You've probably seen the headline: "Man pulls 50,000-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?
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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 a steel wheel doesn't deform as much as it rolls.
A big bus has tires with a coefficient of rolling resistance of 0.006 - 0.01. Making sure that the pressure in the tires is correct, or even a little bit high, can minimize the resistance. So we'll say that for this stunt that coefficient is 0.006.
This means that the force required to pull the bus is 0.006 multiplied by the weight of the bus, or 0.006 times 50,000 pounds. That is 300 pounds. There might be some extra force from brake drag or friction in the driveline, so we'll say it takes 400 pounds of force to move the bus.
This is a lot of force, but it is 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 1.0. If your shoes did have a coefficient of 1.0, 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 1.0, so unless you weighed much more than 400 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, then he could push against the railroad ties. These methods make traction less important because now you are pushing against a vertical surface instead of a horizontal one, so the force you apply against the ladder is in the same direction as the force you apply against the bus. This makes pulling the bus more like lifting a 400-pound weight with your legs.
Your teeth have to be able to hold the rope with 400 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 400 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 400 pounds of force is transmitted down your spine. For instance, if you were lying flat on your back while you pulled the bus, all of the 400 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 400 pounds with your legs while holding it in your teeth sounds a little less impossible than pulling a 50,000-pound bus.
Here are some interesting links:
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