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Speaker 1: Welcome to brainstop from house stuff works dot com where

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Speaker 1: smart happens. Hi, I'm our brain. With today's question, how

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Speaker 1: do people pull large objects with their teeth? You've probably

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Speaker 1: seen the headline man pulls fifty thou pound bus with teeth.

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Speaker 1: It sounds pretty impressive, but let's see how hard it

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Speaker 1: really is to move the bus. Assuming the bus is

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Speaker 1: on level ground, the main force that has to be

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Speaker 1: overcome to move the bus is the rolling resistance of

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Speaker 1: the tires. This force depends on two things, the weight

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Speaker 1: of the bus and the coefficient of rolling resistance of

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Speaker 1: the tires. We know the weight, but where does the

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Speaker 1: rolling resistance come from. The tires on any vehicle deform

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Speaker 1: or squish as they move, and it takes force to

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Speaker 1: make them deform. The less they deform, the less force

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Speaker 1: it takes. For instance, a train wheel has less than

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Speaker 1: one tenth the resistance of a car tire because the

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Speaker 1: steel wheel doesn't deform nearly as much as it rolls.

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Speaker 1: A big bus has tires with a coefficient of rolling

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Speaker 1: resistance of point o six two point o one. Making

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Speaker 1: sure that the pressure and the tires is correct or

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Speaker 1: even a little bit high, can minimize the resistance. So

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Speaker 1: we'll say that for this stunt, the coefficient is point

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Speaker 1: o six. This means that the force required to pull

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Speaker 1: the bus is point o six multiplied by the way

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Speaker 1: to the bus, or point o six times fifty thousand pounds,

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Speaker 1: and that's only three hundred pounds. There might be some

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Speaker 1: extra force from brake drag or friction in the drive

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Speaker 1: line or something like that, so we'll say it takes

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Speaker 1: four hundred pounds of force to move this bus. This

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Speaker 1: is a lot of force, but it's possible for a

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Speaker 1: person to exert this much force with his legs and

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Speaker 1: hold it with his teeth. But the problem now becomes traction.

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Speaker 1: Just like the coefficient of rolling resistance, there is a

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Speaker 1: coefficient of friction between your shoes and the ground. This

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Speaker 1: coefficient determines how much force you can apply in the

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Speaker 1: horizontal direction before your feet slip. About the best coefficient

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Speaker 1: you could hope for is one point oh. If your

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Speaker 1: shoes did have a coefficient of one point oh, then

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Speaker 1: you could apply a force equal to your weight in

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Speaker 1: the horizontal direction. Most likely, though, the coefficient of friction

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Speaker 1: would be less than one point oh, so unless you

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Speaker 1: weighed much more than four hundred pounds, You would not

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Speaker 1: be able to exert that much force against the ground.

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Speaker 1: So the second part of the bus pulling trick is

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Speaker 1: to increase your traction. Sometimes the pullers do this by

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Speaker 1: anchoring a ladder to the road and using the steps

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Speaker 1: of the ladder to push against. If a man is

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Speaker 1: pulling a train, that he could push against the railroad ties.

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Speaker 1: These methods make traction less important because now you're pushing

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Speaker 1: against a vertical surface instead of a horizontal one. So

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Speaker 1: the force you apply against the ladder or the railroad

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Speaker 1: ties is in the same direct and is the force

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Speaker 1: you apply against the bus. This makes pulling the bus

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Speaker 1: more like lifting a four hundred pound weight with your legs.

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Speaker 1: Your teeth have to be able to hold the rope

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Speaker 1: with four hundred pounds of force, but they don't do

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Speaker 1: the lifting. You might be thinking that your neck is

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Speaker 1: the weak link. How can your neck muscles lift four

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Speaker 1: hundred pounds. The answer is you don't really lift the

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Speaker 1: whole weight with your neck muscles, since you have to

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Speaker 1: lean back a lot to push against the ladder for traction,

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Speaker 1: Most of that four hundred pounds of force is transmitted

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Speaker 1: down your spine. For instance, if you were lying flat

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Speaker 1: on your back while you pulled the bus, all the

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Speaker 1: four hundred pounds of force would be transmitted down your spine,

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Speaker 1: so the closer you can get to the ground, the

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Speaker 1: less force your neck muscles have to apply. Pulling the

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Speaker 1: bus is still an impressive feat, but lifting four hundred

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Speaker 1: pounds with your legs while holding it in your teeth

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Speaker 1: sounds a little less impossible than pulling a fifty thousand

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Speaker 1: pound bus. Of other topics, visit how stuff works dot

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Speaker 1: com and don't forget to check out the brain stuff

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Speaker 1: blog on the how stuff works dot com home page.

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