Metal peeling off the bottom

[DrPhyzx]

Another question: is this stainless? How thick? (It looks quite thin - maybe 1/32" - but someone with calipers would have to measure). At that thickness, you can almost bend stainless by hand.

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[tmeyer3]

Right... tell me all about your physics expertise, starting with how you have used Newton's second law to prove your point.

I love it when armchair scientists say stuff like "This is objectively false" and then offer up a bunch of unsubstantiated assertions.

The water from a pressure washer is going much faster than 40 mph and generating large multiples more force per unit area (thousands of PSI). Pressure washers can do a LOT of damage if you aren't careful.

https://pressurewashr.com/speed-of-water-from-pressure-washer-faq011/

Look at it this way. If that folded seam is presenting something like 2 in^2 to water from the road that is hitting it with the pressure of a typical pressure washer (3000 psi), that would be order 6000 lbs. of force, which would slow down the vehicle (which weighs order 6000 lbs) with 1g of deceleration, which is panic braking on dry pavement.

So... Newton's second law actually says "try again".

To be clear, I'm not saying this isn't damage from water. I'm saying
- this is occurring from a lot less force than you feel when driving through deep water
- Tesla could easily have bench-tested for this to identify the design flaw with pretty basic equipment.

I completely agree with you--pressure washers can certainly do a lot of damage and they are certainly decelerating at a greater rate! Science and mathematics has nothing to do with being offended--you said that a power washer generates more force than a puddle would. So sure, I'll nerd spar with you, but keep it objective or I'm out--personal attacks don't count as science, aren't appreciated, and aren't helpful. Peer review and legitimate critiques are always welcome!

Let's get to it!

A puddle that is deep enough to reach that covering contains a huge mass, generating enough force to bend that entire section in an instant. A very powerful pressure washer--one that I couldn't afford--with significant enough mass ejection and speed could do the same. But which would have more force?

Let's do some some napkin math together, let me know your thoughts:

Spoiler: rough math stuff

If we're comparing a pressure washer hitting a car versus a car hitting a 12-inch deep puddle at 40 mph, we're really looking at two very different kinds of force. A typical pressure washer blasts out water at around 2000 psi, which translates to nearly 7,000 newtons of force--but that force is super concentrated in a tiny area, like the size of a pencil eraser. That’s why it can strip paint or blast grime off metal. On the other hand, when a car hits a foot-deep puddle at 40 mph, it's pushing through a wall of water all at once. Even though water is a fluid, it still resists the motion and creates a big impact. If we do the math, the force from the car hitting the puddle is around 12,000 newtons, which is almost double the force of the pressure washer--but it's spread out over a much larger area and happens in a really short burst. So technically, the car creates more total force when it hits the puddle, but the pressure washer feels more intense because it’s focused on a tiny spot.

First, take a typical pressure washer. A decent one might run at around 2000 psi, which is pounds per square inch. If we convert that to metric, that’s about 13.8 million pascals (since 1 psi ≈ 6895 Pa). Now, the nozzle on a pressure washer is pretty small--maybe about the size of a pencil eraser. Let’s say that area is around 0.0005 square meters. To find the force, we use the basic pressure formula:
Force = Pressure * Area
F = 13790000 Pa * 0.0005 m^2 = 6895 N

That’s a lot of force, in a super concentrated spot, which is why pressure washers can damage paint or strip rust. Great!

Now compare that to a car going through a 12-inch deep puddle (which is about 0.305 meters) at 40 mph, or roughly 17.88 meters per second. When the front tire hits that puddle, it pushes a lot of water out of the way really fast. That resistance creates a big impact force. To estimate that, we can use the drag force equation:
Force = 1/2pv^2CA
p (water density) = 1000 km/m^3
v = 17.88 m/s
C (drag coefficient) ~ 1 for a blunt object
A (contact area) ~ 0.25m * 0.3m = 0.075m^2

Now plug it all in:
F = 0.5 * 1000 * (17.88)^2 * 1 * 0.075 ~ 12000 N

So even though it doesn’t look as aggressive as a pressure washer, the total force of a tire slamming into a deep puddle at speed is actually much higher--almost 12,000 newtons of versus 6,895 newtons of force from the pressure washer. But again, the big difference is how that force is applied. The pressure washer focuses all that energy into a tiny point, while the car is spreading it over the entire tire--or in this case the scoop-edge of that cover--and it happens in a flash. So while the car technically creates more force overall, the pressure washer feels way more intense in that small spot.

The total force generated is still much greater from a simple puddle, although more distributed, than a typical pressure washer as even a 12" deep puddle would have a mass far exceeding what a typical pressure washer can eject at once--and that's only calculating the force on the tires, not the undercarriage.

Cheers,
-from one armchair expert to another.

 

[tmeyer3]

Another question: is this stainless? How thick? (It looks quite thin - maybe 1/32" - but someone with calipers would have to measure). At that thickness, you can almost bend stainless by hand.

Confirmed to be alluminum . It's only meant to be an aeroshield

 

[Crissa]

this isn’t really a good comparison. Very different situations.

Sure. But the post I was answering was saying water was incapable of this level of force.

Clearly it is.

-Crissa

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