did the tree break?

Have you seen the 40 to impact with the f150 supercab video from the safety rating people? Its ugly.

 

Not to start something nasty, but that is not the result of a legit 50 MPH impact into a truly solid object. Maybe 50 MPH before you put the brake pedal to the floor, but not at the point of impact. I've hit a massive tree dead-center at around 30 MPH with a different full-size 4x4, and the damage was a good bit worse than that. I'm guessing that your impact speed was around 30-35 MPH.

Assuming that you hit a truly immovable object, the resulting damge would be, due to the immutable laws of nature, equal, more-or-less, to hitting a similar vehicle moving at the same velocity head-on.

Regardless of your education, laws are laws, and that's not a legit 50 MPH blast into an immovable object. Didn't even break a window.

 

exactly what i was thinking. A 50 mph impact into a solid tree would buckle the entire truck. I wish i had the link to the 40 mph head on crash, someone else might.

 

There's another way to look at this. Both sides are saying roughly the same thing, in different ways (based on my engineering degrees, which include a couple in rocket science).

This is a question of dynamics.

The kinetic energy of EACH vehicle is .5mv^2. Energy is neither created, nor is it destroyed. Law of Conservation of Energy. (Same applies if you use the Law of Conservation of Matter).

Let's assume we have 2 identical vehicles moving at the same speed to a head-on collision (just to simplify things). The collisions are perfectly collinear and on a perfectly level, flat surface.

If the vehicles are moving at 60mph, the closing speed is 120 mph. In order to conserve energy, the kinetic energy of one vehicle has to equal that of the other vehicle.

The net kinetic energy is 0. Another way of looking at this is that each vehicle will take the same amount of damage as it is capable of delivering. Both vehicles take the same amount of damage. Let's call this "60 mph" worth of damage.

Let's take a second case. In this case, a vehicle moving 60 mph hits an identical, stationary vehicle. This is much different. Now we have a situation where the stationary vehicle will begin to move from the shock of the hit as a function of the momentum of the first vehicle and the inertia of the second. However, the first vehicle did not take "60 mph" worth of damage. It took less.

In short, I suppose you can look at it this way. If you drive into a brick wall at 60 mph, you do much less than "60 mph" of damage to yourself. Let's just simplify and say you do "30 mph" of damage (although it's not necessarily half--it depends on momentum and inertia).

If you drive into an identical vehicle, both doing 60 mph, then each of you does "60 mph" of damage to one another.

So it seems that, depending on how you look at it, both sides are right. You do inflict more damage by vehicles moving at each other than by 1 moving at another that's stationary. It's just not as black and white as "one is twice the other", although it is helpful to consider that as a possibility.

Next week we'll be discussing how to calculate the curl of a vortex and reviewing NACA foils for their aerodynamic properties at they apply to ricer wings.

--Rip

 

Grynomite, No the tree didn' t bust

Silver-Y2K-SVT,

Good point I should of mentioned There was a 2.5 ft drift of snow I plowed through before impact for what that counts for, It would of slowed the impact, Yes

The point being I walked away from a truck that had $17,000 ish mark for damage bent frame rail mashed Rad support, Almost Everything that had the belt attached to it was busted. They fixed it and I drove from the bodyshop to Metro ford and Leased a 03 King Ranch before something busted on it.

 

No offense, but don't take dynamics or structures courses from Chemical, Electrical, or any other non-mechanical engineers in the same what that you don't go to an ear-nose-throat doctor to have brain surgery performed. Engineers study different disciplines and have different strengths...

Just my advice.

Now, back to the important stuff. Hope the L driver comes through and my condolences to the family of those deceased.

--Rip

 

Grinomyte,

Is that 40 MPH crash into a solid wall, Tree, House, Kenworth, Moose?. The crash physics depend on what you hit. A tree cuts into the truck as you can see quite good If I would of rammed a solid wall it would of given the truck a pie faced look and therefore spread the impact over the whole front of the truck instead of just a 12 inch area were the tree walked in

 

Sorry to hear about your accident, glad that you and pooch walked away unharmed :-) I am amazed at how little damage for hitting a stationary object at 50+ MPH?

Now we must include impact dynamics… In the photo it appears that the vehicle may have hit a pole or tree transferring energy only over a small portion of the vehicle. A head on collusion with another equal vehicle tends to transfers energy over a larger area decreasing the time that it takes to transfer that energy. The shorter the time period transferring energy the more G’s that will be experienced by the occupants and the worse it is for the occupants. The more a vehicle crumples the longer it takes to transfer the energy the better off the occupants; that is why many of today’s vehicles have crumple zones.

Given the question of hitting a stationary pole at 50 MPH or hitting another equal mass vehicle traveling at the same velocity (50 MPH) - Hitting the pole decreases the area of impact increasing the time that it takes to transfer energy resulting in less G forces experienced by the occupants – assuming that the integrity of the occupant area stays intact. Not to mention not involving another vehicle.

Time for this one to go to bed - night all

 

For those of you trying to reason out why the force of an impact between two cars doing 60MPH is the same as one car hitting a stationary object at 60MPH: You need to look at the force _on one vehicle_.

F=MA. The force experienced by a car is equal to the Mass multiplied by the Acceleration. The mass is the same in either situation (for a given vehicle) so it is not important to the calculation.

The questions becomes what is the acceleration? If a car hits a tree and the tree does not move, then the acceleration is a function of vehicle speed and the distance over which it could decelerate. This distance is the crumple zone.

When two cars hit each other, and each of them is doing 60MPH, the force experienced by each vehicle is equal, but opposite (Assuming equal mass and crumple zones and so on. If the mass was not equal then the picture is different). If we look at one car at a time, we can see the forces are the same as if it hit a stationary object that did not move. The single 60MPH vehicle is still brought from 60MPH to 0 in the distance of the crumple zone resulting in the same acceleration. Yes there is a second vehicle, but any forces from that vehicle are involved in bringing _that vehicle_ from 60 MPH to 0 in the distance of it's crumple zone.

In other words, we have (2 * Force) / 2 vehicles. That is to say we have the same force in both cases.

The reason this does not sound right is that we are use to stationary objects "giving" when they are hit and this helps to absorb the impact and lessen the force exerted on the passengers. Few people have seen what happens when a car hits a truly immovable object, but those that have know the results are devastating.

-Don

 

look folks, it's not the same. two vehicles hitting head on, each at 60 mph causes more damage than a single vehicle hitting a wall at 60 mph. trust me. if you like, i'll run a nastran simulation and show you. i guarantee the deformation in the first case is greater than in the second case. anyone want to make a bet?

it's just not necessarily (and not often) twice as much.

btw, force is not mass times speed. it's mass times acceleration. acceleration is the derivative of speed (in this case, miles per hour squared).

--Rip

 

I will not trust what I know to be wrong. Please run the simulation. And yes I would be happy to bet.

-Don

*100th post

 

there is one thing here, both cars will have crumple zones to cushion eachother, that makes a difference.

Pat, the crash is into a solid unmovable wall. My guess is crashing into a solid flat wall face on is much easier on you than crashing into a smaller item.

 

Here is another experiment:

Take two billiard *****. Hit them at each other at the same speed and they will bounce off each other in opposite directions at the same speed they were traveling at initially (minus small diferences due to friction).

Now, hit one of those ***** at a solid wall. It will still rebound at the same speed as it was hit (again minus a small amount due to friction).

If you fired both ***** at each other, and they both stopped immediately upon impact, they would each have to dissipate their speed.

If the ball hits a wall that does not give, all of its energy must be dissipated by deforming the ball itself.

If the ball hits another ball, it must still dissipate all of its energy by deforming _itself_. It can not deform the other ball because that ball is busy deforming itself to get rid of _its_ energy.

This is high school level physics and isn't open for debate. It is a simple fact.

-Don

 

Quite the opposite. Hitting a flat wall dead on is far worse than hitting a tree _in terms of force_.

The tree will penetrate further into the car thus giving the car a greater distance in which to decelerate. This reduces the force of the collision.

The reality of the situation, however, is that having a tree rip into the passenger compartment is probably worse for the occupants than hitting a wall, but that does not change the magnitude of the forces involved.

The crumple zones reduce the force of the collision experienced by each vehicle, but they have the same effect whether you are hitting another car or a solid wall.

-Don

 

You have 2 vehicles both have passengers:

In one scenario both are moving towards each other at 60 mph, in the other scenario one is moving 60 mph and the 2nd car is NOT moving. They hit head on in both scenarios.

In BOTH scenarios all the occupants are going to be injured.

According to what I read here, the injuries to the passengers are the same in both scenarios ... I'm no expert and have no simulations I can run BUT common sense says the injuries will be different. The amount of total energy to dissipate is different.

In one you have the weight of 2 vehicles X the energy of motion of BOTH vehicles to dissipate

In the other you have the weight of ONE vehicle X the energy of ONE vehicle to dissipate. And to complicate things you have 2 crumple zones and 2 sets of airbags in both cases..

I just don't see how the effect on the passengers can be the same...

Not even close

My $0.02

Doug