3 ms·

>I don't understand how anyone can believe it's safer to be unrestrained and free to bounce around the cabin in a crash. Did you read the article? That is the

by blhack 16y ago

>I don't understand how anyone can believe it's safer to be unrestrained and free to bounce around the cabin in a crash. Did you read the article? That is the point; most people share your view. Buses are safer because the tightly packet seats act like little pods of foam, like little safety bubbles. If the bus hits something, it's 4 inches of foam that the kid is going to hit...basic physics, f=ma, a=v/t...the foam increases the time it takes the moving body to slow to a stop, and spreads the force over a longer period of time. Since it would be instantaneous, not aggregate (I'm making both of those terms up, no idea what you would actually call this) force that you care about, a longer time at a lower force == a safer collision. Think about it like this: you have two buckets, one of them is called "child" and the other called "seat". These two buckets contain a liquid called "momentum". If the bus hits a brick wall, all of the momentum from the child is going to get transferred into the "seat" bucket. The interstitial bucket you use to transfer this liquid is called "force". The foam seat means that you're using a little tiny thimble to transfer the momentum from the child to the seat...it takes a long time, and it isn't very big...it's "gradual". It's counter-intuitive, but that is one of the reasons that buses are safe.

wuputah 16y ago
While we're here talking about physics: another big reason why buses are safe are because of their high mass. I was in one accident while on a school bus - another car hit the bus head-on while the bus was stopped. The kinetic energy of the 1 ton car travelling at say 30 mph - 1/2 * m * v^2 - was absorbed by the bus. I won't step through the math, but because of the high mass of the bus, the resulting velocity of the bus+car was quite low (it also helped that the change in velocity was mitigated by brakes/tires/friction, as the driver was ready for the impact). In any case, as a passenger on the bus, the result of the impact was quite minimal - there were no injuries on the bus, and the bus actually drove off afterwards (with no passengers and a bit of sheet metal scraping the pavement). Granted, it's a different story if the bus hits a tree or is hit by a tractor trailer doing 70 mph. Another reason why buses are safe? The bus is operated by a trained professional, most of which have years of experience driving the bus for hours every day.
- klbarry 16y ago
  My own experienceL My school bus when I was young hit into the side of a van and sent it physically flying and rolling. We barely felt it in the back.
- jpr 16y ago
  I have often wondered about the difference in accident rates between professional and other drivers, anyone know if there are any studies done on this?
Jach 16y ago
To compliment your explanation, here's some math. Newton's law: F = m* a = m* dv/dt. If we assume the child and bus seat's masses are constant (no exploding kids, please!), this leads to F = d(m* v)/dt = dp/dt where p is momentum. When the child hits the foam seat, the force will not remain constant, that is it varies with time, leading from the above to dp = F* dt. We can find the change in momentum by integrating Force with respect to Time, leading to (in LaTeX notation) p_f - p_i = \int_{t_i}^{t_f}(F* dt) = I where I stands for Impulse. The time-averaged force F' = 1/delta(t) * I = 1/delta(t) * delta(p), and this is where I think it becomes very clear that we like to have longer collision times for less damage. If we're interested in having a smaller average force for when the child hits the seat, then either delta(t) needs to be longer (the collision takes a longer time, due to e.g. the foam seat, the bumpers on cars, collapsible sections of the vehicle, etc.), or the change in momentum of the child needs to be smaller (e.g. if the collision is inelastic and the seat brings the child's velocity to 0, that is better than the seat bouncing the child back). It's easy to add another layer of foam. Possibly the funnest lab in my high school physics class: shooting an egg at a brick wall and having it survive unharmed by encasing it in some design. (Mine was the Hindenburg of Bubblewrap.)
brian6 16y ago
I'm really astonished at these responses. In this crash, "compartmentalization" seems to work. In other words, if you drive a tank and hit things head on, maybe you don't need to wear a seat belt. http://www.youtube.com/watch?v=HTGMEgibFHU http://www.youtube.com/watch?v=HTGMEgibFHU Now here are some videos where, for some reason, harmful objects didn't line up exactly in front of the bus: http://www.youtube.com/watch?v=4aenzUIixM8 http://www.youtube.com/watch?v=4aenzUIixM8 http://www.youtube.com/watch?v=_0-vi5nOK08 http://www.youtube.com/watch?v=_0-vi5nOK08
SapphireSun 16y ago
The term you are looking for is reduced/exaggerated "impulse" which is defined as change in momentum aka delta(p) = F * delta(t).

by blhack 16y ago

wuputah 16y ago

While we're here talking about physics: another big reason why buses are safe are because of their high mass. I was in one accident while on a school bus - another car hit the bus head-on while the bus was stopped. The kinetic energy of the 1 ton car travelling at say 30 mph - 1/2 * m * v^2 - was absorbed by the bus. I won't step through the math, but because of the high mass of the bus, the resulting velocity of the bus+car was quite low (it also helped that the change in velocity was mitigated by brakes/tires/friction, as the driver was ready for the impact). In any case, as a passenger on the bus, the result of the impact was quite minimal - there were no injuries on the bus, and the bus actually drove off afterwards (with no passengers and a bit of sheet metal scraping the pavement). Granted, it's a different story if the bus hits a tree or is hit by a tractor trailer doing 70 mph. Another reason why buses are safe? The bus is operated by a trained professional, most of which have years of experience driving the bus for hours every day.

klbarry 16y ago
My own experienceL My school bus when I was young hit into the side of a van and sent it physically flying and rolling. We barely felt it in the back.
jpr 16y ago
I have often wondered about the difference in accident rates between professional and other drivers, anyone know if there are any studies done on this?

Jach 16y ago

To compliment your explanation, here's some math. Newton's law: F = m* a = m* dv/dt. If we assume the child and bus seat's masses are constant (no exploding kids, please!), this leads to F = d(m* v)/dt = dp/dt where p is momentum. When the child hits the foam seat, the force will not remain constant, that is it varies with time, leading from the above to dp = F* dt. We can find the change in momentum by integrating Force with respect to Time, leading to (in LaTeX notation) p_f - p_i = \int_{t_i}^{t_f}(F* dt) = I where I stands for Impulse. The time-averaged force F' = 1/delta(t) * I = 1/delta(t) * delta(p), and this is where I think it becomes very clear that we like to have longer collision times for less damage. If we're interested in having a smaller average force for when the child hits the seat, then either delta(t) needs to be longer (the collision takes a longer time, due to e.g. the foam seat, the bumpers on cars, collapsible sections of the vehicle, etc.), or the change in momentum of the child needs to be smaller (e.g. if the collision is inelastic and the seat brings the child's velocity to 0, that is better than the seat bouncing the child back). It's easy to add another layer of foam. Possibly the funnest lab in my high school physics class: shooting an egg at a brick wall and having it survive unharmed by encasing it in some design. (Mine was the Hindenburg of Bubblewrap.)

brian6 16y ago

I'm really astonished at these responses. In this crash, "compartmentalization" seems to work. In other words, if you drive a tank and hit things head on, maybe you don't need to wear a seat belt. http://www.youtube.com/watch?v=HTGMEgibFHU http://www.youtube.com/watch?v=HTGMEgibFHU Now here are some videos where, for some reason, harmful objects didn't line up exactly in front of the bus: http://www.youtube.com/watch?v=4aenzUIixM8 http://www.youtube.com/watch?v=4aenzUIixM8 http://www.youtube.com/watch?v=_0-vi5nOK08 http://www.youtube.com/watch?v=_0-vi5nOK08

SapphireSun 16y ago

The term you are looking for is reduced/exaggerated "impulse" which is defined as change in momentum aka delta(p) = F * delta(t).