Dead simple physics question (still a puzzle to me)

[Bobthefreeradical]

Homework Statement

If two vehicles crash head-on at say 60 mph & the force acting upon each can be described as being as if one vehicle crashed into a stationary obstacle at 120 mph, or?
Clue me in . . PLEASE

Relevant Equations

F=MA

an assumption being that since the M ( that is mass is a constant for the vehicle ) and a being the same as if the vehicle were to have been crashed into a stationary obstacle, that F would be the same as if said vehicle were to have been crashed into a stationary obstacle and not another vehicle. or am I assuming too much?
Please help me out here.

 

[haruspex]

Homework Statement: If two vehicles crash head-on at say 60 mph & the force acting upon each can be described as being as if one vehicle crashed into a stationary obstacle at 120 mph, or?
Clue me in . . PLEASE
Relevant Equations: F=MA

an assumption being that since the M ( that is mass is a constant for the vehicle ) and a being the same as if the vehicle were to have been crashed into a stationary obstacle, that F would be the same as if said vehicle were to have been crashed into a stationary obstacle and not another vehicle. or am I assuming too much?
Please help me out here.

It depends on the nature of the stationary obstacle. Is it another car at rest or something more solid.
Consider two identical vehicles hitting a solid wall from opposite sides at the same speed and same instant. Does the presence of the wall make a difference?

 

[DaveC426913]

Question to make you think: does your scenario happen on Earth?

Because if it happens in space, in free fall, then you're going to have a real tough time distinguishing the difference between two cars each moving at 60 and one car moving at 120. It's all relative after all!

Once you've solved that, you are then free to ask the followup question: if I now transport this experiemnt to Earth, what relevant factors might change the above scenario?

 

[PeroK]

Homework Statement: If two vehicles crash head-on at say 60 mph & the force acting upon each can be described as being as if one vehicle crashed into a stationary obstacle at 120 mph, or?

The most important factor in terms of the collision itself is the relative speed between the cars. Both cars travelling at 60mph is very similar to one travelling at 90mph and one at 30mph; or, one travelling at 120mph and one at rest. The difference is the external factors of the air and especially the road surface.

For two aeroplanes colliding in mid air, there is no road surface. In that case, the total relative speed would determine the collision forces. And the air speed of each plane would be even less important.

 

[Orodruin]

Question to make you think: does your scenario happen on Earth?

Because if it happens in space, in free fall, then you're going to have a real tough time distinguishing the difference between two cars each moving at 60 and one car moving at 120. It's all relative after all!

Once you've solved that, you are then free to ask the followup question: if I now transport this experiemnt to Earth, what relevant factors might change the above scenario?

The most important factor in terms of the collision itself is the relative speed between the cars. Both cars travelling at 60mph is very similar to one travelling at 90mph and one at 30mph; or, one travelling at 120mph and one at rest. The difference is the external factors of the air and especially the road surface.

For two aeroplanes colliding in mid air, there is no road surface. In that case, the total relative speed would determine the collision forces. And the air speed of each plane would be even less important.

The question is not comparing two cars colliding at 60 mph with a car at 120 mph slamming into a stationary car. The question specifies a stationary ”object”. Now, a car could typically be considered an ”object”, but so can many other things. The answer is - obviously - going to depend on what the stationary ”object” is. If it is a fly, not much will happen to the car. If it is a stationary immovable mountain wall the crash is going to be significantly more severe than the case of two cars colliding at 60 mph each. This is essentially the case discussed in #2.

So the bottom line is: It depends on what the OP means by ”object”. @Bobthefreeradical please specify.

 

[phinds]

@Bobthefreeradical I pointed out in another thread where you ALSO failed to answer a very direct and specific question that doing so is bad form here on PF.

@Bobthefreeradical please specify.

 

[DaveE]

I think you'll want to read about "frame of reference" and "Galilean relativity" in the physics world. Basically, for identical objects, the physics will be the same, but how it is expressed depends on the observer's frame of reference.

For different objects... well, it would probably be different.

 

[Herman Trivilino]

Under what conditions will the acceleration of the vehicles be the same?

 

[Bobthefreeradical]

@Bobthefreeradical I pointed out in another thread where you ALSO failed to answer a very direct and specific question that doing so is bad form here on PF.

Sorry about the bad form, Please tell me, what was the question?

 

[phinds]

post #5

 

[kuruman]

So the bottom line is: It depends on what the OP means by ”object”. @Bobthefreeradical please specify.

This is a key question to be answered. In the original statement of the problem, OP mentions two possibilities
1. Two vehicles moving in opposite directions at 60 mph each and crashing head on.
2. One vehicle moving at 120 mph and crashing into a stationary "object".

To @Bobthefreeradical :
You ask whether "F would be the same as if said vehicle were to have been crashed into a stationary obstacle and not another vehicle."

I assume that by F you mean the average force exerted on said vehicle while the momentum transfer takes place. If so, then we can safely say that it would be the same if the stationary obstacle is the vehicle in possibility 1. However, you clearly excludes that possibility. In that case, the answer is "it depends on the mass of the stationary obstacle." There are two extreme cases.

In the first case the mass of the stationary obstacle is much smaller than the vehicle, imagine a bee $\text{(}m\approx10^{-4}~$kg$\text{)}$ hovering in mid air minding its own business. In the second case the mass of the stationary obstacle is much greater than the vehicle, imagine a concrete bunker firmly attached to the Earth ($m\approx 6\times 10^{24}~$kg).

Where, in this range, is your "stationary object"? Do you see why the answer matters if you want to make a comparison between possibilities 1 and 2?

 

[osilmag]

I think what the OP is getting at is the concept of impulse momentum: $I=F \delta t$, where delta t is the elapsed time of the crash. At 120 mph, it would be a different since the delta t would be shorter.

 

[Orodruin]

I think what the OP is getting at is the concept of impulse momentum: $I=F \delta t$, where delta t is the elapsed time of the crash. At 120 mph, it would be a different since the delta t would be shorter.

It would not be shorter if it is still two cars colliding. If it is two cars colliding it is the same setup - just described using a different inertial frame.

Further discussion is pointless unless the OP actually specifies the setup.

 

[phinds]

Further discussion is pointless unless the OP actually specifies the setup.

@Bobthefreeradical this now makes the third time in this thread that you have been specifically asked to clarify your question. Continued failure to do so will not go over well with the moderators.

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