Physics Question

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janikpilot

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janikpilot
This isn't a homework question, I was actually pondering this sitting in traffic on the way home from work today:

Let's say you're in a traffic jam. Traffic is stopped ahead of you, and you're stopped with it. Behind you there is a car coming, and it looks like they aren't going to stop in time before they hit you. If you could theoretically inch so close to the person in front of you that you are just barely touching their bumper, would the kinetic energy be transferred through you and to the car in front? Similar to how a cue ball hitting a line of balls only moves the last one in the line?

Whether or not the car in front of you has their brakes set shouldn't make a difference. I'm just curious if there would be a net force acting on your vehicle, or would it be transferred through you?
 
janikpilot said:
This isn't a homework question, I was actually pondering this sitting in traffic on the way home from work today:

Let's say you're in a traffic jam. Traffic is stopped ahead of you, and you're stopped with it. Behind you there is a car coming, and it looks like they aren't going to stop in time before they hit you. If you could theoretically inch so close to the person in front of you that you are just barely touching their bumper, would the kinetic energy be transferred through you and to the car in front? Similar to how a cue ball hitting a line of balls only moves the last one in the line?

Whether or not the car in front of you has their brakes set shouldn't make a difference. I'm just curious if there would be a net force acting on your vehicle, or would it be transferred through you?
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Well, there will be some transfer, for sure, but not all of it. Results will vary depending on if the car in front of yours mass in comparison to yours and if he is on the brakes.
 
janikpilot said:
This isn't a homework question, I was actually pondering this sitting in traffic on the way home from work today:

Let's say you're in a traffic jam. Traffic is stopped ahead of you, and you're stopped with it. Behind you there is a car coming, and it looks like they aren't going to stop in time before they hit you. If you could theoretically inch so close to the person in front of you that you are just barely touching their bumper, would the kinetic energy be transferred through you and to the car in front? Similar to how a cue ball hitting a line of balls only moves the last one in the line?

Whether or not the car in front of you has their brakes set shouldn't make a difference. I'm just curious if there would be a net force acting on your vehicle, or would it be transferred through you?
Click to expand...

No because there is more friction between the car and ground than a cueball and felt surface.

Example the back of your car still crunches pretty good even thought it is firmly attached to the front of your car

Remember cue ball and those little swinging balls (not those) on your bosses desk that click back and forth are solid. That is why such an elegant transfer of energy. Nothing to crunch.
 
Henning said:
Well, there will be some transfer, for sure, but not all of it. Results will vary depending on if the car in front of yours mass in comparison to yours and if he is on the brakes.
Click to expand...

Let's assume we're equal weight (most mid-size sedans are about equal).

And I beg to differ, brakes shouldn't make a difference. Brakes don't equal a static environment. The car can skid forward if there is enough force. Either way, let's assume we're both on the brakes.

EDIT: Disregard, brakes would indeed mean the force of the car pushing against the car in motion would be considerably less than the force of static friction holding me in place. Thus there would be some crunching.
 
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janikpilot said:
This isn't a homework question, I was actually pondering this sitting in traffic on the way home from work today:

Let's say you're in a traffic jam. Traffic is stopped ahead of you, and you're stopped with it. Behind you there is a car coming, and it looks like they aren't going to stop in time before they hit you. If you could theoretically inch so close to the person in front of you that you are just barely touching their bumper, would the kinetic energy be transferred through you and to the car in front? Similar to how a cue ball hitting a line of balls only moves the last one in the line?

Whether or not the car in front of you has their brakes set shouldn't make a difference. I'm just curious if there would be a net force acting on your vehicle, or would it be transferred through you?
Click to expand...

Cars = quite compressible. Billiard ball = not so much.

Plenty of energy will dissipate in your car before transferring some to the car in front.

Strictly from a liability POV, if you move just before you are hit, or are moving when you are hit, you then become more responsible. If you are hit while stopped, pretty hard for anyone to blame you for any part of the accident.
 
docmirror said:
Strictly from a liability POV, if you move just before you are hit, or are moving when you are hit, you then become more responsible. If you are hit while stopped, pretty hard for anyone to blame you for any part of the accident.
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Solid advice :thumbsup:
 
docmirror said:
Cars = quite compressible. Billiard ball = not so much.

Plenty of energy will dissipate in your car before transferring some to the car in front.

Strictly from a liability POV, if you move just before you are hit, or are moving when you are hit, you then become more responsible. If you are hit while stopped, pretty hard for anyone to blame you for any part of the accident.
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Exactly. You want to be driving something solid. Cars compress far more than billiard balls, so you won't win with this strategy.
 
Big difference between a cue ball and a car is that when cue balls hit one another these are (almost) elastic collisions, when cars collide these are inelastic collisions. If cars were like cue balls there would be no damage to any car - front, middle or rear, doesn't matter. You car is more 'elastic' when you are in neutral (and if the car in front of you is in neutral too, etc.) , so if you see someone coming at you from behind - to minimize damage to your car switch to neutral (and preferably get out of the car...).
 
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If you see that you are about to get rearended, put your head back against the headrest and try to relax the rest of your body, it will hurt less.
 
docmirror said:
Cars = quite compressible. Billiard ball = not so much.

Plenty of energy will dissipate in your car before transferring some to the car in front.

Strictly from a liability POV, if you move just before you are hit, or are moving when you are hit, you then become more responsible. If you are hit while stopped, pretty hard for anyone to blame you for any part of the accident.
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And the audience survey says... Good Answer!
 
One advantage is momentum, however.

If a 1000kg car hits your car at 1000 kg, you're going to accelerate faster than if you are right up against another 1000 kg car (in which case it will be a 1000 kg mass interacting with a 2000 kg mass)
 
write-stuff said:
And the audience survey says... Good Answer!
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Haha! Thanks. I knew there was a good reason to finish that degree - so I could answer trivia question on a web forum. :drink:
 
Maybe if your car happens to be a 1959 Mercury Montclair, which has more solid steel in its frame and bumpers alone than an entire pickup truck of today has in the whole truck. I had one of those back in the late 70s which was formerly owned by an old woman who never drove it after her husband passed away... until she got Alzheimer's and decided to begin driving. She sheared off a concrete parking lot light pole pedestal even with the ground, which only put a dent in the front bumper. After she took out her garage door, support columns and back wall of the garage causing the roof to collapse on top of the car, her son put her into a nursing home and sold me the car for $100. It only had a few scratches from the garage incident. I drove the heck out of it for a couple years and sold it for $300 :lol:
 
Neal Howard said:
Maybe if your car happens to be a 1959 Mercury Montclair, which has more solid steel in its frame and bumpers alone than an entire pickup truck of today has in the whole truck. I had one of those back in the late 70s which was formerly owned by an old woman who never drove it after her husband passed away... until she got Alzheimer's and decided to begin driving. She sheared off a concrete parking lot light pole pedestal even with the ground, which only put a dent in the front bumper. After she took out her garage door, support columns and back wall of the garage causing the roof to collapse on top of the car, her son put her into a nursing home and sold me the car for $100. It only had a few scratches from the garage incident. I drove the heck out of it for a couple years and sold it for $300 :lol:
Click to expand...
The old cars were solid but did a pretty poor job of protecting the passengers and absorbing energy. Interesting to watch the crash test videos where they take an old "solid" car and a new one and set them up for a head on collision. You'd definitely want to be in the new car when watching the results.
 
jesse said:
The old cars were solid but did a pretty poor job of protecting the passengers and absorbing energy. Interesting to watch the crash test videos where they take an old "solid" car and a new one and set them up for a head on collision. You'd definitely want to be in the new car when watching the results.
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Yep, my old Merc had the kind of steering column that would impale you while the metal dashboard structure would likely crush your legs... that is, if you didn't get ejected thru the windshield instead. The massive bumpers bolted to the heavy frame would not absorb any energy in a wreck, just transmit it.
 
If you touch the other vehicle in front of you intentionally before you are hit, you would be causing or causal to an accident first. And I'm sure to my insurance Co. you would be liable if I told them you hit me first in the back, then some guy hit you.

With that said, it should transfer energy from my truck through to the last car behind impact. Combined with OP's car, it would be like the third car is hitting 9000lbs. instead of 3000lbs. The last rear car/driver is going to get ****ed up the worse because it's like hitting a heavier box of metal. The middle car/driver will get screwed up pretty bad because of crumple forces. The last car in the chain or the front car should sustain the least damage because of crumple dynamics and no front collision.

I ''R" no engineer by a long shot... :nonod::redface:
 
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janikpilot said:
Let's assume we're equal weight (most mid-size sedans are about equal).

And I beg to differ, brakes shouldn't make a difference. Brakes don't equal a static environment. The car can skid forward if there is enough force. Either way, let's assume we're both on the brakes.

EDIT: Disregard, brakes would indeed mean the force of the car pushing against the car in motion would be considerably less than the force of static friction holding me in place. Thus there would be some crunching.
Click to expand...

Right, brakes add resistance, that resistance is directly absorbed into crushing.
 
docmirror said:
Cars = quite compressible. Billiard ball = not so much.

Plenty of energy will dissipate in your car before transferring some to the car in front.

Strictly from a liability POV, if you move just before you are hit, or are moving when you are hit, you then become more responsible. If you are hit while stopped, pretty hard for anyone to blame you for any part of the accident.
Click to expand...

Compressability is the key issue in difference. Like I said, there will be some of that effect, but it's not going to particularly protect you.

If you are in a Yugo and pull up to the bumper of the Brinks truck infront of you as the semi slams into the back of you, your Yugo will be spraying blood out the windows before the Brinks truck moves an inch if he's standing on the brakes. Fault is irrelevent when you're squished.
 
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You actually want the crushing to happen. No crushing = you getting a lot of energy transfer to you. Look at the Dale Earnhardt Daytona crash from 2001. Looks like there was little damage to the car. But all that energy had to go somewhere. Then look at the cars that are shredded to pieces. All that energy gets dissipated, and the driver walks away.

You want the trunk to crush right up to the passenger cage, just as you want the engine compartment to do the same. All of that crushing reduces the amount of energy transferred to the passenger compartment, and in turn reduces the energy transferred to you.
 
You position your bike between the two cars ahead of you and watch your mirrors. When the screen-blind cager behind you fails to stop, you take off between the cars in front of you and let him hit one of them.
 
Oh....what you are wanting is an "elastic" collision....like with the billiard ball.

what we get is an "inelastic" collision.....where your car will absorb some of the impact (kinetic) energy. The remaining kinetic energy will be transmitted to the car in front of you.:yikes:
 
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The only question I've had while driving in traffic is "why don't we all just hit the gas at the same time?" :D
 
overdrive148 said:
The only question I've had while driving in traffic is "why don't we all just hit the gas at the same time?" :D
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Because people are morons. There is no reason why when a light turns green, the entire line of cars can't start moving.
 
That's a difficult question to answer. In a low speed collision, you're better off being against the car in front and utilizing its mass to reduce your acceleration. In a high speed collision, you may have less of a chance of being crushed if there is some distance between you and the next car. Since you will absorb the momentum of the colliding vehicle with greater acceleration, there will be less inelastic energy transfer. You generally want more of the latter, but only to the extent you don't get crushed.
 
pilot_dude said:
Two words: crush zones
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The SMART concept uses both. It has crush zones surrounding an engineered cage that transfers the energy forward in the 'non crushable' mode, acting more similar to a fluid wave than solid with your inertia isolated from that energy within the cage so they "bounce" with the cage absorbing the energy keeping it from reaching the occupant. As long as the occupant remains secure and buffered within the cage, the accelerational forces the occupants are exposed to will always remain non fatal to the point of cage failure. When the cage fails though the energy release will be fatal. There is only so much inertial disparity anything can overcome, you can just make things do the best they can.
 
And that's part of why I feel better flying myna mooney, that nice steel cage around the cockpit.
 
FormerHangie said:
If you see that you are about to get rearended, put your head back against the headrest and try to relax the rest of your body, it will hurt less.
Click to expand...

On my bike this is where I zip up between the line of cars or up the shoulder and let the poor schmuck in front of me take the impact.
 
jesse said:
The old cars were solid but did a pretty poor job of protecting the passengers and absorbing energy. Interesting to watch the crash test videos where they take an old "solid" car and a new one and set them up for a head on collision. You'd definitely want to be in the new car when watching the results.
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100% correct. This vid is pretty convincing.

https://www.youtube.com/watch?v=C_r5UJrxcck

Old cars are cool but safe (for the most part) they are not.

Exception to the rule, 1959 Mercedes Ponton, the 1st car that was designed with the combination crush zone/safety cage strategy. The passenger cell holds up very well to a modern offset crash test, even with this old rusty sample. Too bad MB didn't put a collapsible steering column in the car. D'oh!

https://www.youtube.com/watch?v=21BmnLJRxk8
 
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