What’s Wrong with this Picture?

Posted by admin on December 4, 2009; This entry is filed under Alternative Ed, Assessment.
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I love alternative assessments because generally, they are an alternative to crappy assessments. And more often than not, they measure qualities that we claim actually matter. One that sticks in my mind, although I forget who gave it, (Bill Gross at Idealab?) was to give prospective employees a set of obscure hand tools and ask them what they were for. He wasn’t looking for right answers necessarily. He was trying to learn about the applicant’s thinking. In his experience, that simple test was more useful to him than most others. It was a good predictor of who would make a good employee at his design house.

I would be interested in giving the assessment below to anyone applying for an engineering job (or applying to an engineering program, for that matter). It’s from a cartoon I was watching yesterday with my two year old niece. I wonder how many MIT grads would pass it. (Reminds me of this test. You know, the one where they’re given one piece of wire, a battery and flashlight bulb. No cutting the wire into segments allowed.) (Hat tip to Carl Forde for reminding me of it.)

Here it is: From an engineering standpoint, what’s wrong with this picture? (Go ahead. Venture an answer in the comments section.)

Maybe it’s too easy. Maybe not. Back to Nick.

What's wrong with this machine?

Willit run?

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  • kevin
    In addition to the man reader's complaining about piston geometry, wheel diameters, etc., let me also add:

    The drive wheels on this train appear to be in exactly the worst possible place to get any traction. The boiler looking thing in the front is where the weight is normally centered (the cab on the back would normally hardly weigh anything in comparison to a boiler filled partially with water). Also, the thing looks ready to tip forward, since about half of the boiler protrudes forward of the front-most wheels.

    Unless, of course, this is really a retro-styled electric train with lead batteries under the cab and a boiler full of daisies that periodically get ejected from the non-polluting not-smoke-stack on the front. In which case the drive wheels are positioned just fine.
  • Michael, need to get you the 3-D glasses. They'll clear things up.
  • The elephant has been killed by a tree piercing its brainstem. Half its body is missing, and its trunk is suspiciously shrunken, suggesting that it has been dead for some time.

    I am disturbed by the balloon tied to the rhinoceros' nose, and further disturbed by whatever critter is sitting in the car with the rhino.

    There is also an epidemic of hypertelorism (witnessed in a variety of species).

    And why in blazes is anyone exposing a two-year-old to this stuff?
  • lol
    no animals are allowed on the train
  • it won't run because the mechanic isn't on the train
  • Valerie: You state: "I hated questions like this in school because there is so much wrong that the learner is essentially ‘guessing’ which problem the teacher wants them to solve."
    I don't think this question falls into the same category you mention in your comments, of not knowing which problem to solve. The problem does state, "from an engineering standpoint".
    Most of your answers have nothing to do with engineering: statutory law (age of licensee), law and policy (certification, transportation of animals across state borders). I don't think the balloon is an engineering issue.
    It's interesting to note, from the comments, that there is a lot of consensus on what the "problem" was. Not really looking for a right answer, I was more interested in how people analyzed the situation. Keeping it divergent, I think is often more useful than making it convergent. The problem is most of us have been trained to give convergent answers ("the right answer") and then get rewarded or punished depending on our "correctness". I prefer to look at it as information. And no, you will not be punished for giving off topic and ambiguous answers. Your reply gives me an interesting insight into to your way of thinking.
  • Valerie
    That girls is obviously too young for a drivers' license, much less what ever certification is needed to operate a train. A Rhino and a small weasel? thing are her passengers. I doubt she has the proper permits to transport them across state borders.

    Oh Yeah, and some balloon defies Newton's Law and The wheels are joined by a bar at the same spot, though they are of different diameters.

    I hated questions like this in school because there is so much wrong that the learner is essentially 'guessing' which problem the teacher wants them to solve.
  • I wish I had thought to name my post, "The Little Engine That Couldn't."

    That's all.
  • Ooh, what bad English. I meant to write "There's Velcro on the elephant's head."
  • Hey, nothing wrong with the hat: There's velco on the hat and elephants head. Nothing wrong with the balloon: the wind speed is exactly the same as the train.
  • admin
    Oh, and one more form of response bias here; one that may explain the high rate of correctness: most likely those who don't know "the answer" will not venture to give one. That's probably why most tests in school aren't voluntary. Or driving tests.

    Thanks to all who played along. Good stuff.

    Bill F
  • admin
    Looks like most people are noticing the same thing Carl Forde noticed in the first comment, which as far as I can tell, is correct. It's interesting to see the variety of ways the same basic idea was conveyed by many. Of course, other people have noticed other things as well.
    I wasn't sure if this would be too easy. It looks like it may have been. But then again, most people commenting here came over from dan meyer's blog (http://blog.mrmeyer.com/) a place known as a hangout for off-the-bellcurve-(in the right direction)-smart people--so not really an accurate sample.
    Thanks Dan Meyer for playing along and bringing some of the math crowd over.
    Bill F
  • Ken: there could be a hinge where the first drive shaft (the one that's roughly 40 degrees relative to track) connects with piston rod. In which case, it *might* work. It would be too hard to tell from this simplistic picture. But you do have valid points.
    The answer I was thinking of (Carl Forde's) is even simpler than the one you mention, and by the drawing's standards, is unambiguous.
  • Possibly not what you're looking for, but won't 2 differently sized wheels rotate at different speeds? I ask because it looks like the wheels are connected at a "spoke" rather than at the axle... If that's the case (rather than just sloppy drawing or my poor interpretation) then the bar connecting the wheels won't be the same distance apart for very long, and nasty things will happen. Best case: the wheels lock up and the train stops...
  • I think Ken is on to it, I would also like to add that the the way the piston is attached to the two wheels, each a different size, the circle rotations of the wheels would not allow the rotation, based on where the rods are attached to the two wheels. Don't know if I explained that right.......
  • Aaron Hill
    The two wheels are connected by a rod (which appears to be inflexible). However, the wheels are of different sizes, so as the train moves, rotational velocity of the wheels will be different (the wheel with the larger circumference will rotate slower). This will cause great problems if the rod is indeed inflexible.
  • Todd
    I'd have to go with the diameter of the wheels being different yet connected by a fixed rod. The wheels are unable to move at all because the wheels will roll at different angular rates, causing the distance between the points the wheels are connected at to differ, yet the fixed rod prevents a difference in the distance.

    Also, if the train was moving, I think that balloon would be flapping in the wind.
  • Benjamin
    The problem is clearly that the driver of the train is Asian. Asians are horrible drivers and as such would never manage to obtain nor keep a job as one.
  • Better put: the two driving wheels must have the same angular velocity since they are connected by a driving rod that doesn't change size. Since the wheels are of different diameters, the linear velocity at their edges will be different. At least one will slip, or the whole mechanism will jam, and I presume a lot of energy will be lost!
  • MrW
    Outside of the piston, wouldn't the two wheels connected to the piston need to have the same diameter allowing them to turn together?
  • Grey Duck
    Also not an engineer... a math teacher, in fact.

    But I assume there are difficulties with the idea of attaching circles of different sizes with a rigid bar. The forward wheel will make it past the mid point of revolution and begin its return stroke before before the rear wheel will hit its midpoint, which means that one wheel will continue forward while the other begins to rotate backward, forcing them to work against each other.

    Also, if we assume the train is moving, then that balloon must be on a rigid wire, since Newton's laws tell us it should be trailing behind the train a little.
  • Steven Peters
    Don't the two wheels connected by the second rod need to be the same diameter? If not, then the rod will not stay in a constant orientation. Assuming the wheel center positions are fixed, the train wouldn't be able to move.

    I am an MIT grad student, but I did cheat and look at the locomotive animation on wikipedia, which showed wheels of the same diameter and a connecting rod that maintains a consistent orientation.

    I suppose a way of illustrating why it's wrong would be to animate wheels rolling that are connected by a bar. If the wheel centers have to move relative to each other, it should be easy to see that it's problematic.
  • Andrew
    The piston was the first thing that jumped out at me as well, but looking at it more the two wheels seem wrong, too.

    There are two wheels connected by the drive shaft, but they're different diameters. Even if the shaft connected them equidistant from their centers, the differing speed of rotation at the edges (where they touch the track) would mean bad times.

    Not being an engineer, I concede there could be a mechanism I'm not aware of to counter this. But it seems pretty counter-intuitive.

    ~Andrew
  • The wheel-engine connection structure won't work; the wheels won't turn.

    Proportions of all characters are wrong, but it is a cartoon convention, so I would not consider it a mistake as such.

    Parts of the elephant that should have been visible between train's wheels and over the engine are not there. This is more of a "Photoshop Disaster" (TM) than an engineering problem.

    The seat belt of the smallest character is the longest. Let's hope the train does not crash or anything, because that restraint system is full of fail.

    IF the train is supposed to be moving, the air balloon should be blown backward. Maybe the train isn't moving at the moment, though - well, it can't, given the wheel mechanism - and there is no wind.
  • It seems like one of the driven wheels of the engine will need to slip. The driving rod (or whatever it is) is connected at the same distance from the center of both wheels, but the wheels themselves are of different diameters. They'll share a circular velocity (or whatever), but have a different speed on the edges.
  • Tim Erickson
    The picture is fine. The child's eyes are wide with sadness because her train won't move. We can tell it's probably stopped because the balloon is vertical (if it is moving, an unlikely combination of wind and acceleration conspire to make it so).

    I'd add to the other answers that since the two big (drive?) wheels are different diameters, the distance between the two pivot points will change as they roll. If the thing connecting the points is a rod whose length does not change (as opposed to a rubber band, always a possiblility :) the wheels can't turn.

    One has to admire the safety design of the external elephant-shaped air bag....
  • Assuming the train is moving...

    The balloon is perfectly vertical.
  • Derek
    Two wheels of different diameter (and therefore different rotational speeds) are solidly connected to one another.
  • The rod connecting the large and small wheels is connected at an equal distance from the center, meaning that those two wheels must both go around at the same rate.

    But they can't... because the second one would catch up with the first one.
  • Cliff
    Additionally, the drive wheels are of different diameter, meaning that the link between them will bind due to the difference in rotational speed when the train moves.
  • Cliff
    SIMPLY, The train is supposedly moving, but the balloon is floating vertically above the pig (rhino?) holding it.

    the balloon should be trailing behind due to drag caused by the atmosphere.
  • Burt
    You didn't say the train was moving. The balloon on the string tells me the train is stationary. That's why there is no smoke. The balloon's position and lack of smoke are consistent with each other.
  • Kia ora e Bill!

    I'm not an engineer, but since you drew our attention to engineering principles, I feel I should answer in kind.

    The picture shows part of a steam driven mechanism that (from memories of a toy steam-engine I had as a child) simply can't work.

    The piston, which is presumably connected to the piston-rod would have to be at the end of its stroke and near the rear of the piston-cylinder. This is deduced from the geometry of the picture, showing the piston-rod at an angle where it could only connect with a piston in the position I've described.

    Further to this, the other end of the piston-rod is connected to a wheel which, if it turned, as the engine progresses along the track, would require the piston to be pushed (or pulled) clean out of the piston-cylinder.

    As well, the diameter of the piston-cylinder could not possibly permit the free movement of the piston-rod even if other adjustments were made to the mechanism. Again this can be deduced from the geometry of the picture where the whole movement of piston, piston-rod and wheel would take place within the plane of the picture itself.

    Here ends a non-engineer's explanation of why the little engine simply won't work.

    Catchya later
  • I'll also note that the leftmost wheel on the engine is larger than the right wheel, which means that the beam between them would have to change size in order for the train to run. Under standard construction, this is unlikely to happen.

    (It is possible that the beam can change in length, perhaps one sliding rod inside another, with limits. If this is the case, there may be some potential for the whole system to continue moving, but I would guess that the efficiency would be greatly reduced. I haven't actually thought about it, although I assume someone else has, hence the same-sized wheels on most train engines.)
  • Perhaps it is a green train. Maybe there is ome other form of power than coal and therefore no smoke. It could be an electric rail and the engine shape is a nod to a retro look.
    It could be the elephant is actually pulling it?
    More laterally, perhaps it is driven by the power of happiness and imagination.

    The hat enginering is of course floored.
  • Looks like we have a winner. Carl Forde at http://zpd.stormlantern.ca/ responded with the answer I was thinking about. (I'm sure there are others out there, like Mike's above.) I'll post Carl's shortly so as to see what others might come up with.
  • Mike
    She's driving a steam driven train, but there's no coal car or smoke.
  • Charlie: I hadn't thought of the hat. The smallest puff of wind or elephant movement would blow that thing right off. As you correctly point out, it's pretty much useless.
    I have to admit, this "test" is rather ambiguous. It's divergent and convergent at the same time. Answers like yours provide interesting insights.
    I'm curious to see what others may notice. I had something else in mind besides the hat. Thanks for playing along,
    Bill
  • I'll take a go at it. I'd like to point out that making the first comment makes it entirely impossible to benefit in anyway from previous attempts.

    The design flaw in the picture is the hat on the elephant. There really is no way that hat will stay on the elephant's head, provide any warmth, or shade.

    The hat needs to be redesigned.
  • Nope. won't run. The drive wheels are different sizes.
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