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