This Car Can Sure Take Your Breath Away..At 120 Kmph!

[pgustavson]

Originally Posted by lgKido

Sorry, but that's not the correct, and it's my duty to unravel 30 years of your inaccuracy (no offense intended).

If two objects are moving in parallel their relative velocity can be calculated:
For example, two cars are moving in the same direction along a road. Car A is traveling at +30 m/s and Car B is travelling at +20 m/s. Their relative velocity is 30 – 20 = +10 m/s.

If the two cars are moving in opposite directions, the velocity of Car A is +30 m/s and the velocity of Car B is –20 m/s. Their relative velocity is 30 –(–20) = 30 + 20 = +50 m/s.

So in the video, you have Object A (the car) moving at 120 kmh in one direction and Objedt B (the wall) moving at 0 kmh. Simple relative velocity of the pair is 120 - (-0) = 120 + 0 = 120 kmh........

Ok, I'm a civil engineer and studied relative velocities in Engineering Dynamics........

That’s all well and good, but the relative velocities w/r to one another is, well, irrelevant. The system has all the energy that it has regardless of how it’s distributed. In the case discussed above, making a number simplifying assumptions, a car hitting a wall at 120km/hr will experience the same kinetic energy transfer as if it hit another identical car also travelling at 120km/hr directly towards it. In the car/wall scenario 100% of the kinetic energy dissipation is carried by the car. In the car/car scenario, there is twice the energy in the system, but it is dissipated equally between the two cars. Again, this only holds if a lot of simplifying assumptions are made (e.g. no energy goes into heating the wall, both cars absolutely identical, no friction losses, etc….) This problem is a classic one that all physics students are introduced to; and many think that the 2-car scenario is somehow worse than the car vs wall because the relative velocity is twice as high.

[CamaroSkooter]

Hey, I'm a civil engineer too

So, both of y'all are correct. But you're comparing apples to oranges.

If two cars weigh 1,000kg and are travelling towards each other at 60 kph, then car A's speed will look like 120kph to the driver of car B.

However, the formula for kinetic energy is:



Notice how v is squared?

That means that you can't simply add the velocities of two cars travelling 60kph and assume that a collision between the two would be the same as a single car going 120kph hitting an imovable object.

Look at it this way, the two cars going 60kph would have a kinetic energy of:

60kph = 16.66 m/s

0.5(1000)(16.66^2) + 0.5(1000)(16.66^2) = 277,777.77 Joules

The single car going 120kph would have a kinetic energy of:

120kph = 33.33 m/s

0.5(1000)(33.33^2) = 555,555.55 Joules

So, yeah, hopefully seeing the math clears up some confusion

[Sweet Zness]

Ok its been a long time, but if we are going to state that two cars moving at 60 KMH collide in opposite directions is the same as hitting a stationary wall isn't this incorrect because.

1. An object in motion tends to stay in motion. So two cars colliding isn't going to completely stop either object like a wall. Energy will continue to travel through both cars causing more damage than only one car hitting a stationary wall.

2. Kinetic energy is dispersed completely? Kind of like, when you throw a basket ball against a wall it bounces back with X force, but if you throw two basketballs at each other you cause more energy to be dispersed and you get more of a reactionary effect.

Im no expert, Im only trying to remember stuff I learned in school 20 years ago. If Im wrong, Im wrong.

[KXRM]

Originally Posted by CamaroSkooter

...assuming the parked car was bolted to the ground

Nope, no bolting into the ground needed. Again this shows how relative the speed is. Of course there are other factors that play into this that don't make them exactly the same but in the simplistic way to view things this is how it should be viewed.

[rez333]

I have a phD in astrophysics, so I say you're all wrong and I'm right!!



Seriously though, I appreciate people correcting my ignorance with facts, but one's qualifications have nothing to do with the merit of one's arguments.

[CamaroSkooter]

Originally Posted by Sweet Zness

Ok its been a long time, but if we are going to state that two cars moving at 60 KMH collide in opposite directions is the same as hitting a stationary wall isn't this incorrect because.

1. An object in motion tends to stay in motion. So two cars colliding isn't going to completely stop either object like a wall. Energy will continue to travel through both cars causing more damage than only one car hitting a stationary wall.

2. Kinetic energy is dispersed completely? Kind of like, when you throw a basket ball against a wall it bounces back with X force, but if you throw two basketballs at each other you cause more energy to be dispersed and you get more of a reactionary effect.

Im no expert, Im only trying to remember stuff I learned in school 20 years ago. If Im wrong, Im wrong.

Correct. On all counts. But the argument is currently only dealing with theory, not application. The real world has exponentially more variables to attempt to account for in these situations beyond just the two vehicles' speed and mass.

Losses due to heat transfer, losses due to parts coming off the car, losses due to wind drag, the list goes on and on.

So, first you have to explain the most basic stripped-down version of the situation before you can attempt to dispute the real life circumstances.

Originally Posted by KXRM

Nope, no bolting into the ground needed. Again this shows how relative the speed is. Of course there are other factors that play into this that don't make them exactly the same but in the simplistic way to view things this is how it should be viewed.

Well, now you're talking about energy transfer. If a 120kph car hits an identical stationary car, then the moving vehicle will transfer roughly half of its kinetic energy into the stationary vehicle, and the two will begin moving in the same direction as the first vehicle was (assuming the stationary vehicles tires were already pointing that direction). The speed that they both end up going, though, will not be half of the original moving vehicle's speed

Originally Posted by rez333

I have a phD in astrophysics, so I say you're all wrong and I'm right!!



Seriously though, I appreciate people correcting my ignorance with facts, but one's qualifications have nothing to do with the merit of one's arguments.

Right? I merely pointed out that I was also a civil engineer to show that two people with the same education level doesn't necessarily mean that it's applicable in an argument of this type

I should also point out that, Civil Engineers only have a very basic overview of the kind of engineering that goes into understanding crash physics.

[KXRM]

Originally Posted by CamaroSkooter

Well, now you're talking about energy transfer. If a 120kph car hits an identical stationary car, then the moving vehicle will transfer roughly half of its kinetic energy into the stationary vehicle, and the two will begin moving in the same direction as the first vehicle was (assuming the stationary vehicles tires were already pointing that direction). The speed that they both end up going, though, will not be half of the original moving vehicle's speed

I never was making the claim that they would be going half the speed after the crash, all I said was that the force felt will be very similar to if both vehicles were going half the speed at each other. It makes no difference who is moving, what matters is how they move in relation to each other.

[The_Blur]

Originally Posted by KXRM

I'll explain mostly because I am bored.

2 cars, traveling at 60kmph at each other is equal to one car crashing into a wall at 60 kmph. So the video actually shows what would happen if a head on collision occurred with both vehicles traveling at 120kmph directly at one another.

I am not sure where people get this idea that the force of 2 vehicles impacting one another in opposing directions equals half of the force of a solid body like a wall or barrier that does not give or break. I have also seen opposing viewpoints where people believe the force of a head on collision is double that of a collision with a wall. I had this discussion with a buddy of mine recently after he read a thread argument in some other forum. He simply never saw the light as I was trying to explain away his theory. Unraveling 30 years of inaccurate information is difficult for some it seems.

There is of course other factors involved in this discussion to make it true, such as the mass of both of the cars and the safety related crumple zone designs which can make things different. As well as road and crash conditions. But all things being equal, any event that takes you from 120kmph to 0kmph in less than a second will feel the same way. Whether it was a wall or an opposing car traveling the exact same speed.

To put some additional light into the discussion, you could say that this video illustrates what it is like to crash into a identical parked car going 240 kmph. Think about that for a second.

This is a good discussion.

[CamaroSkooter]

Originally Posted by KXRM

I never was making the claim that they would be going half the speed after the crash, all I said was that the force felt will be very similar to if both vehicles were going half the speed at each other. It makes no difference who is moving, what matters is how they move in relation to each other.

I threw the half speed comment as food for thought.

If a moving vehicle hits a parked car, the force felt will not be the same as if the vehicle hit an immovable wall. There will be a transfer of energy from the moving vehicle to the stationary vehicle. After the collision occurs, the moving vehicle will still be moving forward, just at a slower rate of speed. Therefore, it wouldn't be a 120-to-0 kph collision. It would be more like a 120-to-84.85 kph collision (Assuming 100% conservation of energy, which we both know is unlikely)

Granted, the initial impact will be violent in both cases, and the human body would probably be incapable of determining the difference, since you'd probably be severely injured in both instances, but if you measured the collision with scientific equipment, the results would support my above calculations.

[lgKido]

Originally Posted by CamaroSkooter

Hey, I'm a civil engineer too

So, both of y'all are correct. But you're comparing apples to oranges.

If two cars weigh 1,000kg and are travelling towards each other at 60 kph, then car A's speed will look like 120kph to the driver of car B.

However, the formula for kinetic energy is:



Notice how v is squared?

That means that you can't simply add the velocities of two cars travelling 60kph and assume that a collision between the two would be the same as a single car going 120kph hitting an imovable object.

Look at it this way, the two cars going 60kph would have a kinetic energy of:

60kph = 16.66 m/s

0.5(1000)(16.66^2) + 0.5(1000)(16.66^2) = 277,777.77 Joules

The single car going 120kph would have a kinetic energy of:

120kph = 33.33 m/s

0.5(1000)(33.33^2) = 555,555.55 Joules

So, yeah, hopefully seeing the math clears up some confusion

I stand corrected and humbled. You're right! I had a PE tell me once that that's why another engineer should always check your work. He said too often engineers get "locked in" a certain way of thinking and can't see the obvious error they've made. So good job, and thanks.

Oh, btw, I must have been ill during that lecture on 1/2 mv^2.
JK

[KXRM]

Originally Posted by CamaroSkooter

I threw the half speed comment as food for thought.

If a moving vehicle hits a parked car, the force felt will not be the same as if the vehicle hit an immovable wall. There will be a transfer of energy from the moving vehicle to the stationary vehicle. After the collision occurs, the moving vehicle will still be moving forward, just at a slower rate of speed. Therefore, it wouldn't be a 120-to-0 kph collision. It would be more like a 120-to-84.85 kph collision (Assuming 100% conservation of energy, which we both know is unlikely)

Granted, the initial impact will be violent in both cases, and the human body would probably be incapable of determining the difference, since you'd probably be severely injured in both instances, but if you measured the collision with scientific equipment, the results would support my above calculations.

I also never stated that hitting a parked car is the same as hitting a wall. Please read again. All I was stating with that sentence was speed is relative.

A car going 240kmph into a non moving car would create the same forces as 2 cars going at each other at 120kmph. Nothing about immovable walls.

[CamaroSkooter]

I think we're splitting hairs at this point