Speaker 1: Welcome to brainstop from house stuff works dot com where
Speaker 1: smart happens. Hi, I'm our brain. With today's question, how
Speaker 1: do people pull large objects with their teeth? You've probably
Speaker 1: seen the headline man pulls fifty thou pound bus with teeth.
Speaker 1: It sounds pretty impressive, but let's see how hard it
Speaker 1: really is to move the bus. Assuming the bus is
Speaker 1: on level ground, the main force that has to be
Speaker 1: overcome to move the bus is the rolling resistance of
Speaker 1: the tires. This force depends on two things, the weight
Speaker 1: of the bus and the coefficient of rolling resistance of
Speaker 1: the tires. We know the weight, but where does the
Speaker 1: rolling resistance come from. The tires on any vehicle deform
Speaker 1: or squish as they move, and it takes force to
Speaker 1: make them deform. The less they deform, the less force
Speaker 1: it takes. For instance, a train wheel has less than
Speaker 1: one tenth the resistance of a car tire because the
Speaker 1: steel wheel doesn't deform nearly as much as it rolls.
Speaker 1: A big bus has tires with a coefficient of rolling
Speaker 1: resistance of point o six two point o one. Making
Speaker 1: sure that the pressure and the tires is correct or
Speaker 1: even a little bit high, can minimize the resistance. So
Speaker 1: we'll say that for this stunt, the coefficient is point
Speaker 1: o six. This means that the force required to pull
Speaker 1: the bus is point o six multiplied by the way
Speaker 1: to the bus, or point o six times fifty thousand pounds,
Speaker 1: and that's only three hundred pounds. There might be some
Speaker 1: extra force from brake drag or friction in the drive
Speaker 1: line or something like that, so we'll say it takes
Speaker 1: four hundred pounds of force to move this bus. This
Speaker 1: is a lot of force, but it's possible for a
Speaker 1: person to exert this much force with his legs and
Speaker 1: hold it with his teeth. But the problem now becomes traction.
Speaker 1: Just like the coefficient of rolling resistance, there is a
Speaker 1: coefficient of friction between your shoes and the ground. This
Speaker 1: coefficient determines how much force you can apply in the
Speaker 1: horizontal direction before your feet slip. About the best coefficient
Speaker 1: you could hope for is one point oh. If your
Speaker 1: shoes did have a coefficient of one point oh, then
Speaker 1: you could apply a force equal to your weight in
Speaker 1: the horizontal direction. Most likely, though, the coefficient of friction
Speaker 1: would be less than one point oh, so unless you
Speaker 1: weighed much more than four hundred pounds, You would not
Speaker 1: be able to exert that much force against the ground.
Speaker 1: So the second part of the bus pulling trick is
Speaker 1: to increase your traction. Sometimes the pullers do this by
Speaker 1: anchoring a ladder to the road and using the steps
Speaker 1: of the ladder to push against. If a man is
Speaker 1: pulling a train, that he could push against the railroad ties.
Speaker 1: These methods make traction less important because now you're pushing
Speaker 1: against a vertical surface instead of a horizontal one. So
Speaker 1: the force you apply against the ladder or the railroad
Speaker 1: ties is in the same direct and is the force
Speaker 1: you apply against the bus. This makes pulling the bus
Speaker 1: more like lifting a four hundred pound weight with your legs.
Speaker 1: Your teeth have to be able to hold the rope
Speaker 1: with four hundred pounds of force, but they don't do
Speaker 1: the lifting. You might be thinking that your neck is
Speaker 1: the weak link. How can your neck muscles lift four
Speaker 1: hundred pounds. The answer is you don't really lift the
Speaker 1: whole weight with your neck muscles, since you have to
Speaker 1: lean back a lot to push against the ladder for traction,
Speaker 1: Most of that four hundred pounds of force is transmitted
Speaker 1: down your spine. For instance, if you were lying flat
Speaker 1: on your back while you pulled the bus, all the
Speaker 1: four hundred pounds of force would be transmitted down your spine,
Speaker 1: so the closer you can get to the ground, the
Speaker 1: less force your neck muscles have to apply. Pulling the
Speaker 1: bus is still an impressive feat, but lifting four hundred
Speaker 1: pounds with your legs while holding it in your teeth
Speaker 1: sounds a little less impossible than pulling a fifty thousand
Speaker 1: pound bus. Of other topics, visit how stuff works dot
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