Road gritters (A new physics question)

[GoNz0]

Do not try to outrun the grit. That's impossible. Instead... only try to realize the truth.
There is no grit.
Then you'll see, that it is not the grit that moves, it is only yourself.

[scaryjim]

... The grit is magically hanging in the air, moving at 0 mph. ...

Actually, this probably isn't as daft as you think it is. The grit must be spat out of the gritter at a set speed, but the gritter itself is moving forwards. That means that there will be a speed at which the grit leaves the gritter at the exact speed that the gritter is moving, meaning the grit ends up with zero momentum... doesn't it? I'm pretty sure that if you walk past a gritter (or more to the point stand still and turn away as the gritter goes past) at least some of the grit hits you moving in the same direction as the gritter...

[Rave]

Gritters that I have observed spray a fan pattern of grit out the back to ensure good coverage of the road. I'm sure it all has the same forward velocity as the gritter at the point it leaves; some pieces of grit also have a sideways velocity having been chucked sideways by the dispersal mechanism.

Upon leaving the gritter, the grit hits the road, maybe bounces a few times, and then comes to rest on the road. Unless the gritter is moving extremely slowly, I can't see how any of it would ever be travelling in the opposite direction to the gritter.

However, the deceleration of the grit will be proportional to the speed of the gritter- or even the square of the speed of the gritter, if it's moving at speed down an A-Road (wind resistance increases with the square of the speed, and it will be mostly wind resistance slowing the grit down at speeds above 20mph IMO.

So- it depends. In town with the gritter moving <20mph, I'd creep past it slowly, since more grit hitting you at low speed is unlikely to do any real damage. On a dry A-Road, I'd get round it as fast as possible, to minimise time in the firing line.

[jim]

Actually, this probably isn't as daft as you think it is. The grit must be spat out of the gritter at a set speed, but the gritter itself is moving forwards. That means that there will be a speed at which the grit leaves the gritter at the exact speed that the gritter is moving, meaning the grit ends up with zero momentum... doesn't it? I'm pretty sure that if you walk past a gritter (or more to the point stand still and turn away as the gritter goes past) at least some of the grit hits you moving in the same direction as the gritter...

That's true actually... I was only mentioning it as a proof-of-concept, but thinking about it yes. In fact, I think MythBusters did an episode recently where they basically spent all day driving a truck forwards and firing a ball out of a cannon at the back with the aim of matching the speeds and having the ball hang in mid air with no lateral velocity.

Doesn't really change the nature of the problem - since the speed of the grit moving backwards (or indeed forwards) is negligible when you take the speed of the car into account. Certainly true though.