two cars crashing at 50mph = 1 car crashing at 100mph?

No, according to Mythbusters ( http://mythbustersresults.com/mythssion-control ).

They crashed one car traveling 100 mph into a solid wall, and the compared that to crashing 2 cars traveling at 50mph crashing into each other, and say that the 2 cars crashing at 50mph is equivalent to 1 car crashing at 50mph against a solid wall.

I think that is flawed, here's why:

It should have been two cars hitting each other at 50mph.

Then compare that to one car moving at 100mph hitting a stationary car, not an unmoving solid wall that does not compress or have crumple zones.

That would be a far more accurate comparison than the one car hitting a solid wall that does not move.



What do you think? I'm not a mechanical engineer but something seems off to me in the wall comparison vs 2 cars crashing at 50mph against each other. A wall is never the same as a crumbling car at any speed...

Thinking more about this:

valid comparison scenarios:

1) 1 car traveling at 50mph, crashes into a solid wall also traveling at 50mph. Compare to 1 car traveling at 100mph crashing into stationary solid wall.

2) as per above, 2 car traveling at 50mph each crashing into each other. Compare to 1 car traveling at 100mph crashing into stationary vehicle.

A wall does not distribute or absorb energy like a car, so it seems to me Mythbusters made a huge flaw comparing 2 cars crashing at 50mph each, to a car traveling at 100mph crashing into a solid unmoving wall...

Any engineers or any one care to comment?? I think Mythbusters need to fix this test and do it right per my scenarios above.

50mph car crashing into a solid wall is just like crashing into a 50mph car going exactly opposite. Crashing into a solid wall at 50mph has the kinetic energies dispersed over the length of the crumple zone. Crashing in an EXACT car going 50mph is having 2 times the kinetic energies dispersed over 2 times the length of the crumple zone since there are 2 cars.
Steve

whiteNSXs said:

50mph car crashing into a solid wall is just like crashing into a 50mph car going exactly opposite.
Steve

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I'm not sure if it's exactly like a 50mph car crashing into a stationary car though... when you crash into a softer barrier, like a car that crumples, I think less damage is done versus crashing into a sold wall.

For instance, if you drop an egg 10 feet onto concrete side walk, the egg will crack. If you drop an egg 10 feet onto a mattress, it may not crack at all.

The reason is because the mattress absorbs energy differently than the wall.

I think the car is like the mattress, where it will "give" more and be a softer barrier, since it has crumple zones and can fold and dissipate energy, compared to a wall that does not dissipate energy at all and all the energy is transfered back to you.

whiteNSXs said:

50mph car crashing into a solid wall is just like crashing into a 50mph car going exactly opposite. Crashing into a solid wall at 50mph has the kinetic energies dispersed over the length of the crumple zone. Crashing in an EXACT car going 50mph is having 2 times the kinetic energies dispersed over 2 times the length of the crumple zone since there are 2 cars.
Steve

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Ok thinking about this some more, you maybe right, crashing a car into a solid wall at 50mph might equal two cars crashing into each other at 50mph each.

However, the mythbusters equated crashing a solid wall at 100mph to two cars crashing into each other at 50mph each. That again, is flawed, I think....

I think the better test is crashing a car at 100mph against a stationary car, and compare that to the two cars crashing into each other at 50mph each. This is because the stationary car crumples and dissipates energy itself, lessening the energy absorbed by the crashing car travling at 100mph. The solid wall does not crumple and does not help to dissipate energy.

NsXMas said:

I'm not sure if it's exactly like a 50mph car crashing into a stationary car though... when you crash into a softer barrier, like a car that crumples, I think less damage is done versus crashing into a sold wall.

For instance, if you drop an egg 10 feet onto concrete side walk, the egg will crack. If you drop an egg 10 feet onto a mattress, it may not crack at all.

The reason is because the mattress absorbs energy differently than the wall.

I think the car is like the mattress, where it will "give" more and be a softer barrier, since it has crumple zones and can fold and dissipate energy, compared to a wall that does not dissipate energy at all and all the energy is transfered back to you.

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The answer to your question lies ROUGHLY on the total kinetic energies per total length of them being absorbed. A 50mph car crashing into the same stationary car is entirely different from crashing into a same car traveling 50mph at 180 degrees. The total kinetic energies in these 2 cases are 2x different.
Steve

whiteNSXs said:

The answer to your question lies ROUGHLY on the total kinetic energies per total length of them being absorbed. A 50mph car crashing into the same stationary car is entirely different from crashing into a same car traveling 50mph at 180 degrees. The total kinetic energies in these 2 cases are 2x different.
Steve

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Yes I agree, I think... :tongue:

Botton line is I think the Mythbusters made a major flaw in their test and results.

NsXMas said:

Ok thinking about this some more, you maybe right, crashing a car into a solid wall at 50mph might equal two cars crashing into each other at 50mph each.

However, the mythbusters equated crashing a solid wall at 100mph to two cars crashing into each other at 50mph each. That again, is flawed, I think....

I think the better test is crashing a car at 100mph against a stationary car, and compare that to the two cars crashing into each other at 50mph each. This is because the stationary car crumples and dissipates energy itself, lessening the energy absorbed by the crashing car travling at 100mph. The solid wall does not crumple and does not help to dissipate energy.

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I thought you said in the beginning that "They crashed one car traveling 100 mph into a solid wall, and the compared that to crashing 2 cars traveling at 50mph crashing into each other, and say that the 2 cars crashing at 50mph is equivalent to 1 car crashing at 50mph."
So what Mythbusters said was correct.
Steve

whiteNSXs said:

I thought you said in the beginning that "They crashed one car traveling 100 mph into a solid wall, and the compared that to crashing 2 cars traveling at 50mph crashing into each other, and say that the 2 cars crashing at 50mph is equivalent to 1 car crashing at 50mph."
So what Mythbusters said was correct.
Steve

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Hmmm.... I wonder. Is it equivalent to 1 car crashing at 50mph to a solid wall, or equivalent to 1 car crashing at 50mph to an identical car sitting still?

I guess I'm thinking it's not apples to apples comparison. Maybe the entire comparison by Mythbusters is incorrect, is my thinking.

your guys posts crack me up as it equates to this: It's not a question of where he grips it! It's a simple question of weight ratios! A five ounce bird could not carry a one pound coconut.

It's Mythbusters. Science is not included. :biggrin:

NsXMas said:

Hmmm.... I wonder. Is it equivalent to 1 car crashing at 50mph to a solid wall, or equivalent to 1 car crashing at 50mph to an identical car sitting still?

I guess I'm thinking it's not apples to apples comparison. Maybe the entire comparison by Mythbusters is incorrect, is my thinking.

Click to expand...

My understanding of the statement is one car at 50mph crashing to a solid wall is equivalent to 2 cars crash head-on both at 50mph. So that's correct.
Steve

quack said:

your guys posts crack me up as it equates to this: It's not a question of where he grips it! It's a simple question of weight ratios! A five ounce bird could not carry a one pound coconut.

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Come again?!
Steve

Ok, your local resident ME here. First off, I'll say this. If you follow the pure and straight simple physics and engineering equations of kinetic energy, momentum, and elastic collisions, then yes, what the Mythbusters is saying is correct. I won't get into the equations (unless requested) however, I'll give you a simple analogy to help demonstrate the equations. If you have a clay ball and threw it at a wall, it would hit the wall, deform and stop at that position. The wall in return applied an equal and opposite force to completely cancel out the force of the clay ball and to completely stop it's movement (all deformations included). Now say I took two clay balls of equal size, weight and velocity and threw them at each other. If they are completely identical, where they would meet, they would completely cancel each other's motion out and stop moving. So if you focus on one of the balls you'll see that it encountered an equal and opposite force to completely cancel out the force of the ball moving and stopped it's movement in the exact same way the wall did. See picture below:



In this picture, the first frame is a standard clay ball moving with force (F) and to the right you'll see a wall that it is hitting. The only way to stop the ball perfectly in place is if it sees an exact and equal force in the opposite direction; we'll call that -F because it is the exact same value as F but in the opposite direction. If -F was greater than F, then the clay ball would bounce back and start heading in the opposite direction. If -F was less than F, it would still continue to move in the same direction (just slower) because there wasn't sufficient force to stop it.

In the next set of pictures below, we have the same clay ball moving with force F again. However, in the next frame to the right, it is hitting an identical clay ball. Note that again it is seeing an exact and equal force -F in order to stop it in place. Again, if -F was greater than F (the opposing clay ball had more mass or was faster; more energy), then it would have pushed the ball back. If the force -F was less than F (opposing clay ball has less mass or going slower; less energy) then the ball would continue to move right.

Where you have to use a bit of rationalization is that keep in mind that the force F isn't a perfect even number. Because the clay balls (or car) is what is called an inelastic object, the force it applies varies over time as the object deforms. So in reality, it would look like this:



If the object was an elastic object, like a billiards ball, the graph would be just a sharp spike to a fixed force F. But as the object deforms the force will vary over time. The key to note that in the case of the wall and opposing clay ball, the force of the equal and opposite force -F will match exactly the same as force F over the same period of time. It has to because the ball with stop completely at the end of the collision.

Hope that helps!

NsXMas said:

Hmmm.... I wonder. Is it equivalent to 1 car crashing at 50mph to a solid wall, or equivalent to 1 car crashing at 50mph to an identical car sitting still?

I guess I'm thinking it's not apples to apples comparison. Maybe the entire comparison by Mythbusters is incorrect, is my thinking.

Click to expand...

1 car crashing into a solid wall at 50 mph is not the same as 1 car crashing into an identical car stationary. The main difference will be the ending position of the moving car after the collsion. The car hitting the wall would be stopped at the wall (Force F will be cancelled out by an equal and opposite Force -F). The car hitting the stationary car would have travelled well past the point of collision which is a clear indication that the force in the opposite direction (-F) is less than the force of the moving car (F).

They aren't the same.

The kinetic energies of the two systems are different...

The 100mph system has double the energy, and even if it was moving car versus stationary car it'd be a ridiculous collision in comparison.

What if the car was traveling 50 MPH on a conveyor belt spinning at 100 MPH in the opposite direction?

Malibu Rapper said:

What if the car was traveling 50 MPH on a conveyor belt spinning at 100 MPH in the opposite direction?

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hahah that reminds me of this physics based myth buster: http://www.youtube.com/watch?v=IbRcg3ji_Pc&feature=player_embedded

The force required to stop a car moving at 50mph is 1/2 the force required to stop a car moving at 100mph. Regardless of where/how the stopping power is generated.
There would only be a difference if the stopping power was in excess of what's needed.

Thus, mythbusters are correct.

quack said:

your guys posts crack me up as it equates to this: It's not a question of where he grips it! It's a simple question of weight ratios! A five ounce bird could not carry a one pound coconut.

Click to expand...

I love swallows. :biggrin:

whiteNSXs said:

Come again?!
Steve

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Monty Python, Steve:smile:

Vega$ NSX said:

1 car crashing into a solid wall at 50 mph is not the same as 1 car crashing into an identical car stationary. The main difference will be the ending position of the moving car after the collsion. The car hitting the wall would be stopped at the wall (Force F will be cancelled out by an equal and opposite Force -F). The car hitting the stationary car would have travelled well past the point of collision which is a clear indication that the force in the opposite direction (-F) is less than the force of the moving car (F).

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Ok this is what I'm getting at.

I guess Mythbusters drew an incorrect analogy by saying two cars crashing at 50mph each is equivalent to 1 car hitting a wall at 100mph. The better analogy should be to compare it to 1 car hitting another car (stationary) at 100mph.

I'd be interested in the results of this test and see what it would look like.

jond said:

The force required to stop a car moving at 50mph is 1/2 the force required to stop a car moving at 100mph. Regardless of where/how the stopping power is generated.
There would only be a difference if the stopping power was in excess of what's needed.

Thus, mythbusters are correct.

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I don't see how this is correct. A car traveling at 100mph has exponentially more energy than a car traveling at 50mph. It's not a doubling, it's exponential.

By your post, mythbusters is wrong.

Mr Payne said:

They aren't the same.

The kinetic energies of the two systems are different...

The 100mph system has double the energy, and even if it was moving car versus stationary car it'd be a ridiculous collision in comparison.

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Speed is double, kinetic energy though is much more than double.

NsXMas said:

Ok this is what I'm getting at.

I guess Mythbusters drew an incorrect analogy by saying two cars crashing at 50mph each is equivalent to 1 car hitting a wall at 100mph. The better analogy should be to compare it to 1 car hitting another car (stationary) at 100mph.

I'd be interested in the results of this test and see what it would look like.

Click to expand...

The formula for kinetic energy is:

F = ½ m * v^2

So if a car of a fixed mass is hitting a wall, the total force applied to the wall (in an inelastic equation) will be equal to the force above.

If that same car were to be halted in the exact same spot by another car going in the opposite direction using an equal force, then the force would be exactly the same.

If you look at a car going 100 mph into a wall, notice how the velocity component is square and therefore the forces applied would be much greater.

So you are correct: The Mythbusters analogy for two cars hitting each other at 50 mph is NOT equivalent to one car hitting a wall at 100 mph. Rather it would be more correct to say that two cars hitting each other at 50 mph is more equivalent to one car hitting a wall at 50 mph.

However, it isn’t entirely accurate to say that two cars hitting each other at 50 mph is similar to one car hitting a stationary car at 100 mph. As you can see the forces are far greater at that speed. However, in reality it completely changes the dynamic of the crash. One major limiting factor is the coefficient of static friction of the stationary car will limit how much force the stationary car can apply back to the moving car. The maximum force the stationary car can apply back to the moving car will be far less than the force of the moving car as the tires can hold only so much force before they break loose. The 100 mph car will plow into the stationary car, but will end up pushing it very far and the coefficient of dynamic friction will act as a dampener (or airbag) for the moving car and will help distribute the force over a longer period of time. Two moving cars crashing into each other will be more of an impact (relatively speaking) where the force will immediately peak, whereas a car hitting a stationary car will have a much longer and drawn out force curve relative to time.

NsXMas said:

I don't see how this is correct. A car traveling at 100mph has exponentially more energy than a car traveling at 50mph. It's not a doubling, it's exponential.

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Is it? I thought it's just:
Force = Mass*Acceleration

Should be linear, no?