Dot Physics

First, I think we should all be using smaller lighter cars. They get better gas mileage, they are better for the environment and maybe even reduce traffic? (ok, not sure about that last one – but it is possible). In Buzz Out Loud 918, they were discussing Toyota working on new car bodies built from sea-weed based plastics (from a wired article). One point that was brought is the comparison between old plastic bodies (the pontiac fiero comes to my mind) and these new plastics that should be much stronger. Well, here is the problem. Light strong bodied cars can be dangerous. Really? Yes.

First, what makes a heavy car safer? One thing to consider is what happens to the people inside the vehicle during a crash. Essentially, people get injured due to excessive acceleration. Wikipedia has a description of human tolerances to g-forces. Not too fond of the term ‘g-force’, but oh well. If a car has a high acceleration, the people inside can have an even higher acceleration (people inside and car don’t have to stop in the same amount of time). Ok, let me set up a situation of a big car colliding with a small car.

i-10900ef1c834a3be39060677af4f689e-collision-1.jpg

The acceleration (I will call the positive x direction to the right) for the big car will be:

i-4a55e6886729f7dbc5916e6050310926-abig-1.jpg

I left some things out. First, the units for this acceleration are just “units”. I don’t know the time of the collision, and I don’t really want to go into unit conversions. Also, this is just the acceleration in the x-direction. The acceleration for the big car is to the right because that is the direction of the force exerted on it by the small car. (if you want to review collisions – check this out.) Now what about the small car?

i-46901ee6e9bef7f4b76a74d102727317-asmall-1.jpg

Even though the two cars have the same initial speed and the same final speed, the car changes speed AND direction and thus has a much greater acceleration. For the collision, the time of the collision has to be the same for both cars. Further, the people inside the car will have to accelerate – but I will assume they have similar restraining systems.

What if the two cars had giant air bags on the front of them or something? This would increase the time and decrease the acceleration even if the final velocities were the same.

Ok, now I am going to crash a car into an immovable tree.

i-f7b6db2c31980b573bb7ede193136c16-treecrash.jpg

I am trying to represent in the above picture “crumple zones”. In order to decrease the acceleration of the car (and the people inside) is to increase the distance (and thus time) over which the collision takes place.

So, what if you have a really strong body? Well, maybe they could still design crumple zones – but it doesn’t really make the car safer to have a strong body. Of course, if the car was made of paper that wouldn’t be safe either. The harder body panels might be more resistant to more superficial stuff (like grocery cart collisions).

One final note about cars and technology made from seaweed. When they overheat, you know they are ready to eat.

Comments

  1. #1 Dave
    February 26, 2009

    So, what you’re saying is that we should all bolt multi-thousand gallon water tanks [1][2][3] on the front (sides, rear…) of our vehicles to increase the mass and to provide crumple zones.

    [1] Weren’t there some vehicles manufactured in the 1970s or so that had water filled bumpers?

    [2] Ok, so you’d have to make sure they didn’t freeze in winter, and didn’t slosh.

    [3] Maybe Mercury would be even better (Denser and still incompressible), err, except in the event of a collision.

    Dave

  2. #2 Rhett
    February 26, 2009

    Dave,

    I like the big water filled bumpers idea. I might try that.

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