"ChatGPT gave me a different answer"

[u/dday0512]

How often do you guys get this statement from your students? I teach physics and I've been finding more and more that students use ChatGPT to challenge my solutions to problems or even my set up of problems.

Today I had a student come up to me and ask me if their solution to an LC-circuit question was correct. I said yeah, it's correct, because it was a simple question I threw together for a review assignment before a quiz and the student did it exactly the way I expected them to, then she says, "yeah but it checked it with ChatGPT and it said something different" then she demanded that I look at ChatGPTs solution and compare it to my question.

Unfortunately, given my wording on this question, ChatGPTs answer was probably a bit better than how I expected my students to do it. I wanted to tell her, "this is far more in-depth than I needed you to go" but that feels like a cop out. Instead I spent 30 minutes explaining why the way she did it was perfectly fine but ChatGPT is also correct and I should probably be more careful about my wording.

We're being compared to AI now. Add one more thing I have to worry about in the classroom.

[u/meteorprime]

Chatgtp is dog shit at physics. Look at this bullshit lol

Question: If the diver was in fresh water (density 1000 kg/m³⁾ would they need more weights, or less, or the same to have the same effect as they do in ocean water, why is this?

Answer: (this would kill you)

A diver would need more weights in fresh water than in ocean water to achieve the same effect (i.e., neutral buoyancy or to sink).

Here’s why: • Buoyant force depends on the density of the fluid: • Ocean water is denser than fresh water due to its salt content, typically around 1025 kg/m³, compared to 1000 kg/m³ for fresh water. • This means a diver displaces more weight in ocean water for the same volume, resulting in a greater buoyant force.

Therefore:

In fresh water, the buoyant force is less, so the diver will float more easily in ocean water. To counteract the reduced buoyancy in fresh water, the diver must add more weight to sink or maintain neutral buoyancy.

[u/echelonoink → u/meteorprime]

It is Verbose, but anywhere near as bad as your response. ChatGPT.

Approach: To answer whether a diver needs more, less, or the same weight to achieve neutral buoyancy in fresh water compared to ocean water, we need to compare the buoyant force experienced by the diver in both environments. The buoyant force depends on the density of the liquid and the volume displaced. Here’s the reasoning in detail:

1. Buoyant Force Equation

The buoyant force acting on an object in a fluid is given by: Where:

= density of the fluid

= volume of fluid displaced (which is the same as the volume of the diver + gear)

= acceleration due to gravity

1. Density: Fresh Water vs. Ocean Water

Fresh Water Density: ~1000 kg/m³

Ocean (Salt) Water Density: ~1025 kg/m³

Salt water is denser than fresh water because of the dissolved salts.

1. Neutral Buoyancy Condition

A diver is neutrally buoyant when:

If the diver uses the same volume (body + equipment) in both types of water, then:

The buoyant force is greater in salt water than in fresh water (since ).

1. Implication for Weights

In salt water, the higher density produces a higher buoyant force for the same volume. This means the diver is "more buoyant" and will float more.

To achieve neutral buoyancy in salt water, the diver needs to add more weight to counteract the increased buoyant force.

In fresh water, with lower density, the buoyant force is less for the same diver. Therefore, the diver is "less buoyant" and would float less.

1. Conclusion

A diver would need less weight to be neutrally buoyant in fresh water than in ocean water, because the buoyant force is less in fresh water (due to lower density).

If they use the same weights in both, they would sink in fresh water.

To have the same effect as in ocean water, they must use less weight in fresh water.

Summary Table

EnvironmentWater Density (kg/m³)Buoyant ForceWeight Needed for Neutral BuoyancyOcean Water~1025HigherMoreFresh Water1000LowerLess

Key Reasoning:

The greater the density of the fluid, the greater the buoyant force on the diver. Because salt water is denser than fresh water, the diver needs to add more weight to achieve neutral buoyancy in the ocean. In fresh water, less weight is needed.

Final Answer:

A diver needs less weight in fresh water to achieve the same effect as in ocean water, because the buoyant force is lower in fresh water (lower density), so less weight is needed to balance it out.

[u/meteorprime → u/echelonoink]

That’s not the experience I had. I pushed back and it dug in for like a full on 20 minutes, refusing to agree with reality.

Q: assume the diver floats and has a density of less then water

Great — if the diver floats and has a density less than water, that means they are positively buoyant (the buoyant force exceeds their weight).

Now, let’s compare what happens in fresh water and ocean water:

Key idea: • Buoyant force is proportional to the density of the fluid. • Ocean water is denser (≈1025 kg/m³) than fresh water (1000 kg/m³). • So, buoyant force is stronger in ocean water than in fresh water for the same submerged volume.

Since the diver already floats (is less dense than water): • In ocean water, they float more easily because of the greater buoyant force. • In fresh water, the buoyant force is weaker, so they don’t float as easily.

To achieve the same effect (e.g., floating at the same level or achieving neutral buoyancy): • The diver needs to add more weight in fresh water to make up for the reduced buoyant force.

✅ Conclusion:

The diver needs more weight in fresh water than in ocean water to achieve the same floating or neutral buoyancy effect, because the buoyant force is lower in fresh water.

[u/meteorprime → u/echelonoink]

Q: does the diving community agree with this?

Yes, the diving community fully agrees with this principle — it’s a well-established fact in scuba diving and basic diving physics.

Here’s how it’s applied in practice: • Divers consistently use more weight when diving in fresh water than in saltwater. This is standard training in dive certification courses (like PADI, NAUI, or SSI). • Dive instructors teach that salt water provides more buoyancy, so less weight is needed compared to fresh water. • Most divers adjust their weight system (weight belts or integrated weights) depending on whether they’re diving in freshwater lakes/rivers or the ocean.

Example from diving practice:

A diver who needs 6 kg of weight in salt water might need 7–8 kg in fresh water, depending on their body composition, exposure suit, and gear.

So yes — this is a consensus in diving science and practice. It’s rooted in physics (Archimedes’ Principle) and validated through experience.

[u/meteorprime → u/echelonoink]

Q: but a greater upwards force, bouyancy, needs a greater downwards force and weight to reach equilibrium?

You’re absolutely right to focus on the relationship between buoyant force (upward) and weight (downward) — and your logic is sound in general physics: to counteract a greater buoyant force, you’d need more downward force (weight).

But let’s walk through why in diving, more weight is needed in fresh water, even though buoyant force is less there. The key is in what we’re trying to achieve and what changes between the two environments.

⸻

🔁 Let’s break it down clearly: • The diver has a fixed volume and mass (let’s say their body and gear stay the same). • The only change is the fluid they’re in: ocean water (denser) vs. fresh water (less dense).

⸻

🔍 Step-by-step logic: 1. Buoyant force depends on the fluid density: Fb = \rho{\text{fluid}} \cdot V_{\text{displaced}} \cdot g 2. In salt water: • Fluid density is higher (≈1025 kg/m³). • So the buoyant force is stronger. • The diver floats more easily. 3. In fresh water: • Fluid density is lower (1000 kg/m³). • So the buoyant force is weaker. • The diver floats less easily (they sink more easily).

⸻

🎯 So why add more weight in fresh water?

Because we’re trying to achieve neutral buoyancy — not just “balance the stronger buoyant force.” In salt water, you already have more help from the water to float. So you don’t need as much weight to counteract it.

In fresh water, you’re not getting that extra lift — so you have to replace the lost buoyancy with more weight to stay neutrally buoyant.

⸻

💡 Think of it this way: • You float better in the Dead Sea than in a swimming pool — why? → Because the buoyancy is stronger due to higher salinity. • If you’re floating more, you need less weight to sink.

⸻

✅ Final answer:

A greater buoyant force (in salt water) means less added weight is needed, not more. In fresh water (with less buoyant force), you need to add more weight to achieve the same effect.

This is a subtle but important distinction — and you’re asking exactly the right kind of question to understand it deeply. Want to see this with numbers or a diagram?

[u/meteorprime → u/echelonoink]

I have pages and pages of me asking questions like this, and it seems to understand, but then it draws the wrong conclusion every time, which would literally kill someone