So if it's say 1500 kg trucks you get Car 1 issuing 1,875,000 Joules of energy into the other object. Car 2 transfers 7,500,000 Joules of energy into the other car. Since both cars come to a complete stop, the kinetic energy must be transfered into the other vehicle as damage, not movement.

No. 1,875,000 plus 7,500,000 joules (9,375,000 altogether) are converted from kinetic energy to "damage" (sound and friction). But there's no rule that says that all of car 1's energy goes into car 2 (rather than being partially dissipated in car 1 itself), or vice versa. In the zero-momentum frame, the situation is completely symmetrical. Each car will sustain about 4,687,500 joules of damage.

If you'd like, I'll scan and post the pages from my physics book.

If it genuinely concurs that all car 1's energy will leap out into car 2 causing car 1 no ill-effects, and vice versa, then just scan the title page. It'll let us know which authors to avoid

Peter

The car travelling twice as fast will do about 4 times as much damage to the slower car as it will get.

By this argument, you could hit a stationary car going as fast as you like and only the stationary car would be damaged. "It should be obvious to even the most casual observer" that this is complete bollocks.

Peter

See? This is why I gave up on physics in University.

I so wanted to say something like this, but after a fortnight of lots of my common sense being refuted I thought I should hang back and wait...

I definitely agree that the force on A from B has to equal the force on B from A. So the damage really all comes down to construction, angle/position of collision, speed difference, and what happens to the cars after the impact.

OK, sorry I'm so late into this thread. I would let it die, but I think I can shed some light on the subject (I am a mechanical/strength engineer, or at least I used to be).

The problem with this discussion is the word "damage" and some confusion about the nature of "inelastic" collisions. These things are related. "Inelastic" means that the cars don't bounce off of one another. It means that all of the energy of the impact is *absorbed* by the bodies rather than remaining kinetic energy.

In an elastic situation (as if the cars were billiard balls), the two cars bounce off of one another and sustain *no* damage. Their speeds change is all. Like billiard balls. I can't remember the relative speeds used in the example, but in a perfectly elastic situation, the kinetic energy is conserved and the slower vehicle speeds up and the faster one slows down.

The inelastic situation is different. When we speak of inelastic collisions, we are entering into the realm of "deformable body mechanics". That means the bodies in question "deform" or go beyond the elastic region of the material and "bend" or "break". As they do so, they absorb the energy of the collision. [Technical folks will remember that the total energy absorbed is the area of the hysteresis in the stress/strain curve.] This is why cars are designed to "crumple". By designing a car to crumple, the energy of an impact is dissipated by the crumpling, if you will. If you're clever about how you design the vehicle to crumple, you can lengthen the duration of the energy transfer quite significantly so that the forces are significantly smaller.

So, you can see the relationship between the inelasticity and "damage". Damage is what happens when the kinetic energy of the impact is absorbed by the bodies and the materials deform or fracture.

You can see that "damage" is a function of design. *If* the vehicles are of identical design (which the won't be, because the front of one car is hitting the rear of another) and the impact is totally symmetrical, then they should sustain nearly identical damage. This is never the case, obviously.

The speed that the vehicles are travelling relative to the road is irrelevant. As mentioned above, it is their speed relative to each other that matters.

The high school physics teacher is correct that the faster car has more kinetic energy, but what matters is the relative speed and the energy that must be absorbed to bring the cars to the same speed. All things being equal (which they never are), the cars will share the impact energy and sustain the same damage.

To answer the question about the baseball bats: it is because the systems are not of identical design, as mentioned above. Swing two bats at each other and see what happens.

Hope this helps,

Jim

Ah bugger it all. This is why I fix computers for a living. This all just makes no sense. I mean, When I was playing baseball, if I ran full throttle and creamed the catcher, He sustained far more "damage" than me. More than a couple were knocked out, and I barely felt it. As was mentioned in other threads, I was slight of build as a child, so if it was mere mass, I'da always lost, especially since most catchers were simply power hitters and therefore a little on the beefy side.

Basically, I DO understand what you're saying, it just doesn't make sense to me. So I give up.

I'm guessing you "aimed" at those poor beefy catchers? You didn't lead with your face, did you? You might've hit them low, or with your feet? Did you guys just bump bellies?

:-)

Grumble..... I dunno, I just kept running, ignoring them. 90% of the time they dropped the ball and I was safe.