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Really dumb question that my dumb brain just wants answered....and I have like totally crap math skills or knowledge.

So yesterday driving home it started to hail.

GRAPE sized hail started coming down (wish I could tag the FB image I uploaded).

We pulled over under some trees as soon as we could.

My physics thoughts were this:

If hail is falling at the speed of gravity....

1) I assume that the 40+ MPH winds can accelerate the hail past the speed of gravity

2) If driving, does the speed your vehicle is going *add* to the speed of impact, or does it multiply the force of impact...or both or neither?

Depends on the direction of the wind. And whether or nor you're driving uphill, downhill, or flat.

Bran Dawri wrote:
Depends on the direction of the wind. And whether or nor you're driving uphill, downhill, or flat.

Oh good lord I don't know....lol.....don't try to confuse the mathless nerd even more!!

We were driving South on flat ground..initally at 55mph then 40mph then 20mph then 0mph....wind from the east at, say, 40mph.

Does that seriously help or were you being silly?

 TheMeanDM wrote:

1) I assume that the 40+ MPH winds can accelerate the hail past the speed of gravity
2) If driving, does the speed your vehicle is going *add* to the speed of impact, or does it multiply the force of impact...or both or neither?

1) There is no "speed of gravity". The force of gravity accelerates the ball, continually increasing its speed, until it balances the counter-acting force of air resistance and the ball reaches its terminal velocity. High winds in storm systems keep ice particles in the air longer, giving them a chance to accumulate more ice until they get too heavy and just fall. They also change the angle at which the hailstone falls.

2) If driving, you vehicle adds a horizontal momentum vector to the impact. How much it adds is determined by your speed, the angle the hailstone is falling at, and the angle of your windshield. The specifics would make a decent problem for first-year or high school physics students.

Correction: Terminal velocity, not escape velocity. Ice isn't going into space...

See!

I have already been corrected and educated! Thanks for the gravity lesson....I was totally messing that up!

Hail falls at terminal velocity for its physical type. Gravity accelerates it up to that speed, and no further. This depends on density and shape, among other things:
https://www.nssl.noaa.gov/education/svrwx101/hail

For hailstones that one would typically see in a severe thunderstorm (1-inch to 1.75-inch in diameter), the expected fall speed is between 25 and 40 mph.

If you are stationary, the hail hits you at that speed.

If you drive into it, you are adding a component of your horizontal vector to the vertical vector of the hail. This makes a diagonal speed, which is where the other questions come in. Wind adds a horizontal component to the hail's speed. Are you driving into the wind (adding to it) or away from the wind (cancelling it)?
And so on.

So to summarize, hail coming down is one vector by gravity. Car driving adds a horizontal one, wind coming from the left adds a third direction.
Or depending on how the mechanics work (high school was a *long* time ago) we might be able to consider the two separately - ie the front of the car is hit by gravity + forward motion, disregarding the wind as it's coming at nominally a right angle, and the side of the car by gravity + wind.

Just a caveat, the hail may not have enough time to reach 'terminal velocity,' depending on how high it formed and the ground elevation in the local area. It has to have enough time to reach that top speed.

Also, given the weather conditions, you might have some thermals coming off the warmer ground (especially pavement if the storm is rolling into an area that was sunny and hot), which will provide some upwards force that may also slow the hail. (though conversely, in areas adjacent to the thermals, the cold air going down will help falling hail accelerate to terminal velocity. This is one of the factors that may be contributing to the speed variation (25-40mph) in the link Skinnereal provided.

https://en.wikipedia.org/wiki/Thermal

I don't know anything about any of this, but it's pretty interesting to read.

Physics makes my head hurt. Of course, I follow along with it, because its the law!

Its pretty easy to use vectoral summation to calculate out the velocity based on the square of the hypotenuse sort of triangle math, as the down and side elements are (thank god) right angles to each other.
Since you square velocities to give their contributions to relative velocity's squared velocity the contribution from the downward fall (40 mph squared or 1600 mph^2) is directly analagous to the kinetic energy contributed from the fall vs the forward movement of the car. A 40 mph car adds just as much Ke, on that velocity triangle, gives a double square of the velocity -- thus double the kinetic energy exactly.
An 80 mph car wll add 6400 mph^2 to the 1600 from the fall of the ice, and increase that Ke from 800 to 4000, a five fold increase with a doubling of speed. Because the doubled speed quadruples that contribution of Ke from forward motion, of course!

So nutshell? Try to be hit by really small hailstones. Cause an increase in mass of the hailstone from 2 grams to perhaps 400 grams (those exist!) is a 200 fold increase in the impact kinetic energy, which (as you flee away at 80mph) means your car is suddenly being hit 1000 times as hard by the impacting sky ice. So if in doubt, hit only the very tiny hailstones.

Dukeofstuff wrote:
So if in doubt, hit only the very tiny hailstones.

I will tell the clouds to not hit my car with grape-sized hail next time...just pea sized


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You are all awesome, thanks for the replies!! Much appreciated!