Since I’ve never taken physics, I have no clue how to I would even go about solving something like this. But I was curious so I thought I’d ask. This question comes in two parts, the hypothetical and the real world.
Assume a rainstorm where it’s falling straight down and a wind of 0 MPH. Assume a frictionless glass surface for a windshield. At what speed (Y) must your car be going with a windshield angle of X when the water starts to travel upwards?
In the real world, can you actually get water to hover at a fixed point on your windshield?
No simple answer to your question. Lack of friction is a non issue as a drop of water doesn’t behave like a BB of ice skating on the surface. The size and shape of each raindrop , the surface tension and viscosity of water and the density of the atmosphere would all be factors. In fact if you had a “perfect” theoretical smooth surface it would react differently than glass.
You’re probably right in that there is no easily discoverable answer. There are enough variables in there to account for or eliminate that the answer ceases to have any real world application. Yet there must be one. Water does travel up my windshield at a certain speed. I just haven’t noticed what that speed is, given what conditions, because I figured that paying attention to the road might be a better use of my time.
That is, when moving forward, the rain looks to the car like it’s falling at a slanted angle. The faster the car goes, the closer to horizontal the angle is. At some speed for the car, the rain will be hitting the windshield precisely perpendicular to the glass. If you go faster, it will start going “up” the windshield.
Unfortunately this ignores wind turbulence, the effects of buildup of the water on the windshield, etc.
However, for the question we just need to find out how fast we need to go for the rain to hit the windshield at the appropriate angle.
At perpendicular to the glass, the angle from vertical for the rain is also X (some minimal geometry here). Hence the ratio:
(horizontal velocity)/(vertical velocity of rain) = tan(X)
emarkp, did you create those websites for this answer? If so, that’s cool! The first link doesn’t work, however.
Anyway, my original, fantasy-world problem assumed zero windspeed and the rain coming straight down. How does the speed of my car affect the angle or speed of impact?
I’m not sure about that. If I read the OP correctly, it asked about drops of water on the glass surface travelling up or down. If so, the initial speed of the raindrops are irrelevant. What matters is the gravitational force on the water and the wind resistance. If they cancel each other exactly (or rather, the component of each force tangent to the glass surface cancel each other) then the drops of water will “hover” - i.e. stay in one place on the glass.
This is not a trivial question, because wind resistance depends on the size of the water drops as well as their shape. Not only that but the shape of the water drops change depending on the wind.
The question gets even more complex, because the water is sitting partially in the ‘boundary layer’ of air flowing across the windshield. If the drop is small enough, it may be totallly within the boundary layer and not feeling a lot of wind resistance in the first place. That’s why sometimes you’ll see a tiny aphid or something on the windshield hanging on even when you’re going 60 mph - it’s inside the boundary layer, and no wind forces are acting on it at all.
Even small things like the design of the windshield wipers will have a huge effect, because they trip up the air flow and cause it to be turbulent. Another factor will be how clean the glass is.
I was thinking about emarkp’s answer, as to what would happen if there was no atmosphere.
In this case, the initial angle of impact of the particles will be the resultant vector of the velocity of the particle going down, and the velocity of the windshield hitting it. However, that doesn’t mean the particles will constantly crawl up the windshield. If the car is going fast enough, the resultant vector may well cause the particles to initially move upwards, but if the vehicle is going at a constant speed then the particle will travel up a ways, but the constant force of gravity will eventually pull it back down. Once it uses up its momentum from the initial collision, it will flow back down the windshield.
If all the particles were the same shape and mass, and the vehicle was going in a constant direction at constant speed, you’d get a ‘standing wave’ of particles at the maximum height, and fresh particles travelling up while other particles travel down. I imagine it would look pretty strange.
A raindrop ice sphere about the size of a .177" BB, something that would slide around on glass with extremely little friction My point was that friction in the traditional sense is different than a blob of water on a surface.
I’m not going to contribute to the answer on this because I myself am only a junior in highschool, but I am in Honors Physics II.
It seems there is another VITAL piece of information missing here. Such as the angle and the windshield. In a more streamlined car, for instance, the velocity needed to propel the rain upwards is far less than in, say, a school bus, with nearly vertical windshield panes.
