# How much weight required to submerge this object?

• backsdraw
In summary, the conversation discusses the best way to submerge a 2000L plastic tank for a short film, using a combination of pulleys and weights. There is a debate about whether using a pulley will provide enough force to pull the tank down, and whether using water-filled drums as weights will be effective. Ultimately, it is decided that using steel weights is the best option.
backsdraw
Hi all,

So A quick question for all the smart minds out there to ponder (plus I'm stumped and need help).
I'm helping a friend out with a short film and we are trying to be cost efficient.

I need to submerge a 2000L plastic tank (circular and approx 1.5 high) and then release it to make it look like it's popping out from the ocean.
The tank we have is 3 metres deep and we have 3000kg of steel weights which we have a friend making a low profile steel rack for stacking the weight and to attach pulleys.
We will run a line from the tank through the pully attached to the weight stack and back to a forklift to pull the tank under.
Or we get a turfer and do a direct pull from the weight stack.

I slightly understand the principle of water displacement and realize that the steel will weigh less under water due to it's volume/water displaced.
A rough guesstimate would be 600L of displacement so the weights will be approx 2400kg or 2.4 ton underwater.

I know this weighs more then the object we are trying to submerge so excuse the silly question...but is that enough weight firstly to pull the tank directly down with the turfer...
Or if we go with the pulley and forklift method, because it's 2 forces from above, does the weight at the bottom need to be doubled?

Hope you can understand my explanation and thanks for any help.
Backsdraw

As long as the pulley doesn't jam, you should be okay. You're remembering the mechanical advantage of two pulleys from way back in school giving you an ability to lift twice the weight of the actual pulling force.

Thanks Bystander...so I'm thinking of the 2:1 principle or something like that... but it doesn't apply in this situation because it's a single pulley purely to enable pull from another direction? (so essentially the pulley isn't there and I'm just pulling a direct weight).
If this is correct, would it not matter at all where the pull once through the pulley was from then? ie...directly above, directly to the side, 45 degrees etc etc...I assume it doesn't but just checking...

For some reason known to only my brain, I still think I don't have enough weight on the bottom but that's all I can get!

You don't want to hook up to the people who drive in tractor pulls, or the weight will come off the bottom just because your tank won't sink quickly enough, but a nice slow sedate drawdown in quiet water should be very well behaved.

Haha...I'll be the calm driver...thanks again.

And just one more question...what happens if you use steel 44 gallon drums filled with water for weights...since the volume is the same as the weight, does this just completely cancel it out and the actual underwater weight is only the steel or is there some other calculation for this scenario?

backsdraw said:
the actual underwater weight is only the steel
No magical advantage --- that's why novelists always fill oil drums with cement to dispose of victims.

Aha...so it's pointless to use a water filled drum as an underwater weight...cheers

## 1. How is the weight of an object related to its ability to float or sink?

The weight of an object is directly related to its ability to float or sink in a fluid. This is because of Archimedes' principle, which states that the buoyant force acting on an object is equal to the weight of the fluid that the object displaces. If the weight of the object is greater than the weight of the fluid it displaces, it will sink. If the weight of the object is less than the weight of the fluid it displaces, it will float.

## 2. How can I calculate the weight required to submerge an object?

To calculate the weight required to submerge an object, you will need to know the density of the fluid it will be submerged in and the volume of the object. Multiply the density of the fluid by the volume of the object to get the weight of the fluid displaced. This weight will be equal to the weight required to submerge the object.

## 3. Can the shape of an object affect how much weight is required to submerge it?

Yes, the shape of an object can affect how much weight is required to submerge it. Objects with a greater surface area will experience a greater buoyant force and therefore require less weight to submerge. Objects with a smaller surface area will experience a smaller buoyant force and require more weight to submerge.

## 4. What other factors besides weight can affect an object's ability to float or sink?

Besides weight, other factors that can affect an object's ability to float or sink include the density of the object, the density of the surrounding fluid, and the shape and size of the object. Objects with a lower density than the fluid they are submerged in will float, while objects with a higher density will sink. Additionally, objects with a larger surface area will experience a greater buoyant force and therefore float more easily.

## 5. Can the weight required to submerge an object change in different fluids?

Yes, the weight required to submerge an object can change in different fluids. This is because the density of the fluid will affect the buoyant force acting on the object. For example, an object may float in water but sink in a more dense fluid like saltwater. The weight required to submerge the object would be greater in the saltwater due to the increased buoyant force.

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