Physics Question of the Day

Did the water level go up, down or stay the same.

  • Increase

    Votes: 0 0.0%

  • Decrease

    Votes: 14 73.7%

  • Stay the same

    Votes: 5 26.3%
  • Total voters
    19
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harrisonm

harrisonm

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This was a question on a physics test when I was in college over 50 years ago. I liked it so much; I have always remembered it.

You place a rowboat in a swimming pool and fill it with large rocks to the point where it is about to capsize. Then you mark the water level on the side of the pool. Then you remove all the rocks from the boat and throw them into the pool. All the rocks are now at the bottom of the pool. You mark the water level on the side of the pool again. Did the water level go up, down or stay the same.
 
I believe the water level would go down compared to the rocks being in the boat. It would seem the hull of the boat has more surface area and would displace more water with the rocks in it, than the rocks just being in the water. Sounds good anyway. lol
 
Weight of rocks combined with the total volume of the boat vs total volume of the rocks alone...
I'd have to agree with RRR
The water level would go down.
 
Last edited:
Interesting question. I see Rob/RRR's point but I'd err to the side that the water level is the same.
 
RustyRatRod said:
I believe the water level would go down compared to the rocks being in the boat. It would seem the hull of the boat has more surface area and would displace more water with the rocks in it, than the rocks just being in the water. Sounds good anyway. lol
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Yup. The boat floats and so is displacing an amount of water equal to the weight of the stones. If the stones sink, they're displacing less water than they weight, which is less than when they're in the boat which is why they.. sink.
 
The question revolves around the density of the rocks. If its the same density as water. the level stays the same, it's greater the water lowers, if lower it rises.
The boat displaces weight of water.
The rocks in the pool displace volume of water.
The rocks sinking indicates they are denser than water.
That is my story and I'm sticking to it.
 
If the rocks cracked the concrete or tore the liner at the bottom of the pool, water level went WAY down. If no damage to bottom of the pool, the water level went down slightly from when the rocks were in the boat.
 
What I find interesting is what possible reason could a person have to waste their time thinking about such crap. Ask a physicist he’ll crunch the numbers and give you an answer
 
Ahoey said:
What I find interesting is what possible reason could a person have to waste their time thinking about such crap. Ask a physicist he’ll crunch the numbers and give you an answer
Click to expand...
Yes, a physicist can calculate the needed angles, and speed to put a 1.62oz 1.68" diameter round ball with dimples into a 4.25" diameter hole maybe 100 yards away.
Tiger Woods can put in the hole in less than 60 seconds.
 
Ahoey said:
What I find interesting is what possible reason could a person have to waste their time thinking about such crap. Ask a physicist he’ll crunch the numbers and give you an answer
Click to expand...
True. A Real Physicist would not worry about a boat in a pool with rocks. But remember that this was a question on a physics test in college. I always thought it was an interesting question, and I thought I would share it.
Every Physicist in the world has taken a basic physics class and learned concepts like this as a stepping stone towards their eventual PHD. It's just like math. I like math. I took a lot of it and used it a lot over the years. You can have a PHD in math, but at some time, you had to have taken Algebra. Algebra is easy, but it is a stepping stone to geometry, trigonometry, calculus and beyond.
 
Ahoey said:
What I find interesting is what possible reason could a person have to waste their time thinking about such crap. Ask a physicist he’ll crunch the numbers and give you an answer
Click to expand...

No physicist would “crunch the numbers” to solve this problem. Absolutely no numbers are needed to solve it, just a solid understanding of the concept of density.

And that’s why the question is asked, because if you think you need numbers to answer the question, you don’t fully understand the concept of density or how it relates to real world problems.

Putting numbers into equations is easy, knowing which numbers and which equations requires practical understanding.
harrisonm said:
True. A Real Physicist would not worry about a boat in a pool with rocks. But remember that this was a question on a physics test in college. I always thought it was an interesting question, and I thought I would share it.
Every Physicist in the world has taken a basic physics class and learned concepts like this as a stepping stone towards their eventual PHD. It's just like math. I like math. I took a lot of it and used it a lot over the years. You can have a PHD in math, but at some time, you had to have taken Algebra. Algebra is easy, but it is a stepping stone to geometry, trigonometry, calculus and beyond.
Click to expand...

Exactly. Being able to answer the question shows that you understand the concept and its practical application, which is more important than the monkey skill of plugging numbers into an equation.
 
OK, I had to change my vote. Originally thought 'stay the same', but then looked up the answer and it's indeed 'level decreases'. I understand why now as well, so learned something.
Click to expand...
 
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