[psu ody fan]

OK, earlier this morning I was sitting at a red light and noticed a Jeep Grand Cherokee approaching me from behind at a high rate of speed. At first, it didn't look like he was going to stop without becoming intimately familiar with my rear bumper, so I quickly checked the other lanes, only to find out there really was no place for me to escape. Fortunately, JGC driver realized his pending situation, mashed the brakes and stopped just short of my bumper in a cloud of squealing tires and dust.

Since I didn't pay much attention in science class, this situation got me thinking........ If I knew I was about to be rear ended (with no chance of escape) am I better off to keep my brake foot planted to the floor (thus absorbing 100% of the impact) or would I be better off to take my foot off the brake to sort of dissipate the force of the impact. I realize this drastically increases the chances of me then being projected into the car in front of me and damaging the front of my car, but from a physics standpoint, which option would lessen the chance for injury? and what about damage to the vehicles?

 

[Steve Pert]

I have a nearly identical physics question that I often wonder about.

Given the same situation and given that there's a stopped car in front of me, would it be better to remain in the same place (with space between me and the car in front), or to have my vehicle move forward to touch the vehicle in front before impact.

Think of the desk toy with the five steel balls that swing back and forth. Of two stationary hanging balls that touch, when the swinging ball impacts the first hanging ball, the energy passes to the next hanging ball. The initial impacted ball doesn't move, but the second ball flies away.

So if my vehicle is touching the vehicle in front, is any force of the impact transferred to the vehicle in front of mine? Is the damage to my vehicle less than if there is no vehicle touching the front of mine?

Or do I end up with damage to both ends of my car instead of just the rear?

 

[SuperDad]

To the first question....

Damage to the rear of your car will be slightly decreased if your foot is not on the brake. I'd advise you keep the pedal down enough to keep the brake lights on, so the police still think you were braking for real.

Assuming there is someone in front of you(and you are not touching the front car), damage to his rear and your front will obviously increase if your brakes aren't trying to stop your car.

If there is no one in front of you, stomp on the GAS, and you might be able to avoid all these problems at the same time.


From an injury perspective, the absolute worst thing you can do for the OTHER driver (behind you) is to nudge up against the car ahead of you before the impact. Its also probably the best thing to do to avoid injuries to yourself.

From a damage perspective, if you nudge the car ahead of you before impact, you are likely to dramatically increase damage to the back of your car, and may eliminate damaging the front car(and the front of your car) entirely.

Of course, you probably won't be thinking of all these things when the JGC is roaring up behind you. I'd just always lighten up on the brake if it looks like its gonna be a bad crash or there's no one in front. If not so much, I think I might try to save the front of the car by holding the brake completely locked up.

 

[dvpatel]

I don't know man. I am just too old and also too busy to think of answers to difficult physics questions.

 

[davedrivesody]

The fact of a rear-end crash is that your car has to absorb the kinetic energy of the car crashing into you. Some energy will go to crumpling your bumper and sheet metal. The rest will rapidly propel your car forward. This sometimes gives the occupants whiplash, especially if they are not looking straight ahead.

However, standing hard on the brake will convert some of that remaining energy into heat from friction between either the pads and the rotors or between the tire and the pavement. In any case, your car will accelerate less rapidly and will travel less. So your potential injury will be less. However your car will take more damage in the rear end.

 

[herrhaus]

Shift from D to R and mash accelerator pedal to floor and thus avoid all damage to front of your vehicle (that way it's has a higher salvage value).

 

[Mahansm]

Question one: You are less likely to sustain injury if you apply the brakes hard. The friction between your tires and the road will decrease the acceleration you experience but will increase the rear end damage to your vehicle. It will also increase the damage to the front end of the other vehicle and increase the risk of injury to the occupants.

Question two: To minimize accelerations experienced, you should be hitting the vehicle in front as the vehicle from behind strikes you. Assuming equal masses, optimum speed is 1/2 that of the vehicle approaching from the rear. The impact forces front and rear will be equal and you will experience no net accelerations during the collision (to a very crude first approximation). Damage will be extensive and the resultant legal/insurance settlement will be very complex, as much depends on who hit who first.

Worst I've seen was a 5-car multiple front/rear collision. 4 were stopped and the momentum from the fifth vehicle striking the rear of the string caused the other three impacts.

 

[0dyfamily]

Shift from D to R and mash accelerator pedal to floor and thus avoid all damage to front of your vehicle (that way it's has a higher salvage value).

THAT is funny. Man, I almost choked on my coffee and got some on the keyboard...and I don't even drink coffee that much.

OF

 

[spacecst4now]

Best thing to do is to keep you head back on the head restraint to minimize whiplash, stomp on the brake, and flip him the bird. The crumple zones on the van should absorb most of the energy. If you are shoved into the car in front of you, than you may be responsible for the damage to that car.