Thoughts From the Front Porch

Which causes more damage; colliding with a stone wall, or head-on with another identical vehicle going the same speed?  We hear it all the time–”they ran together with a combined speed of (whatever), so it’s no wonder nobody survived”.

If you recall a previous Phunky Physics story, a moving object must disperse all its energy before it can come to a stop.  If you have two such objects, both must lose all that energy; twice as much as one.  According to common belief, a head-on would be geometrically more destructive.  Is this fact, or an illusion?  Let’s do the science.

True, there is twice as much energy, but it is divided among two objects.  It’s possible to draw a theoretical line between both objects, beyond which neither will pass  because the opposing object won’t let it.  Therefore, all the energy of both objects will be dispersed on that line, but in no way shared by neither object.  The equal forces on both sides of said line assure that the line will remain still while those objects exert their kinetic energy on each other until all is used up.

Let’s assume that two identical cars begin moving straight toward each other from a dead stop.  Their speed reaches, say 60mph each, and they hit each other head-on.  Using our convenient “damage units” from a previous story, that means each vehicle has to dump 3600 DUs, total 7200DU.  If you divide that by the number of cars, it’s still 3600 per, which is the same energy either car would have expended by hitting our theoretically immovable wall.

This implies that the total expenditure of inertia would be the same as both objects crashing into an immovable stone wall, separately.  All the energy gets expended, and the impact point never moves.   For the damage to be any greater would mean that extra energy was created or transferred from somewhere else.  Simply butting two cars together won’t do that.

Makes sense, doesn’t it?

Views From Benny Hill is a series by Jerry Smith

Views From Benny Hill