Episode 143: Mythssion Control

Air Date: May 5, 2010

Two cars crashing into each other at 50 mph will result in the same damage (for each car) as a single car hitting a wall at 50 mph.

confirmed

In their small scale tests, the Mythbusters compressed clay at 1x and 2x speeds. Their results showed that two objects hitting each other at 1x speed will cause 1x damage. In their full scale tests, the Mythbusters crashed two cars into a wall at 50 and 100 mph as references. They then had two cars going at 50 mph collide into each other. After surveying the results, it was clear that the two cars suffered damage identical to the car that crashed into the wall at 50 mph. The Mythbusters explained that was possible through Newton’s third law of motion. Although the total force was doubled by having two cars, that force also had to be divided between both cars during the crash.

It is possible to literally knock someone’s socks off. (Revisited from Knock Your Socks Off)

partly confirmed

The Build Team tested various types of socks on both hairy and hairless legs to see which sock could be pulled off the easiest. The Build Team then concluded that a non-elastic woven wool sock on a hairless leg would be the best possible combination. They then built ballistic gel legs covered in lamb skin to simulate human legs. The Build Team tested several punches: the uppercut, the right hook, and the body blow. However, none of the punches managed to remove the socks, even with superhuman strength. In order to see what it would take to knock a person’s socks off, they hit Buster with a girder mounted on a moving truck. While the truck failed to knock Buster’s socks off at 40 mph (64 km/h), they saw on reviewed footage that the socks were sliding off; a second run at 65 mph (105 km/h) finally sent the socks flying off Buster’s feet. While the myth was confirmed, the Build Team pointed out that they had to use an astronomical amount of force in order to achieve it.

57 Comments

  1. Laird says:

    I rather think that “an astronomical amount of force” is the whole *point* of the saying “Knock your socks off.” It’s hyperbole! No one *really* believed a simple punch from another human being could actually do this, did they..?!

    Next, you’ll be testing the “myth” of “Knock me over with a feather” or “Mountains out of molehills.” Oh, wait – you already just did that one!
    :-p

  2. Elmo says:

    Umm.. when I take my shoes and socks off, I point my feet downward, which offers less resistance. In the case of your test, you had to use so much force that you severed one of those rigid feet.

    Secondly, if I were boxing, or otherwise fighting, I would have my shoes on. I wonder if any shoes would drag on the socks and drag them off at a lesser speed (and a foot that could actually bent to minimize the bend around the heel).

    But I’m with Laird, otherwise.. it’s just an expression.. boasting, or the like.

    • KADC says:

      You’re right about the shoes, but that’s not the only problem with the experiments to knock someone’s socks off in this episode.

      The core of the issue is that a pair of dry socks have very little mass, and what really happens is not that someone’s socks are knocked off but rather that someone is hit with enough force to knock them out of their socks provided that the socks have enough inertia to overcome the friction of the legs and stay in place.

      There are four elements to this myth, three of which they overlooked in their experiments and a fourth they need to re-examine should they ever give this myth a third (and hopefully final) look.

      1) Shoes
      It’s not very realistic to suggest that someone would be getting into a fight wearing just socks on their feet, and more importantly, the friction inside a pair of shoes with enough inertia to stay put is going to have the effect of pulling the socks off the legs.

      Anyone who has seen the shoes lying on the ground at the scene of a car accident knows that it doesn’t take much (compared to the 65 mph they required in this episode) to knock someone out of their shoes, and as go the shoes so too (at least sometimes) go the socks.

      The high laced boots that boxers wear aren’t going to come off, however, so I’m afraid they’re going to have to take the fight outside the ring when re-examining this myth.

      2) Sock length
      In the experiments they chose the material with the least friction, but shorter socks, or socks that had rolled down, say, due to movement you’d expect to occur during or leading up to a fight, would also have less friction.

      3) Sock weight
      A heavier material is going to give the sock more mass and therefore more inertia, but anyone taking their socks off knows that a damp or wet sock, either from sweat or stepping in a puddle, weighs a lot more than almost any kind of dry sock, and if the socks are damp/wet than the shoes will be too.

      Tripling or more the weight of the shoes/socks combination compared to dry socks alone should translate into a third or less force needed to be transferred to the body to launch our hapless victim out of their footwear, possibly putting this myth back in the realm of human-strength possibility.

      4) Momentum
      The larger and stronger the boxer and the smaller and lighter the “victim”, the greater the momentum that will be put into the “victim’s” body, however, if the idea of replicating what might look like beating up a child doesn’t sit well, there are other, possibly more effective, means of transferring energy into the target body, such as a football tackle for example. I don’t think this myth is specific to being punched so much as being “hit”, so if all else fails I think using a car is a perfectly legitimate part of this myth.

      Though it may not be as exciting as smashing cars or blowing things up, I’m afraid the Mythbusters team really didn’t do a very good job of testing this myth under the realistic circumstances where it might actually occur.

      Does that warrant a third go-around? Obviously that’s entirely up to them; though, I know I’d never be able to let something like this go if I knew I’d blown it the first two times.

  3. Tiexandrea says:

    you’d sooner kill the guy than knocking his socks off. the high speed of it was pretty damn entertaining though.

  4. Ab says:

    Why compare two cars hitting each other to a car hitting a wall? The wall doesn’t move so will obviously cause more damage at the lower speed. You made the mistake of changing 2 variables and then making a direct comparison – basic testing theory.

    It should have been a 50mph car hitting a stationary car compared to two cars hitting at 50mph – and yes there would be more damage as combined speed is 100mph.

    • Non-Scientist says:

      Yes I agree 100%. I think Mythbusters made a big mistake here.

      • cm says:

        But of course what was disputed was that “Two cars crashing head on at 50 mph is the same as one car crashing into a wall at 100 mph”. And that is just what was tested. And it’s not the same.

        • Nathan Crang says:

          Mythbusters is right! If the cars are of equal weight, speed and the same model (rigidity) then the head on collision will be the same as them crashing into a concrete wall. When the cars hit each other at 50 mph, they can only equalise each other. Bringing them to a stationary stop immediately at a fixed point. Whether that fixed point is caused by a concrete wall or another car with the same energy is irrelevant!

        • graham says:

          yes two cars hitting each other 50 mph will only cause as much damage as one car hitting the wall at 50 mph. NOW the reason for this is that the wall is solid where as the car has what i call a crumple zone now that is the variable that was changed now if you take something solid like a wall and send that at 50 mph and a car at 50 mph then what should happen is that the car would end up like a car hitting the wall at 100 mph the reason this didn’t happen with the cars is because they both crushed the same amount because f the crumple zone if you measure the two cars and the do the maths you will find this 100mph at wall 15ft=8ft car at wall 50mph 15ft=12ft now 2 cars at each other at 50mph 15ft+15ft=12ft+12ft=30ft-24ft=6ft now one car =15ft-6ft of the crumple zone of 2 cars at 50mhp so 15ft-6ft=9ft which is close enough do the maths

      • Perry says:

        I also agree….my students sometimes watch myth busters and when I am teaching about relative velocities this episode always comes up. They should do an experiment with a car colliding with a parked car in neutral instead of a wall.

        • Adam says:

          But that is not the myth. The myth was “Two cars crashing into each other at 50 mph will result in the same damage (for each car) as a single car hitting a wall at 50 mph.”

          If you really want them to do what you said, then why don’t you go to the website and submit a myth?

  5. bustersbuddy says:

    when you guys were pulling the socks off torys feet, his feet were in the air. busters feet were on the ground. i think this makes a difference. i think you guys should do more tests with busters feet off of the ground.

  6. fdh says:

    I liked this episode, but I think it might be easier than it looks. I’ve never had my socks knocked off (that I recall) and it wasn’t in a boxing match but I was once hit by a car and although I don’t really remember what happened to my socks, my shoes came off and were thrown pretty far, despite being tied. That might be where it comes from. No, it didn’t do me, the car, or my bicycle any good at all. I remember seeing a shoe fall from the sky. And a windshield. I remember a windshield.

  7. Jerry Kruss says:

    Now that you crashed 2 cars headon at 50 mph.

    What happens if the 2 cars are different speeds.

    IE: 1 car is 50 mph and the other is 30 mph?

    • Michael says:

      They should suffer damage comparable to a single car hitting a wall at 40MPH.

  8. Robert Macias says:

    Ab does make a valid point. Changing two variables and then making a direct comparison as the stationary wall will not move when hit but a stationary car will at both 50mph and 100mph speeds thus giving a different result. So even though you are right about the damage caused between two different variables one being stationary and the other being in motion its unwise to compare the two because the car in motion has an equal and opposite force and the wall doesnt. I havent used physics in a while so I think I’m right.

    • Alex says:

      What? the wall DOES have an equal and opposite force. When the car hits the wall at 50 miles per hour, the wall pushes back with the same force.

      • Non-Scientist says:

        Wrong, the wall does not have crumple zones and distribute energy the same as a car. It is not a valid comparison.

        • Jordan says:

          Guys! Jamie’s original statement was that two cars hitting each other at 50 mph would be equivalent to one hitting a SOLID WALL at 100 mph, not a stationary car. Fans claimed that it would actually be equivalent to one hitting a SOLID WALL at 50 mph, not 100.

          Smashing a car moving at 50 mph into a stationary car has NOTHING to do with this.

        • Michael says:

          What do crumple zones have to do with anything?

        • Perry says:

          the wall does exert an equal and opposite force as the car does, regardless of crumple zones…

  9. Claude Abraham says:

    MB did it right by crashing into a wall at 50 & 100 mph, vs. 2 cars at 50 mph. It’s all about momentum & kinetic energy. The KE for a vehicle is 0.5*m*v^2, where m=mass, & v=velocity. Into a wall, the wall does not move so that the car absorbs all the KE. With 2 cars, they have twice the combined KE, but it divides in 2. Sorry Ab.

  10. Bill Gallagher says:

    The mock-up test using the pendulum and clay wasn’t completely accurate. Pull back a pendulum 45 degrees and record speed, then pull back to 90 degrees you will have more than double the speed. relying on gravity to generate your speed you must remember it is an acceleration. a brick hitting the ground from 100 feet will be moving more than 2X a brick dropped from 50 feet upon impact.

    • Greg says:

      That’s not right. A pendulum will swing with pretty much the same period, regardless of how far from vertical it is. This is the reason pendulums are so good for timekeeping.

      A pendulum swinging from 90 degrees travels twice as far as a pendulum swinging from 45 degrees in the same time. Therefore it is travelling twice as fast (assuming we ignore wind resistance and friction).

      This isn’t quite true due to something called the circular error, but will be true to within a few percent. Certainly for one half swing of the pendulum (as tested on MB) it’s accurate enough.

      A brick dropped works differently because it is dropped in a straight line. But a brick dropped from 100 feet won’t be travelling twice as fast as a brick dropped from 50 feet. It will be travelling faster, but not twice as fast.

      Gravity’s acceleration is about 10 meters/second/second. So every second, it will increase the speed of an object by 10 meters/second.

      If the brick dropped from 50 feet hits the ground in 1 second. Then after 1 second, the brick dropped from 100 feet will now be 50 feet in the air. It will then drop through the remaining 50 feet in less than 1 second so gravity will have less time to accelerate it. It will be travelling faster then the first brick when it hits the ground, but not twice as fast (I think it will be about 40% faster, but I could be wrong about that).

      • Bundy says:

        has anyone missed the point with the car crash that in the original experiment it was 2 trucks hitting a stationary object in the middle of them, Regardless of Jamie’s statement 2 cars Hitting a car in the center is going to have the equivalent force of a 100 mph crash into a wall

        • Michael says:

          Yes, but the force is distributed between the two objects equally, so each car will be affected the same as if it hit a stationary object while traveling at 50MPH. You’re only correct if we assume that one car is able to transfer all of its energy into the other without having any trasferred back.

    • AySz88 says:

      Greg is incorrect for large angles. Actually, the Mythbusters did the math and pulled the pendulum back the correct amount, 49 degrees (from vertical), not 45. So the Mythbusters did it right.

      • AySz88 says:

        Oops, after reading a bit more, Greg does mention that there’s an error – but it doesn’t really matter anyway, since the Mythbusters did the exact math and didn’t use the approximation.

  11. Glen V. says:

    Regarding the car collisions: p(momentum) = m(mass) times v(velocity). The force generated F = dp/dt = change in momentum over time. For example, catching a thrown egg gently (slow change in momentum) results in less force generated, keeping the egg intact. Stopping the egg suddenly (rapid change in momentum) will generate more force, breaking the egg. Two 50 mph cars colliding will result in more force generated than one 50 mph car hitting a wall (the wall contributes no momentum). However, the 100 mph car and wall collision will result in more force generated than the two 50 mph car collision because the wall collision occurs over a shorter time period (the wall does not “give”). The damage to the 100 mph car is therefore greater than the sum of the damages in the two car collision. However, this can not be accurately measured by measuring the car length as the amount of crumpling will not be a constant as different parts of the car become involved with more severe compression. Regarding the pendulum portion of the experiment, a pendulum swing is not linear with amplitude (distance from vertical),but instead varies in a complex way with the sine of the angle.

    • Michael says:

      Wrong.

  12. T says:

    I’m glad they revisted this experiment and that they used something skin like. It may seem a bit gruesome but one of the reasons people’s shoes (and socks) get knocked off is because human skin stretches when they are hit by a large force. When a car hits someone’s chest or stomach the skin is pulled tight in other areas away from the impact…ie the feet. This makes the feet smaller and hence the ease of shoes and socks coming off when tied or fitting tightly under normal wear.

    I was frustrated with the original airing of the experiment because they did not once address the skin aspect of this. As someone who has seen people get blown out of their shoes and socks in traffic accidents with cars traveling as little as 20 mph, I know that this myth should have been confirmed.

  13. Army SGT says:

    I spent a few months healing at Walter Reed Army Medical Center after being blown up in Iraq. It was there that I met a couple other Soldiers who were in fact blown out of their boots and socks by road-side bombs while conducting dismounted (foot) patrols. Body armor obviously had a part in their survival.

  14. Logak says:

    Ab, you missed the point entirely. Remember what brought this about? It was Jamie’s statement during the Compact Compact revisit that two vehicles hitting each other head-on at 50 miles per hour would be like one of those vehicles hitting the wall at 100 miles per hour. The direct comparison WAS the myth. Would they get different results by hitting a stationary car? Most likely, but again, that wasn’t what was being tested.

  15. Johannes says:

    @Ab

    That was the point. 100% of the total impact energy goes into the car hitting the wall, because the wall doesn’t absorb any energy, opposed to when 2 cars impact it splits the total impact energy 50/50

  16. MichaelT says:

    What they should have done is keep the car still and smash it between two moving walls. In that way it would be more like the original myth where the car was squashed between two big trucks which didn’t deform much. That would be much closer to what Jamie was actually comparing against.

  17. BMan says:

    Socks

    Elmo at the very top bring up something interesting. It stands to reason that the mass of shoes on the feet would help strip the socks off the feet. This would work if the resistance between the material inside the shoe and the sock was greater than the resistance between the sock and the skin of the foot and leg. As T and Army SGT state, people can in fact be knocked out of their shoes with great force.

    *Punching* someone out of their socks MAY be possible in freakish and perfect conditions, but I wouldn’t hold my breath.

    Maybe they could do this one yet again and use some shoes.

  18. PeterH says:

    When Grant says “woven socks” I think the poor lady who graciously hand *knitted* them must have cringed as much as we did. Please, Mythbuster, detail!

    PH

  19. Dane Higgins says:

    Bill I beg to differ, the brick dropped at let’s say 500 ft., in comparison to a brick dropped at 1000 ft., might reach terminal velocity based on different variables such as mass. Therefore, if I were to drop a brick at 100 ft, it would not fall at double the speed of the brick dropped at 50 ft. This is proven when you drop two objects at the same time from different heights. Both objects will hit the ground at the same time.

    • Greg says:

      I think you’re getting confused with the experiment of dropping two different weights (but both the same size/shape) at the same time. The weights will hit the ground at the same time.

      If you drop two objects from different heights, then assuming the wind resistance for both objects is the same, then the lower object will hit the ground first.

      Of course, air resistance will come into play, but that won’t be a very significant factor at the speed of a swinging pendulum. I doubt air resistance will be hugely significant for a brick dropped from 100 feet, although from 1000 feet it would be.

      It’s worth nothing that terminal velocity has nothing to do with mass, but simply on the aerodynamics of the object in question.

  20. CanadianReich says:

    With your clay tests run 1 more experiment with the clay in the middle of the two hammers. Im thinking when the hammers hit the clay it will mush similar to that of a “100mph” clay not a “50mph” clay.

  21. Non-Scientist says:

    I think your test on the cars were wrong.

    It should have been two cars hitting each other at 50mph.

    Then compare that to one car moving at 100mph hitting a stationary car.

    That would be a far more accurate comparison than the one car hitting a solid wall that does not move.

  22. Non-Scientist says:

    Thinking more about this:

    valid comparison scenarios:

    1) 1 car traveling at 50mph, crashes into a solid wall also traveling at 50mph. Compare to 1 car traveling at 100mph crashing into stationary solid wall.

    2) as per above, 2 car traveling at 50mph each crashing into each other. Compare to 1 car traveling at 100mph crashing into stationary vehicle.

    A wall does not distribute or absorb energy like a car, so it seems to me Mythbusters made a huge flaw comparing 2 cars crashing at 50mph each, to a car traveling at 100mph crashing into a solid unmoving wall…

    • cm says:

      Non-Scientist, that was not the myth to be (re)busted.

    • Michael says:

      The wall most certainly does absorb energy, it just doesn’t crumple.

      Non-scientist, to you is it a myth that a mosquito and a school bus exert equal force on each other if the mosquito hits the bus windshield?

  23. RMGreen says:

    I’m with T above on SOCKS. I wonder if not only the elasticity of the skin is in play as a variable, but also the fact that the body is primarily liquid — the impact on the body may make the feet minutely smaller at the time of impact, and combine with the inertia and friction coefficient of socks in shoes. I too have seen shoes come off in car accidents…

  24. Matt says:

    Socks:

    I agree with Elmo. I was yelling at my tv when you didn’t have the toes pointing downward thus creating less resistance.

  25. Samuel says:

    I agree with what has been said about the collision.
    My AP physics class took up this problem and came up with the following:
    Every car is built to have a certain amount of crumple space so that a lot of the force can go into break the car instead of the driver. Designers use this to protect the driver or passengers and allow the car to “fold” into itself. With this extra distance, the collision is doing the same amount of work (F*d) but over a larger dictance so the force on the passengers is smaller. The Work done by the two car collision is of the same magnitude but done over a larger distance because you have twice the crumple space because of the two cars.
    Some people say that the wall should have been moving but that still only has one car’s cumple space. Comparing the two- car collision to a car with a wall is flawed because you are only testing how the change in crumple space effects the two cars in the collision.
    If you had a wall that had the same amount of crumple space as one of the cars, only then, could you try two cars at 50 mhp compared to a car going 100 mph and a “stationary” wall. That would prove newton’s third law and the law of conservation of momentum.

  26. Sebastian says:

    Re car crash:
    in an ideal situation damage would be related to amount of kinetic energy transferred. Ek= 0.5*m*(v^2). so if car at 50mph has X kinetic energy then a car at 100mph has 4*X energy whilst two cars both going at 50 will have combined energy of 2*X. (a single car at 71mph would also have 2*X energy).
    Now in the real world not all energy is transferred as damage to the participants of the collision.

    in the original episode Jamie said 2 vehicles colliding both at 50mph was like a vehicle hitting at 100mph, in terms of “total” damage it would be more like 1 colliding at 71mph.

  27. DavidK says:

    I don’t think the ankle on the dummy flexed. The should have tested the force to pull the socks off a dummy to make sure they were the same as the human.

  28. Kristoffer says:

    All laws of physics are the same in all inertial systems. Therefore:

    1 car at 100 mph against a wall at 0 mph = 1 car at 50 mph against a wall at 50 mph.

    1 car at 100 mph against a car at 0 mph = 1 car at 50 mph against a car at 50 mph.

    How much damage 1 car against another will do compared

    • Kristoffer says:

      to 1 car against a wall will depend on the type of cars. With new safe cars it will do less damage and with old unsafe cars with bad energy absorbing materials it will do more damage.

      This episode made me dissapointed since any first year engineering student can figure out the answer.

  29. Maxie says:

    How fast can a Golf as in those they use for racing go to knock a pedastrian of his bycicle. Can it be at the speed of 26miles acording to the driver.

  30. David says:

    To people worried about the crumple space – when two cars are impacting the crumple space looks like it’s doubled, but there are two cars crumpling. In an ideal scenario (both cars constructed exactly the same, moving at the same speed) the frontmost of the car will instantaneously stop moving as soon as the two cars impact, and the rest of the car will crumple with the frontmost point stationary

    If the wall had a crumple space added in, as the car stopped the frontmost point would be moving, and the car would probably end up taking less damage

  31. patricia overaitis says:

    my husband and I argue about Newton 3rd Law of Motion. If we carry 500 pounds of water in our tanks of an RV and we must make an emergency stop or crash, what kind of damage can be done by the water. I say it is more because the water is a moving object. He claims there is no difference is the water is moving and the RV is stopped suddenly or if the RV is still moving. Would like an answer. He refuses to entertain any idea other than he own. He first told me the weight of water was only 1 pound per gallon so you have an idea of what I am up against. I am afraid to travel with full tanks and the manual advises against it. Thank you for your help.

  32. Wade Mahlo says:

    If a car is is going along the road at 50mph and the is a sheet of paper in the middle of the road it makes no difference weather there is a car on the other side going at 50mph with the same mass or the wall the paper will not move if you give the 50mhp car I unit of energy the same car doing 100 mph as 4 units of energy

  33. Daniel Tilbury says:

    About the “knock you’re sockes off” episode. Their dummy Buster was seriously flawed. The knee joint on it went past full extension, straight. Extremely overlooked variable. This would cause a change in any momentum transferred from the initial contact to the sock when the leg bends past extension, as human legs do not bend upwards towards our faces.

  34. SuperGumby says:

    They should revisit Compact Compact again, regardless of Jamie’s statement.

    The effect on the compact, from 2 semi’s travelling at 50mph, in opposite direction, is greater than the effect of a single semi at 50mph.

    In this ‘revisit’ episode the ‘pendulum test’ was performed in a manner that would test the impact to the objects in motion, not an object between them. This is correct for the interpretation of testing against Jamie’s statement, but not the original myth (Compact Compact).

    A ‘proper’ Compact Compact ‘pendulum test’ would place the clay between the wall and the moving object (for speed X and 2X) or between the two objects (at speed X).

    Yeah, the show was a repeat in AU just recently.

    and Compact Compact should be revisited to see if you can get a more ‘dead on’ collision.

    The ‘test track’ used for this revisit could possibly be set for a better test of the original myth. Put a car in the middle and send two large objects (they don’t have to be semi’s, but that would be nice) at it from either end of the track (at 50mph). Compare that impact to the impact from 1 large object heading at 100mph with a car between it and the wall.

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