Today I was on a City bus.

And Newton's first Law hit me hard.

I was standing. And the bus stopped suddenly. I fell down. Because of Newton's Law of Inertia, which states:

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

For example, when the bus started to move my body has the tendency to move so when the bus stopped I swayed forward and fell down.

I want to know without holding the steel bars in the bus is it possible to cancel out this extra force ${F}_{\text{net}} = 0$? I know you can use friction. But then the bus is moving and the bus is moving you. You have no way to walk inside the bus with the industrial boot that has the ultimate friction on the bus floor because you don't know the time and place when and where the bus would stop.


1 Answer 1


The industrial boots would not help.

In fact, it was the friction force that caused your fall in the first place. If it was not for the friction, you would just slide forward.

If the friction, even from regular shoes, was acting on your center of mass, it would be sufficient for stopping you from falling or much sliding, but, since the friction acts on your feet, it creates a torque relative to your COM and that causes rotation and fall.

Of course, you can reduce the probability falling by spreading your feet along the direction of the movement. This will help in several ways.

First, it will lower your COM and thus decrease the magnitude of the torque.

Second, the leg pointing forward will transmit some of the friction force toward your COM, also reducing the net torque.

Third, for your body to go down, it would, first, have to go up, which, would, at a minimum reduce the speed of the fall, or, very likely (along with other factors), prevent the fall altogether.

  • $\begingroup$ Can you balance that torque by leaning appropriately and causing an opposite torque? It would require good timing and judgement but motorcyclists do something similar when cornering. $\endgroup$
    – badjohn
    Aug 15, 2018 at 10:21
  • $\begingroup$ @badjohn I can see how that might work, at least, in theory. Spreading your feet and, perhaps, shifting your weight backwards is more practical, though. $\endgroup$
    – V.F.
    Aug 15, 2018 at 12:35
  • $\begingroup$ Yes, it is a more theoretical rather than a practical approach. It is hard to achieve since you generally don't know when the braking will occur or how intense it will be. In practice, I use your strategy. $\endgroup$
    – badjohn
    Aug 15, 2018 at 13:12

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