How do people pull large objects with their teeth?

Have you ever seen those folks that can pull a whole truck with their teeth? Tune in as Marshall Brain explains the physics behind pulling large objects with your teeth in this episode.

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Episode Transcript

Speaker 1 (00:00):
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.

(00:24):
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

(00:45):
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.

(01:06):
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

(01:27):
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

(01:49):
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

(02:11):
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.

(02:32):
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

(02:54):
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

(03:15):
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

(03:36):
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

(04:02):
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