Collision course

The combined energy doesn't matter, though, since we will be looking at the effect on each car.

Christian

 

Ok, im not going to say its wrong, but please explain how the damage caused by two cars colliding at 10m/s, is the same as if only one of them is in motion and crashes into the other with 10m/s ?

Because i can't see how that other 50 kj of energy just diappears !

Best regards, KBO.

 

If I can step in here and put forward something...

moving car hits stationary car. 50kj of energy is shared between them. For argument's sake call it 25kj each.

2 moving cars collide. 100kj of energy is shared between them. For argument's sake call it 50kj of energy each.

Am I correct?

 

Yes you are.

 

Thanks. So a collision between 2 moving cars does cause more damage to each car than a collision between a moving car & a stationary car? (when all moving cars have the same speed, mass etc)

 

Ricky that was exactly what i meant !

The combined force of an collision between two cars moving at 10m/s is 100 kj ! (each 50 kj)

And the combined force of a car crashing into a non-moving car at 10m/s is half that ! (50 kj) each 25 kj !

God bless your well formulated english, as opposed to my poor english !

Best regards, KBO.

 

I think I was answering too quickly to your first post. I didn't remember to count conservation of linear momentum in here. That complicates (or easier) things a bit...

Yes. We have to remember that momentum is conserved in this kind of collisions. So, after hitting a stationary car, both cars probably move and have therefore kinetic energy. Some of that kinetic energy possessed by moving car has been translated to KE of that stationary car. Therefore, all KE originally possessed is not consumed in this collision.

But when two identical cars (same speed and mass) colliding each other head to head and stick together (non-elastic collision), all KE is consumed in this collision and is probably divided equally to destruction of these cars.

 

Christian and KBO, please try to keep a lesson in motion physics a bit civil here! Do not provoke each other or call each other childish or unwilling to understand. You will have to face it Christian, not everyone knows as much as you.

 

Oh that was sorted out long ago !

However he insisted that two cars colliding with 10m/s arent going to creat more KE than if one is motionless ! Wich i just can't get to add up with anything !

Now lets see who is right, it doesnt matter who is, but this must be solved !

Best regards, KBO.

 

The point is, that the car in motion has the same amount of kinetic energy as each one of the two cars in motion, and upon impact, will suffer the same damage as each of the other cars.

To give another example - we have a tank weighing 50 tons. This is hit head-on by a shell weighing 10 kg. and travelling at 2,000 km./h. None of these numbers are unrealistic. The principle is the case as the two cars colliding.

The kinetic energy of the shell is ((10 * 555.556^2) / 2) = 1,543,209.877 J. This kinetic energy will damage the tank, we can all agree on that.

Now, let's imagine that the tank is moving at 50 km./h. This will give it a kinetic energy of ((50,000 * 13.889^2) / 2) = 4,822,530.864.

Thus, if the speed of the oppisite object is to inflict any damage, then the second scenario will mean that the tank is damaged more than three times as much from the impact of the shell because i is moving.

 

But in this case both objects dont weigh the same, so the other will simply because of its much larger mass stop the much smaller Shell dead in its tracks, much the same way as if it was standing still.

 

A good explanation on this can be found here: http://www.physicsclassroom.com/mmedia/ ... cthoi.html

KBO

 

When this has been sorted out how about some figures for a 38,000 ton Rodney colliding with a 50,000 ton Bismarck at 23kts?

:smok:

 

Yet we see that the shell penetrates in both cases, so there is a difference between the example and the illustration above.

Christian

 

Well the shell's energy is also distributed over a 'much' smaller area !

 

Ah, surely some of the energy from the moving car will be absorbed by the stationary car? Otherwise the stationary car would be undamaged...

 

Well, I asked my dad... (he is an engineer)

He said that when they are both going 60 kmh the impact is bigger.

If one is stationary alot of the energy would be absorbed into movement... it would move backwards when hit.

Also, Anthony Beevor writes about this tactic being used by Russian T-34's.
The crews would ram them into German Panthers and Tigers, kamikaze-like.

 

Wich is exactly what i implied !

Well at last we got it solved ! If an engineer doesnt know it, then who does ?!

Best regards, KBO.

 

After teaching some engineerstudents, I cant agree with you...

 

Well a 'good' engineer has to know, or else the world is on the wrong track !