Why do you stumble forward when a bus starts moving?

[shortydeb]

So I was taking the bus the other day and i started thinking about this.
If you're standing in a bus, and the bus starts to move, you'll stumble forward if you're not creating enough force opposite the direction of the bus' movement. So why does this happen? This is my theory: The static friction from the floor of the bus causes you to move forward along with the bus. But there's also the force from the air behind you, that is caused by the back of the bus pushing the air inside the bus forward. So the air inside the bus, being pushed by the back of the bus, pushes on you. So there are two forces acting on the passenger, the static friction from the floor of the bus, and the force from the air behind the passenger, whereas on the bus itself, there's only one force acting on it in the beginning, which is the static friction from the road. So since in the beginning, there are two forces acting on the passenger and one force acting on the bus, you accelerate at a higher rate than the bus, causing you to stumble forward.
Am i completely wrong about this?

 

[Hootenanny]

I would say it was more that the force of static friction creates a torque about your centre of mass causing you to fall backwards. I would guess that you stumble forward as a reflex action in order to prevent you falling over.

 

[BryanP]

Well let's see what's happening.

When you're in a platform (in this case, a bus) that is accelerating and you have friction to keep you on the floor, then you will feel the force of the acceleration.

Depending on how you're standing, you can support yourself from tumbling over. Basically, you want to keep yourself from having any rotational motion.

Thus if you stumble over, it's because the force of the acceleration is causing your body to "pivot," which is why you'd probably want to shift the way your legs are.

 

[BryanP]

I would say it was more that the force of static friction creates a torque about your centre of mass causing you to fall backwards. I would guess that you stumble forward as a reflex action in order to prevent you falling over.

I agree. Stumbling into the direction of the bus's movement is a reflex action. The static friction is definitely more than "air" pressure, and if it weren't for some reflex, a person would normally fall in the opposite direction of the bus's movement.

 

[Hurkyl]

Jerk (the derivative of acceleration) plays a big role too. If the acceleration were smooth, you could easily compensate: e.g. by leaning forward while the bus accelerates. But when there's a high jerk, you can't react fast enough. (Of course, you can still manage if you can anticipate the jerk)

 

[BryanP]

True. So, reflex action?

 

[pixel01]

When the bus starts to move, your feets move as well with a certain acceleration, so your body is to pushed backward. Your body tries to overcome this acceleration (by hands, legs, body etc..), but then the acceleration of the bus is reduced (it starts to move at constant speed) and your body is stumbled forward by your own effort.

English is not my first language so I hove I explain it clear.

 

[neu]

It is quite clearly a reflex action since if you replace the person on the platform with a ball, the ball would undoubtably "move" in the reverse direction to the bus's motion in the bus's reference frame.

The nature of the muscular reflex of a person is that anything less than overcompensation and leaning forward will give the sensation of falling over.

Interestingly, (you might already know this but it surprised me) if you have a balloon, say, tied to one of the seats on the bus then it will move forward by a displacement proportional to the bus's acceleration. This is due to increased air pressure in the rear of the bus during acceleration (which is why the scenario used is often of a plane taking off where you can't affect things by opening a window, and where acceleration is much larger).

 

[Borek]

Reflex action was my first idea, but I was not sure if I understand the question properly. Note, that bus starts and moves for a short time using 1st gear with more or less constant acceleration, then driver shifts to 2nd - and this is a short moment when there is no acceleration - but your muscles are already working to keep your position constant against acceleration... This is more or less connected with the jerk that Hurkyl referred to.

 

[Hootenanny]

I agree with both Hurkyl & Borek that it is the magnitude of the jerk(s), rather than simply the acceleration which is the significant factor. I think that it would be fairly easy to compensate for a constant acceleration, it is when this acceleration changes (i.e. the jerk that Hurkyl refers to) where your muscles need to react quickly to keep you standing.

 

[pixel01]

Similar thing happens when the bus stops, you stumble backward

 

[TVP45]

pixel01 has the right answer. There are standard balance tests where patients stand on a plate that suddenly accelerates forward or backward. The typical patient sways opposite the direction of the acceleration, then overshoots in returning to equilibrium (as would be seen in most simple negative feedback systems). Buses, having automatic transmissions, will usually have the greatest acceleration at the start.

 

[BryanP]

Yeah the jerk is strong.

Buses need to have an engine that can accelerate quite quickly from a stop as well as an engine that can provide enough torque to haul the passengers as well as the bus itself (while it's accelerating). This is most likely why you experience that high amount of jerk (causing you to stumble forward as a reflex action)

 

[LowlyPion]

I think pixel01 is right on this. I've ridden many buses and noted this very thing and observed that I can anticipate the lessening of acceleration as the bus gets to speed and compensate to avoid any stumbling. After all in this frame it's F=ma and when the "a" goes to zero upon reaching speed, so does the F against your body - an F which has been changing your inertia through your legs as the frame accelerates - and if you haven't anticipated its cessation by lessening your own bias in maintaining your equilibrium - acts as a forward pushing sensation on your body by virtue of your pushing or leaning into or against something no longer there.

Another interesting observation occurs with a helium filled balloon in a closed car. When you accelerate or decelerate the balloon will move in the direction of the acceleration (or away from the deceleration). The apparent explanation for this seems to lay in air pressure differences. In the case of acceleration - from the air being slightly compressed by acceleration against the back of the car - higher pressure applying a forward force to the back of the balloon. (Reversed of course for deceleration.)

Cheers and happy commuting.