WEBVTT - How do people pull large objects with their teeth?

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<v Speaker 1>Welcome to brainstop from house stuff works dot com where

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

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<v Speaker 1>do people pull large objects with their teeth? You've probably

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<v Speaker 1>seen the headline man pulls fifty thou pound bus with teeth.

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<v Speaker 1>It sounds pretty impressive, but let's see how hard it

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<v Speaker 1>really is to move the bus. Assuming the bus is

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<v Speaker 1>on level ground, the main force that has to be

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<v Speaker 1>overcome to move the bus is the rolling resistance of

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<v Speaker 1>the tires. This force depends on two things, the weight

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<v Speaker 1>of the bus and the coefficient of rolling resistance of

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<v Speaker 1>the tires. We know the weight, but where does the

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<v Speaker 1>rolling resistance come from. The tires on any vehicle deform

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<v Speaker 1>or squish as they move, and it takes force to

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<v Speaker 1>make them deform. The less they deform, the less force

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<v Speaker 1>it takes. For instance, a train wheel has less than

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<v Speaker 1>one tenth the resistance of a car tire because the

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<v Speaker 1>steel wheel doesn't deform nearly as much as it rolls.

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<v Speaker 1>A big bus has tires with a coefficient of rolling

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<v Speaker 1>resistance of point o six two point o one. Making

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<v Speaker 1>sure that the pressure and the tires is correct or

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<v Speaker 1>even a little bit high, can minimize the resistance. So

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<v Speaker 1>we'll say that for this stunt, the coefficient is point

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<v Speaker 1>o six. This means that the force required to pull

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<v Speaker 1>the bus is point o six multiplied by the way

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<v Speaker 1>to the bus, or point o six times fifty thousand pounds,

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<v Speaker 1>and that's only three hundred pounds. There might be some

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<v Speaker 1>extra force from brake drag or friction in the drive

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<v Speaker 1>line or something like that, so we'll say it takes

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<v Speaker 1>four hundred pounds of force to move this bus. This

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<v Speaker 1>is a lot of force, but it's possible for a

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<v Speaker 1>person to exert this much force with his legs and

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<v Speaker 1>hold it with his teeth. But the problem now becomes traction.

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<v Speaker 1>Just like the coefficient of rolling resistance, there is a

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<v Speaker 1>coefficient of friction between your shoes and the ground. This

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<v Speaker 1>coefficient determines how much force you can apply in the

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<v Speaker 1>horizontal direction before your feet slip. About the best coefficient

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<v Speaker 1>you could hope for is one point oh. If your

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<v Speaker 1>shoes did have a coefficient of one point oh, then

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<v Speaker 1>you could apply a force equal to your weight in

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<v Speaker 1>the horizontal direction. Most likely, though, the coefficient of friction

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<v Speaker 1>would be less than one point oh, so unless you

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<v Speaker 1>weighed much more than four hundred pounds, You would not

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<v Speaker 1>be able to exert that much force against the ground.

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<v Speaker 1>So the second part of the bus pulling trick is

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<v Speaker 1>to increase your traction. Sometimes the pullers do this by

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<v Speaker 1>anchoring a ladder to the road and using the steps

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<v Speaker 1>of the ladder to push against. If a man is

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<v Speaker 1>pulling a train, that he could push against the railroad ties.

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<v Speaker 1>These methods make traction less important because now you're pushing

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<v Speaker 1>against a vertical surface instead of a horizontal one. So

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<v Speaker 1>the force you apply against the ladder or the railroad

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<v Speaker 1>ties is in the same direct and is the force

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<v Speaker 1>you apply against the bus. This makes pulling the bus

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<v Speaker 1>more like lifting a four hundred pound weight with your legs.

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<v Speaker 1>Your teeth have to be able to hold the rope

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<v Speaker 1>with four hundred pounds of force, but they don't do

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<v Speaker 1>the lifting. You might be thinking that your neck is

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<v Speaker 1>the weak link. How can your neck muscles lift four

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<v Speaker 1>hundred pounds. The answer is you don't really lift the

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<v Speaker 1>whole weight with your neck muscles, since you have to

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<v Speaker 1>lean back a lot to push against the ladder for traction,

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<v Speaker 1>Most of that four hundred pounds of force is transmitted

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<v Speaker 1>down your spine. For instance, if you were lying flat

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<v Speaker 1>on your back while you pulled the bus, all the

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<v Speaker 1>four hundred pounds of force would be transmitted down your spine,

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<v Speaker 1>so the closer you can get to the ground, the

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<v Speaker 1>less force your neck muscles have to apply. Pulling the

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<v Speaker 1>bus is still an impressive feat, but lifting four hundred

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<v Speaker 1>pounds with your legs while holding it in your teeth

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<v Speaker 1>sounds a little less impossible than pulling a fifty thousand

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<v Speaker 1>pound bus. Of other topics, visit how stuff works dot

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<v Speaker 1>com and don't forget to check out the brain stuff

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<v Speaker 1>blog on the how stuff works dot com home page.

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