Does Throwing an Object Overboard Affect Pond Water Level?

AI Thread Summary
Throwing an object overboard from a boat affects the water level in a pond. When the object is in the boat, it contributes to the weight and volume of water displaced. Once thrown overboard, the object sinks, displacing less water due to its higher density compared to water. The conclusion is that the water level will fall when the object is removed from the boat. This reasoning clarifies the misconception that a denser object would displace more water when thrown overboard.
TomK
Messages
69
Reaction score
14
Homework Statement
Archimedes' Principle.
Relevant Equations
upthrust = weight of water displaced
density = mass/volume
volume of object submerged = volume of water displaced
mass x g = weight
I was asked this question:

Assume you're sitting in a boat (you and the boat, together, are a mass M) which also contains a heavy object (of mass m). The boat (inc. you and the heavy object) is floating in a pond (a fixed body of water, rather than open sea). If you throw the object overboard (it sinks) and remain seated in the boat, will the water level of the pond rise, fall, or stay the same?

Below, I have given my proposed answer, and I wanted to ask if it made sense:

When the object is in the boat: weight of displaced water = (M+m)g = (M+Vd)g
This is because masses M and m are both floating.

volume x density = mass
d = density of object
V = volume of objectWhen the object is thrown overboard: weight of displaced water = (M+1000V)g
This is because the volume of submerged object = volume of water displaced by object.

Assumes density of water = 1000kgm^-3Comparing the two equations: We know d (density of object) is greater than 1000, because the object sinks in water.
When the object is thrown overboard, the weight (and volume) of displaced water decreases.
Therefore, the water level must have fallen.

Is this the correct conclusion to make? Could you prove this scenario in-practice, or would your boat not stay afloat with the object in it?
 
  • Like
Likes berkeman and Delta2
Physics news on Phys.org
TomK said:
Is this the correct conclusion to make?
The reasoning is correct.
TomK said:
would your boat not stay afloat with the object in it?
Do you think that having an object denser than water in a boat would necessarily make it sink?
TomK said:
M+1000V
Better to write M+ρwV, or, if you must substitute numbers, quote units: M+(1000V kg/m3).
 
Thank you for the response. This answers my question.

I think my misconception came from thinking that the object (more dense than water) would have to be sufficiently heavy to displace more water when thrown overboard.

However, my working shows that, under the assumed circumstances, m could really be any value and the water level would still change (though it would have to be heavy to make a measurable difference in real-life).
 
Thread 'Voltmeter readings for this circuit with switches'
TL;DR Summary: I would like to know the voltmeter readings on the two resistors separately in the picture in the following cases , When one of the keys is closed When both of them are opened (Knowing that the battery has negligible internal resistance) My thoughts for the first case , one of them must be 12 volt while the other is 0 The second case we'll I think both voltmeter readings should be 12 volt since they are both parallel to the battery and they involve the key within what the...
Thread 'Correct statement about a reservoir with an outlet pipe'
The answer to this question is statements (ii) and (iv) are correct. (i) This is FALSE because the speed of water in the tap is greater than speed at the water surface (ii) I don't even understand this statement. What does the "seal" part have to do with water flowing out? Won't the water still flow out through the tap until the tank is empty whether the reservoir is sealed or not? (iii) In my opinion, this statement would be correct. Increasing the gravitational potential energy of the...
Back
Top