the preferences and shooting style of the archer. The same goes for the construction of bow, which needs to be much more fit to the user than a firearm in order to shoot well. Bow length, draw length and draw weight are the three biggest factors by which bows will vary.

Bow length is obviously important because length determines the size, which must fit to the size of the shooter and his needs. Power increases with bow size, however, a recurve style bow will generate more arrow energy than a straight bow of the same size. This is because of the curves near the limbs, which allow for greater energy storage and more efficient energy transfer. Because of this, the recurve bow was preferred in situations (such as on horseback or in wooded terrain) where extra length could affect an archer's mobility.

A related characteristic is draw length - the distance the archer pulls back the string, measured from the nock point on the bow. Bow size and the size of the archer determines draw length, which in turn affects the size and length of the arrows used. A basic method for estimating appropriate draw length is to divide the archer's arm span by 2.5. Having too much or too little draw can affect accuracy and shooting consistency.

The draw weight, which is the max resistance of the bow when being pulled back, is also very important for the archer to consider. A higher draw weight means more strength is needed to pull and hold the bowstring in tension, but it also means a more powerful shot. provides a nice chart that correlates draw weight (DW) to archer weight:

Small child (50-70 lbs) --> 10-15 lbs DW

Child (70-100 lbs) --> 15-25 lbs DW

Most women, boys from (100-130 lbs) --> 30-40 lbs DW

Women above average strength; youth boys (130-150 lbs) --> 40-50 lbs DW

Most men (150-180 lbs) --> 55-65 lbs DW

Muscular young men and larger men (>180 lbs) --> 60-70 lbs DW

Compare these numbers to back in Medieval times, when estimates say the draw weight of the longbowmen ranged anywhere from 80 to 180 pounds! Pretty unbelievable.

Today, archery and bowhunting has advanced to the use of the compound bow, utilizing pulleys and wheels to generate more power and accuracy with less exertion. Unlike recurve and longbows, which have a linear relationship of draw weight to draw length, compound bows have a parabolic relationship, because the levering system of these bows allows for a lower draw weight when extended past a certain draw length.

Despite such advances in technology, the art of the recurve bow has been preserved in the Olympics, keeping alive a fascinating skill and practice that has been around through the ages.


The Mechanics of Arrow Flight

Bow Sizing and Adjustment Guide

15 comments; last comment on 08/13/2014
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