Flash Overs

[plugwash]

In the absence of background information, a film of a tennis ball hitting a 1 ton block of concrete at 5mph would not enable you tell whether one, the other or both were moving. However, I presume that you would not expect the tennis ball to suffer the same amount of damage if it were dropped at 5mph onto the concrete block as it would if the concrete block were dropped at 5mph onto the ball - or would you?!

It all depends on what the ball is sitting on.

If the ball is sitting on a surface that can barely hold it's weight and a heavy concrete block is dropped on it at 5mph i'd expect the ball to be fine. The surface supporting the ball would break and the ball would be accelerated so it was moving at a speed greater than that of the concrete block.

On the other hand if the ball is sitting on a concrete driveway which is effecitvely an immovable object then things are going to end up much worse for the ball.

Similarlly in a low speed collision between a person and a car i'd expect the biggest hazard would not be from being accelerated by the impact but from being caught between the car and something that is effectively an "immovable object".

 

[ban-all-sheds]

Energy is measured in ft-lbs, torque in lb-ft.

[sigh]

And 1 lb-ft of torque is what?

Why, could it possibly be the torque created by one pound force acting at a perpendicular distance of one foot from a pivot point?

When I think of the times I've tightened wheel nuts on a car using a wrench about 1' long, I'm inclined to disagree that the energy in a handshake is anywhere near 400 ft-lbf.

[/sigh]

 

[JohnW2]

It all depends on what the ball is sitting on. If the ball is sitting on a surface that can barely hold it's weight and a heavy concrete block is dropped on it at 5mph i'd expect the ball to be fine. The surface supporting the ball would break and the ball would be accelerated so it was moving at a speed greater than that of the concrete block.

Indeed. In fact, whether one talks vertical or horizontal, the question of what is restraining the 'object hit' is crucial. If there is no restraint at all, the only damage done will be that resulting from inertia.

What's getting me confused is the energy side of all this. Start with a person and a car with no relative velocity between them. Viewed from any fixed reference point, both would be stationary, so both would have zero kinetic energy. Now inject energy and accelerate one of the objects until it has a velocity of 5mph relative to the fixed reference. It will now have kinetic energy (relative to that fixed reference, and also the other, 'stationery', object), equal to the energy that had to be put into it to accelerate it. Now the 'moving' object hits the 'stationery' one. Some damage is done to both the objects, consequent upon the kinetic energy being converted to work, as the applied force moves through a distance as the object 'crumples', and both objects then fairly rapidly 'come to rest' (relative to the fixed reference point, and each other). The kinetic energy of both objects (relative to the fixed reference, or each other) is now again zero - so all the previous kinetic energy has been converted into work (hence damage, heat etc.).

The damage done is a consequence of conversion of the kinetic energy into work. Had it been the person that had been accelerated to 5mph, then the total amount of kinetic energy (which vanished after the impact) would be much less than had the car been accelerated to 5mph. Put another way, far more energy would have had to be put into the car (as compared with the person) to accelerate them to 5mph. So why would the amount of damage done be the same in the two cases?

Kind Regards, John

 

[ban-all-sheds]

What's getting me confused is the energy side of all this.

You say that, but then go on to give an example which does not indicate any confusion.

Start with a person and a car with no relative velocity between them.

So, a moving car with the person in it?

Viewed from any fixed reference point, both would be stationary

Not any fixed reference point. Viewed by an observer standing on the pavement as the car drove by, neither car nor occupant would be stationary.

But I digress - the rest of what you wrote was fine.

The damage done is a consequence of conversion of the kinetic energy into work. Had it been the person that had been accelerated to 5mph, then the total amount of kinetic energy (which vanished after the impact) would be much less than had the car been accelerated to 5mph. Put another way, far more energy would have had to be put into the car (as compared with the person) to accelerate them to 5mph. So why would the amount of damage done be the same in the two cases?

It would not be.

Maybe this question (to all) would help.

You have to choose between undergoing one of the following:

1) Being hoisted (in a prone position) to a height of 13' 4½" above the front of one of these which has been stood on its tail, and dropped onto it.

2) Standing with your back against a solid sturdy wall and having the same vehicle driven into you at 20mph.

Which would you prefer?

 

[JohnW2]

What's getting me confused is the energy side of all this.

You say that, but then go on to give an example which does not indicate any confusion.

Start with a person and a car with no relative velocity between them.

So, a moving car with the person in it?

That's not what I was thinking of. I was thinking of the person and car separate, both with no velocity relative to a fixed reference point and with no velocity relative to one another.

Viewed from any fixed reference point, both would be stationary

Not any fixed reference point. Viewed by an observer standing on the pavement as the car drove by, neither car nor occupant would be stationary.

See above for explanation of what I meant.

But I digress - the rest of what you wrote was fine.

....So why would the amount of damage done be the same in the two cases?

It would not be.

Well, I'm glad you agree, but there's still something in here which is confusing me.

It seems to me that some of the problem is probably due to the fact that we are (I am) assuming that, following a car-person collision, both will 'come to rest' very soon after the impact and thus all of the kinetic energy will have been converted into something else (damage, ultimately probably heat etc.), which may well not be true (in the absence of braking etc.). If the same collision occured in space, both would carry on moving at much the same rate as the car had been moving, hence with little loss of total kinetic energy.

Kins Regards, John

 

[plugwash]

I think the key is that the "initial collision" with a car at 20MPH (and without your back to a wall) is no worse than running into a wall at 20MPH.

BUT in the case of runing into a wall a lot of the energy will be dissapated in that initial collision and any secondary collisions will be likely be less bad than the intitial collsiion.

In the case of being hit by a car there is still a lot of energy in the system after the initial collision. Depending on the precise geometry of the accident you could easilly end up under the car with the remaining energy being dissipated by dragging your body along the ground while subjecing it to crushing downard forces.

IIRC modern cars are deliberately designed to increase the chance that people go over the top rather than underneath to avoid precisely that scenario.

 

[Spark123]

I don't think it matters too much, having a face full of electrical splatter is going to hurt!

 

[ban-all-sheds]

I think the key is that the "initial collision" with a car at 20MPH (and without your back to a wall) is no worse than running into a wall at 20MPH.

The wall is vital.

When you are dropped onto the front of the truck, your speed relative to it is 20mph, and you come to rest almost immediately.

You need the wall to ensure that when the truck is driven into you at 20mph it too stops almost immediately.

In the case of being hit by a car there is still a lot of energy in the system after the initial collision. Depending on the precise geometry of the accident you could easilly end up under the car with the remaining energy being dissipated by dragging your body along the ground while subjecing it to crushing downard forces.

IIRC modern cars are deliberately designed to increase the chance that people go over the top rather than underneath to avoid precisely that scenario.

Absolutely, and my example was not meant to be relevant to real-life collisions between vehicles and people, it was just to show that the mass of the moving object is of great significance.

 

[JohnW2]

I think the key is that the "initial collision" with a car at 20MPH (and without your back to a wall) is no worse than running into a wall at 20MPH. ... In the case of being hit by a car there is still a lot of energy in the system after the initial collision.

Yes, I think that's the answer, which is essentially where I got to in the final paragraph of my last post. In fact, in both cases, the kinetic energy 'lost' (converted to something else) during that initial impact will only be that which is involved in the crushing/squashing/whatever of the person (and, to a much lesser extent, what hit the person) as a result of a 20mph impact with something hard. In the case of being hit by a car, what happens to the car's residual kinetic energy following the initial impact will, as you imply, depend upon many things. In many/most practical situations, of course, much of that energy will be turned into heat as the result of braking at some point during the incident. If the person merely 'stuck' to the front of the car as it continued to move, one assumes that no further damage ought to arise.

Kind Regards, John

 

[JohnW2]

I don't think it matters too much, having a face full of electrical splatter is going to hurt!

It is, indeed, and I think the main issue is likely to be thermal (i.e. burning) rather than anything to do with the force/energy of the flashover - which is probably therefore largely a red herring (although someone did choose to introduce it!).

Kind Regards, John.

 

[ban-all-sheds]

It is, indeed, and I think the main issue is likely to be thermal (i.e. burning) rather than anything to do with the force/energy of the flashover

I think you'll find that the thermal component has energy.

 

[JohnW2]

It is, indeed, and I think the main issue is likely to be thermal (i.e. burning) rather than anything to do with the force/energy of the flashover

I think you'll find that the thermal component has energy.

Of course it has, but you know jolly well what I meant - despite all this discussion about cars or walls hitting people's faces, facial damage due to an electrical flashover is going to be dominated by burns, not 'mechanical' ('blast') injuries.

Kind Regards, John.

 

[ban-all-sheds]

I knew what you meant.

But one thing this topic has highlighted is the importance of saying what you mean with accuracy.

As well as the generic issues with equating energy in different forms, and of introducing a time dimension and making it work.

I might well be able to generate 1hp for 30 seconds (IHNI where that would put me on the scale from couch potato to Chris Hoy), but I could not lift 7.37 tons through a distance of 1' in that time.

 

[JohnW2]

I might well be able to generate 1hp for 30 seconds (IHNI where that would put me on the scale from couch potato to Chris Hoy), but I could not lift 7.37 tons through a distance of 1' in that time.

Eh? Of course you could - that's what 1hp for 30 seconds (aka 16,500 ft.lb) means. You'd obviously need some gears/pullies/whatever, but if you could generate 16,500 ft.lbs of work in 30 seconds, then you could lift 7.37 tons through 1 foot in that time.

Looked at from that viewpoint, I suspect that (no matter what gearing)working at that sort of power is not within the capability ballpark of either you or me With a manual ('chain') hoist, it would undoubtedly take me more than 30 seconds to lift the average car engine by 1 foot - and that certainly ain't 7.37 tons

Kind Regards, John.

 

[Porque223]

but I could not lift 7.37 tons through a distance of 1' in that time.

Yeah, you can, using some type of machine that does not add power but does add mechanical advantage. A scissor jack comes to mind and you could use such a device to measure the hp your arms can put out.

These machines in the mechanical domain are like a transformer that does 'impedance' matching in the electrical domain, as does a megaphone in the 'pressure domain.'

If you really want to get weird in the rotational mechanical domain, how and why does a crankshaft harmonic balancer work?