Thread: car vs telephone pole

i'm trying to figure this out:

i have a telephone pole made out of pine, 5m tall, 28cm in diameter, with
a mass of 570 kg/cu.m.
which means the telephone pole weights 2576kg.
the car weights 2359kg, and travels at 16m/s, which means it has an energy of 301,9kj.
so in theory, if the vehicle hits the pole in outer space, the car would come to a complete stop, and the telephone pole would be accelerated at 16m/s.
on earth this is not how things happend.

the car hits the telephone pole, and the telephone pole remains in place, because of friction.
another factor is that the energy of the car is only expended on a small fraction of the telephone poles surface area, about 0.03sq m, which grows to 0.15sq m as the car crumples around the pole.

so my question is, how much pressure does the telephone pole exert on the car?
and how do i calculate the friction and structural integrity of the telephone pole into this?

oh and another question:

is this formula
the opposite or reciprocal, or whatever its called of

Originally Posted by dejawolf

... ... i have a telephone pole made out of pine, 5m tall, 28cm in diameter, with a mass of 570 kg/cu.m ... ...

It should be over 800Kg/ M**3

Originally Posted by dejawolf

... ... the car would come to a complete stop, ... ...

Not necessarily complete stop. Newton's second law of resultant force states that The rate of change of momentum of a body is proportional to the resultant force acting on the body and is in the same direction, not necessrly it transfer all the force.

Originally Posted by dejawolf

... ... so my question is, how much pressure does the telephone pole exert on the car? ... ...

According to your statement it shoud excert 301,9kj exactly!!!!!!!!!

It should be over 800Kg/ M**3

no, its a pine tree:
http://www.simetric.co.uk/si_wood.htm

right then. another example.

i have a tank which weights 62300kg, traveling at the same speed, hitting the same tree.
its energy is 7981kj.
the trees relative energy is 304,13kj.

question:

is it correct to subtract the trees energy from the tanks energy to find out how much the tank slowed down from the trees impact?

Conservation of energy (and momentum) holds in any case. Draw out the situation immediately before and immediately after the collision. In both pictures the total energy (and momentum) will be equal. Having said that, I'd have a tough time working it all out myself.

Originally Posted by MagiMaster

Conservation of energy (and momentum) holds in any case. Draw out the situation immediately before and immediately after the collision. In both pictures the total energy (and momentum) will be equal. Having said that, I'd have a tough time working it all out myself.

I would watch telling someone that conservation of energy is a fact, without some actual testing.

Take a heavy steel lump hammer and hammer in a steel stake into the ground. Now take a raw hide mallet of equal size and hammer in the stake. If someone was buried in the ground, a foot under the surface, you might hit him with the steel rod with one blow. Because the rod just positively accelerates through the soil. Like you hammered it through Jello.

If you want to say that it is because the raw hide transferred all the energy and the steel lump hammer did not. I am ok with that.
But you really need to have a grasp of this actuality of just how much energy the steel lump hammer does not deliver. Before you tell someone that both hammers of same weight will deliver the same force.

I did a test when I was a kid. I took a couple large rocks and threw them into an old floating wet wooden boat bottom. They just went buuuuump. They really did not even bounce. One or two rolled the others just kind of plunked into the dirty muddy bottom.

Then we took the same size rocks with the same force. And threw them into the water. And we rocked and damaged boats in the canal as they raised and lowerd, and moved their mooring weights up and down as much as eight to ten inches. The boat that was damaged was sunk bellow a bulk heading support and then raised up underneath. Destroying the side of the boat.

We actually did this with one giant rock for each scenario, and had the same effects.

So although as Newton says, for every action there is an equal and opposite reaction, he meant the whole universe would absorb and create the equality. Not necessarily the immediate area.

Sincerely,


William McCormick

Originally Posted by MagiMaster

Conservation of energy (and momentum) holds in any case.

You can't really use conservation of energy in an inelastic collision, as some of the energy will go into crushing the metal. Conservation of momentum will hold, though.

Oh right. Well, what he said then. (I'm not much of a physicist.)

Originally Posted by dejawolf

is this formula
the opposite or reciprocal, or whatever its called of

It's an equivalent formula, ie, both of them say the same thing.