Road gritters (A new physics question)

[simonfitba]

After reading MSIC's fans, and physics thread I have a question that has always puzzled me.

When driving, is it best to overtake a road gritter quickly or slowly? What does less damage to my car?

It's tricky because there is time of exposure to the grit, speed of the stones etc.

What do you think?

[Salazaar]

Personally I don't think there's any good way to overtake a gritter but if one's out then you're probably in icy conditions so slowly would be preferable.

[duc]

Slow down and allow some room. You have to remember that the road ahead has not been gritted.

[simonfitba]

For the purpose of this question, we'll say road conditions are perfect. (Gritter only out because cold snap is predicted).

I'm only wondering about the damage caused by the grit hitting my car.

[aidanjt]

Low speed, low revs. That'll reduce the energy of the grit kick-up.

[PeterStoba]

Surely the grit leaves the gritter at the same speed wether your going fast or slow?

So driving past it faster would reduce the time you are exposed to the grit, causing less damage?

I'm obviously missing something?

[jim]

I'd say slowly. Whatever happens, you're going to get hit with *some* grit.

The question is, how much damage will it cause. The slower you go, the less impact it will have on the car, so even if you get hit with thousands of bits of grit, it probably won't have a noticeable effect.

If you're going quickly, you might only get hit by a quarter as many pieces, but if each of them takes a speck of paint off, then your car's going to look like it's just staged a hedgehog barrel-rolling competition.

[simonfitba]

Surely the grit leaves the gritter at the same speed wether your going fast or slow?

So driving past it faster would reduce the time you are exposed to the grit, causing less damage?

I'm obviously missing something?

But if you speed past the gritter, won't the grit hit your car harder?

[jim]

Surely the grit leaves the gritter at the same speed wether your going fast or slow?

So driving past it faster would reduce the time you are exposed to the grit, causing less damage?

I'm obviously missing something?

Yeah, the grit leaves the gritter at the same speed, but impact momentum will incorporate the speed you're driving at.

Consider an equivalent situation:

You walk down a road into oncoming traffic at 3 mph. You are hit by a car coming in the opposite direction. Would you like it to be travelling 5 mph or 80 mph?

[Biscuit]

i tend to just plant my fot as soon as all the slow pokes get out the way... i go for the quicker you go through the less pieces hit your car approach myself.

[aidanjt]

Yeah, the grit leaves the gritter at the same speed, but impact momentum will incorporate the speed you're driving at.

Consider an equivalent situation:

You walk down a road into oncoming traffic at 3 mph. You are hit by a car coming in the opposite direction. Would you like it to be travelling 5 mph or 80 mph?

Indeed, the noise of the grit hitting is disconcerting, but really the slower you go past the less force the car and grit collides.

[PeterStoba]

I can't see how it'd hit the car harder? it's still travelling at the same speed and hitting the same car side on.

Your 5mph or 80mph puts you in the position of the grit? your not bothered about the grit, it's the car...

[lego batman]

Grit > paint though.

[SeriousSam]

I've just sat here for half an hour going through the mechanics equations needed to work this question out and I suddenly realised that which variables are important is entirely dependant on how you define "damage". Then you'd have to decide on a relative scale on which to place all of your definitions. As an example would damage to paintwork be worse that grit in an airfilter or somewhere else vunerable. This relative importance would be entirely dependant on both the type of car and the drivers own perception of risk. So the number of answers is dependant on the number of permutations of drivers and cars... even just in the UK thats going to be a very big number.

Setting that aside to look at one instance the final answer is actually going to give you an optimum speed with every other speed making the damage worse. This is because some types of damage will be worse if you drive through faster and others if you drive through slowly. Consequently the answer to whether or not you to drive faster or slower actually requires you to set an initial velocity against which to measure faster or slower.

So in the end it's all relative

[jim]

I can't see how it'd hit the car harder? it's still travelling at the same speed and hitting the same car side on.

Your 5mph or 80mph puts you in the position of the grit? your not bothered about the grit, it's the car...

It only hits the car side on for a fraction of a second, when the car is perfect in-line with the gritter.

Let me try and rephrase it.

You're following a gritter. Grit is constantly flying up and hitting your car, even when you're some distance behind it. The grit isn't actually moving that quickly itself - let's call it 3 mph.

So if you parked near it, with the car moving at 0 mph, the car would be hit by grit at 3 mph.

Now let's consider a different scenario. The grit is magically hanging in the air, moving at 0 mph. A car is driven into the grit at 30 mph - so the speed of the collision is 30 mph. Which bit was moving is irrelevant - the speed of the collision is what matters.

So if we now combine the two - the grit is moving backwards at 3 mph, and the car is moving forwards at 30 mph, the speed of the collision is 33 mph. So the faster the car moves, the faster the collision is. The faster the collision, the more energy that is expended on the body of the car as the grit is brought to an abrupt stop. That energy will be passed into the paintwork, and if sufficiently high will rip it off the metal. So the slower you go, the lower the speed of the collision, and the less energy there will be, so the less damage that will be caused.

Admittedly, for a fraction of a second you'll be perfectly side on and grit will hit the side of the car, at which point the car's speed is more or less negligible, but it's such a short space of time that it's not worth worrying about. The approach is the problem.

[PeterStoba]

It only hits the car side on for a fraction of a second, when the car is perfect in-line with the gritter.

Let me try and rephrase it.

You're following a gritter. Grit is constantly flying up and hitting your car, even when you're some distance behind it. The grit isn't actually moving that quickly itself - let's call it 3 mph.

So if you parked near it, with the car moving at 0 mph, the car would be hit by grit at 3 mph.

Now let's consider a different scenario. The grit is magically hanging in the air, moving at 0 mph. A car is driven into the grit at 30 mph - so the speed of the collision is 30 mph. Which bit was moving is irrelevant - the speed of the collision is what matters.

So if we now combine the two - the grit is moving backwards at 3 mph, and the car is moving forwards at 30 mph, the speed of the collision is 33 mph. So the faster the car moves, the faster the collision is. The faster the collision, the more energy that is expended on the body of the car as the grit is brought to an abrupt stop. That energy will be passed into the paintwork, and if sufficiently high will rip it off the metal. So the slower you go, the lower the speed of the collision, and the less energy there will be, so the less damage that will be caused.

Admittedly, for a fraction of a second you'll be perfectly side on and grit will hit the side of the car, at which point the car's speed is more or less negligible, but it's such a short space of time that it's not worth worrying about. The approach is the problem.

I see what you mean now. All I could think of being side on for some reason.