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I'm having a discussion with several folks about the results of a head on collision of two similar vehicles that have about the same weight and crush properties. I stated that if both vehicles collide head on, at 60 MPH, the impact force and damage to each vehicle would be roughly the same as hitting a solid unmoveable object at 60 MPH, because each vehicle absorbs roughly 50% of the damage.

They claim that because both vehicles are traveling at 60 MPH, their combined speed of 120 MPH would increase the damage to twice what it would be if the vehicle hit a solid object at 60 MPH.

How can I explain this to them?

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Phi for All    4746

How can I explain this to them?

Newton's Third Law of Motion.

Total force is doubled, but then you have to divide it between the two cars.

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Psycho    33

Newton's Third Law of Motion.

Total force is doubled, but then you have to divide it between the two cars.

So if that is the case and it is based on newtons 3rd law, why is that the case? Surely there should be 2 50mph opposite reactions if they crash head on?

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TonyMcC    87

So if that is the case and it is based on newtons 3rd law, why is that the case? Surely there should be 2 50mph opposite reactions if they crash head on?

See if this helps:- Imagine a solid wall that is free to move along the ground. Perhaps in the form of a vertical thick steel plate on casters. Imagine the two cars coming together one on each side of the wall. The wall would not move and both cars will suffer the same amount of damage as if they had hit a solid immovable wall.

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Phi for All    4746

So if that is the case and it is based on newtons 3rd law, why is that the case? Surely there should be 2 50mph opposite reactions if they crash head on?

There are. Two 50mph crashes totaling 100mph of impact (2x60=120 for the OP's example), spread along a straight line and divided equally between the two cars. The force from car A acts on car B, and the force from car B acts equally on car A. The force starts at the front end of each car and travels to the back of each car, leaving each with a single 50mph (60mph) impact.

Think of it this way. Imagine clapping your hands together with just enough force where it almost hurts, but not quite. Now imagine slapping just one hand against the wall with the same force. And finally, imagine slapping just one hand against the wall with twice the force. If it almost hurt the first two times, what would it feel like with twice the force?

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md65536    307

I'm having a discussion with several folks about the results of a head on collision of two similar vehicles that have about the same weight and crush properties. I stated that if both vehicles collide head on, at 60 MPH, the impact force and damage to each vehicle would be roughly the same as hitting a solid unmoveable object at 60 MPH, because each vehicle absorbs roughly 50% of the damage.

They claim that because both vehicles are traveling at 60 MPH, their combined speed of 120 MPH would increase the damage to twice what it would be if the vehicle hit a solid object at 60 MPH.

How can I explain this to them?

Another way to think about it or explain it is that the head-on with each traveling 60 MPH would be like a car traveling 120 MPH slamming head on into a stationary car (in neutral). This would be softer than hitting say a wall (lower deceleration over a longer period of time compared to say brick wall), and the total mass of 2 mangled car should continue moving at half speed (60 MPH -- lower overall deceleration compared to say a brick wall).

If you explain it using the example of a 120 MPH car hitting a movable stationary car, it's clear to see that this isn't the same as hitting an immovable obstacle. With the former your velocity changes by 60 MPH; with the latter it changes 120 MPH.

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Thank you everyone. Excellent info and I agree with all. Now, if I can just convince the others. ☺