Physics Question

Ben-e

Captain Critter!
Not sure if the more mathematically minded people of farkin will be able to settle an arguement a mate and i have been having.

We were talking about car crashes, and the impact/force relative to speed.

My mate reckons that if two cars travelling at equal speed crash into eachother head-on, say 50km/h, that the impact speed will be 100km/h - the same as a car hitting a brick wall at 100km/h, according to him, and the damage will be roughly the same.

My counter to his arguement is that although the impact speed will remain the same, there will be greater damage between two cars than a single car hitting a brick wall.

Who's right? (BTW, theres beer riding on this)
 
i'd say more damage to a car thats hitting the wall as the car you hit crumples & absorbs(a tiny bit) of the impact, whereas the entire force of the crash is felt by the single car in the single car smash.

same amount of damage, but halved between the cars if they hit each other;)
 

sockman

Likes Bikes and Dirt
according to my phyiscs teacher, if 2 identiccal cars are traveling at 50 kmh and hit head on, its the same as a car hitting a wall at 100. so yes your mate is correct
 

Ben-e

Captain Critter!
according to my phyiscs teacher, if 2 identiccal cars are traveling at 50 kmh and hit head on, its the same as a car hitting a wall at 100. so yes your mate is correct
What about two masses travelling towards eachother, rather than one mass travelling and another stationary?
 

tu plang

knob
mmm im with vicious. his idea of relative velocity is correct. when it comes to damage you have to look at impulse and momentum - to put it simply, impulse is the rate of change of momentum - I=mvf -mvi (mass, final velocity and initial velocity) and this is also equal to F*t (force and time.) so you have your cars travelling at there relative velocity, so they both have a given momentum, if you compare that to F*t we see that the greater the value of t, the lower the value of F (force)

the fact that 2 cars will crumple more and take a greater amount of time to cause that change in momentum will mean that the impact will be less harmful to occupants.
 

hayd

Likes Dirt
i'd say more damage to a car thats hitting the wall as the car you hit crumples & absorbs(a tiny bit) of the impact, whereas the entire force of the crash is felt by the single car in the single car smash.

same amount of damage, but halved between the cars if they hit each other;)
Sorry fairly sure ur wrong. If both cars are traveling at the same speed and colide head on its the same as one car hitting a wall at twice that speed.
Note: this is only true if it is a direct hit and the cars lock together, because some velocity will remain if the cars bounce off from eachother.

Sorry, but I hope that answers ur question
 

Mojo

Likes Dirt
if the cars are the same weight travelling at the same speed it will be the same force as one car travelling twice that speed and hitting a wall. the only difference will be the way the cars cruple, because of the diffence in shape to a wall and the front of a car. if the car was travelling twice the speed and hit the same parked car. the same senario
 

Ben-e

Captain Critter!
Sorry fairly sure ur wrong. If both cars are traveling at the same speed and colide head on its the same as one car hitting a wall at twice that speed.
Note: this is only true if it is a direct hit and the cars lock together, because some velocity will remain if the cars bounce off from eachother.

Sorry, but I hope that answers ur question
Hmm, Tu Plang had a formula to back his theory up. What do you base this on?
 

ajay

^Once punched Jeff Kennett. Don't pick an e-fight
Hmm, I dont know the equations, but im with Tu plang and VF.

Picture it this way, if a car hits a brick wall at 100kphr, the car will effectively take the full force of the collision.

If 2 cars hit at 50kphr then, logically it would be more similar to a car traveling at 100kphr and colliding with a stationary car (not a wall). The stationary car will absorb a fair wack (lamens terminology) of the impact, therfore lessening the force on the moving car.

You've all seen what happens in that instance, so no, its not the same thing.
 

Ben-e

Captain Critter!
Hmm, I dont know the equations, but im with Tu plang and VF.

Picture it this way, if a car hits a brick wall at 100kphr, the car will effectively take the full force of the collision.

If 2 cars hit at 50kphr then, logically it would be more similar to a car traveling at 100kphr and colliding with a stationary car (not a wall). The stationary car will absorb a fair wack (lamens terminology) of the impact, therfore lessening the force on the moving car.

You've all seen what happens in that instance, so no, its not the same thing.

Ok, how about if the wall had a similar bulk/weight to the car? Would this make any difference?
 

S.

ex offender
Hmm, I dont know the equations, but im with Tu plang and VF.

Picture it this way, if a car hits a brick wall at 100kphr, the car will effectively take the full force of the collision.

If 2 cars hit at 50kphr then, logically it would be more similar to a car traveling at 100kphr and colliding with a stationary car (not a wall). The stationary car will absorb a fair wack (lamens terminology) of the impact, therfore lessening the force on the moving car.

You've all seen what happens in that instance, so no, its not the same thing.
Nah, assuming both cars are the same size, shape and mass, it'll be pretty well identical to hitting a brick wall unless they manage to get past each other (eg front quarter panels hit and cars go partially sideways). The reason for this is that while the other car can deform to absorb the kinetic energy of your car, yours is doing the same for the other car. The end result is that the collision is pretty much symmetrical - the fronts of the cars will collide at a fixed point and due to the identical forces from either side of that impact point, will not move either way. Thus, it's the same as hitting a brick wall at 100km/h.

Relative velocity is everything. If you're doing 100km/h and you run into a brick wall moving in the same direction as you are (relative to the ground) but it's going 50km/h, the effective impact speed is 50km/h. Likewise, if it was moving towards you, the effective impact speed is higher. This is also why it's dangerous to drive extremely slowly in traffic - you increase the relative velocity between you and the vehicles around you!
 

S.

ex offender
Ok, how about if the wall had a similar bulk/weight to the car? Would this make any difference?

By saying it's a wall, you're assuming it doesn't move at all (well, significantly), and that the whole impact is absorbed ONLY by the car's body. As soon as it moves or deforms or gets knocked down then you're not comparing apples with apples.
 

ajay

^Once punched Jeff Kennett. Don't pick an e-fight
Ok, how about if the wall had a similar bulk/weight to the car? Would this make any difference?
OK, sorry, assuming that the brick wall was a fairly solid unit ie; the car wouldnt bust through it)

And considering the crumple zones of the car etc then, no it wont make a difference.

As I said, I dont know the equations, but considering the massive differences between a car and a wall...

Ahh, I'll pull out of this one.
 

robmundall

Banned
I didn't read the whole thread so I am not sure if it has been answered. but, for every force there is an equal and oppisote reaction therefore hitting a brick wall at 100km/h is the same as having a head on crash at 100km/h.
 

Ben-e

Captain Critter!
Nah, assuming both cars are the same size, shape and mass, it'll be pretty well identical to hitting a brick wall unless they manage to get past each other (eg front quarter panels hit and cars go partially sideways). The reason for this is that while the other car can deform to absorb the kinetic energy of your car, yours is doing the same for the other car. The end result is that the collision is pretty much symmetrical - the fronts of the cars will collide at a fixed point and due to the identical forces from either side of that impact point, will not move either way. Thus, it's the same as hitting a brick wall at 100km/h.

Relative velocity is everything. If you're doing 100km/h and you run into a brick wall moving in the same direction as you are (relative to the ground) but it's going 50km/h, the effective impact speed is 50km/h. Likewise, if it was moving towards you, the effective impact speed is higher. This is also why it's dangerous to drive extremely slowly in traffic - you increase the relative velocity between you and the vehicles around you!

Ok, it took me a little while to decipher, but i get what your saying - i owe my mate a case and he owes me one.


I found your speal interesting - about the wall and the car travelling in the same direction but colliding at different speeds. I never thought of it like that.
 

Chalkie

Likes Dirt
If in both the two cars colliding experiment and the car vs. the wall one, the cars had no crumple zone, then yes they would be the same. HOWEVER if they are normal cars, two cars colliding head on will be much less damaging. Let me confirm/add to whats been already said.

(assume both cars have equal MASS of 1000kg and speed)

Car vs. Wall

Velocity = 100km/h = 27.78m.s^-1
Time to Stop = 1s (lets say that the crumple zone of a car lets it stop in 1second to make things easy)

Acceleration of the car: (V=velocity)
a = Vinital - Vfinal
......Change in time

= 27.78 - 0
....1

=27.78m.s^-2

Therefore the force on the car:
F = m.a (F=force, m=mass of car, a=acceleration)
= 1000kg . 27.78m.s^-2
= 27780 N (N=units of force)

Car(A) vs. Car(B)
Lets just use Car(A) as the force on both cars is the same.

Velocity = 50km/h = 13.89m.s^-1
Time to Stop = 1s + 1s = 2s (take into consideration both crumple zones of each car)

Acceleration of the car:
a = Vinital - Vfinal
......Change in time

= 13.89 - 0
....2

=6.945m.s^-2


Therefore the force on the car:
F = m.a (F=force, m=mass of car, a=acceleration)
= 1000kg . 6.945m.s^-2
= 6945 N (N=units of force)

Therefore in Car(A) vs. Car (B) the force is 4 times less (as 27780/6945 = 4).

So in Car vs. Wall the impact is not only worse, but 4 times worse.

So summing it up, your both partly wrong... Car vs. Wall is much worse. I think that's right, can someone confirm for me here. If there's still an argument go out and give it a shot :D
 
Last edited:

S.

ex offender
If in both the two cars colliding experiment and the car vs. the wall one, the cars had no crumple zone, then yes they would be the same. HOWEVER if they are normal cars, two cars colliding head on will be much less damaging. Let me confirm/add to whats been already said.

(assume both cars have equal MASS of 1000kg and speed)

Car vs. Wall

Velocity = 100km/h = 27.78m.s^-1
Time to Stop = 1s (lets say that the crumple zone of a car lets it stop in 1second to make things easy)

Acceleration of the car: (V=velocity)
a = Vinital - Vfinal
......Change in time

= 27.78 - 0
....1

=27.78m.s^-2

Therefore the force on the car:
F = m.a (F=force, m=mass of car, a=acceleration)
= 1000kg . 27.78m.s^-2
= 27780 N (N=units of force)

Car(A) vs. Car(B)
Lets just use Car(A) as the force on both cars is the same.

Velocity = 50km/h = 13.89m.s^-1
Time to Stop = 1s + 1s = 2s (take into consideration both crumple zones of each car)

Acceleration of the car:
a = Vinital - Vfinal
......Change in time

= 13.89 - 0
....2

=6.945m.s^-2


Therefore the force on the car:
F = m.a (F=force, m=mass of car, a=acceleration)
= 1000kg . 6.945m.s^-2
= 6945 N (N=units of force)

Therefore in Car(A) vs. Car (B) the force is 4 times less (as 27780/6945 = 4).

So in Car vs. Wall the impact is not only worse, but 4 times worse.

So summing it up, your both partly wrong... Car vs. Wall is much worse. I think that's right, can someone confirm for me here. If there's still an argument go out and give it a shot :D
Nope, you're missing the fact that when the two cars collide, they both take THE SAME time to decelerate, because each of them takes up half of the total distance that they travel in the impact (btw - impact of 1.0 seconds? You have a 14 metre long crumple zone? Consider how far a car travels in one second at 100km/h). You don't add the two times together.
 
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