Calculating Force to Lift Cuboidal Object in Water: 1.1 Kg

[foggyeyes]

I have a cuboidal object of dimensions 15cm x 7cm x 10cm.
Its volume is 1050 cm³

Since density of water, is 1g cm^-3, the object will have to weigh 1.05 Kg to sink.

Assuming it sinks to the bottom and weighs 1.1Kg, and has to be lifted by 4, electric DC Motors.
Will the motors have to exert enough force to lift the WHOLE 1.1 Kg, or Do they have to lift 1.1 Kg - 1.05 Kg, ie. 0.6 Kg.

 

[sophiecentaur]

Good question. I haven't looked at the numbers but the principle is clear.
As the object rises through the water, some water is moving down, to fill the space below and is doing work on the object. Hence, the motor only needs to supply the remaining work. Upthrust does the rest.
The energy to provide this work, from the water, was 'put there' when the object originally sank. No violation of energy conservation. :-)

 

[Buckleymanor]

Ignoreing friction and presuming the numbers are correct they have to lift 0.5 Kg.

 

[foggyeyes]

Its a practical question, not a theoretical one.
Adding the impact of friction to it, can you estimate how much work needs to be done ?

Also, how can i modify the cuboid to give it an aerodynamic shape?

 

[Buckleymanor]

Its a practical question, not a theoretical one.
Adding the impact of friction to it, can you estimate how much work needs to be done ?

Also, how can i modify the cuboid to give it an aerodynamic shape?

Well you can still ignore friction in the case of it being a practical question.
If it don't lift useing 0.5 Kg of force any extra amount of measured force used to lift. will tell you how much friction is in the system.
A gentle tug with a spring balance should give you results.

 

[sophiecentaur]

If it really is a practical question then you would have to specify how fast the object would be rising in the water before you would know the drag. If you don't care how long it takes then there you can ignore the viscosity of the water because frictional forces in the motors would be greater. As this is not a theoretical matter but a practical matter, you'd also need to measure the performance of the motors etc.
BTW, you should be talking in terms of N, and not kg in your calculations because it's forces that are involved and you would need that in order to determine work and energy (in J). That may account for your apparent error when you wrote 0.6kg in the op? There may be an excuse for those poor souls who have to use pounds and things but, for SI users, there need be no confusion; a value of 9.81 for g will give you the right answers.

 

[foggyeyes]

It doesn't matter how fast it rises, as long as it isn't terribly slow.
I know, ill probably have to experiment.
I meant 0.06 Kg. Was a typo.
If i have 5V DC Motor, How do I calculate how much force it generates ?

 

[sophiecentaur]

You DON'T mean 0.06kg.
You are discussing a FORCE - i.e. the weight of 0.06kg so talk in terms of Newtons (i.e. approx 0.6N). Thats what Newtons are there for. You won't be able to discuss Work or Energy until you do.
Pardon my shouting but you really must behave like a proper Scientist! :-)

As for your motor - just saying it is a 5V motor tells you nothing about the force it can produce. You need to know its torque when stalled or, possibly its power rating. Both of these are available for many motors that you can buy. Otherwise you will need to measure them. That's another little measurement job to keep you occupied during the long winter evenings.

 

[sophiecentaur]

Why don't you relieve us of the suspense and tell us a bit more about the system you're discussing? It would be better fun all round and, who knows, you may get some other ideas.

 

[ak47_hardik]

I'm also working with him . we are trying to make something that goes down 40cm in water and comes up on its own after 2-3 mins . we figured out the going down and automation part but we still need help with the coming up ... so we thought of using small waterproof fans as rotors or propellors to get our robot to come up.

 

[sophiecentaur]

Rather than pulling up with strings, why not use a swim-bladder type system (like fish use) or a flotation tank (like submarines use) which varies the density of your submersible? It strikes me that the load is constant so it would be relatively easy to have a pump inside the 'craft' which pumps out water from a flotation tank, to rise and let's it into sink. Or, you could have a motor to compress / expand an air-filled bladder to make it rise or sink. Even baking soda could work!
This system could be autonomous and not need any connection (suspension) with the surface. There are many small pumps I know of, which operate at 12V (not the 5V of your choice, perhaps, but who knows? I have never needed to look)
The advantage of using a flotation / displacement system rather than fans /propellers is that it wouldn't need to be driven constantly to keep it up.

 

[ak47_hardik]

the idea seems a nice one ... but do you think it will be easy to accommodate a floatation tank as well as the circuit that we are making that will activate after 2mins ?

Also could you tell me how will baking soda help ? i know that it reacts with any acid to give out CO2 but how will our circuit be able to trigger off the reaction?

both of your ideas seem nice but before i can talk further i will have to go through the working of ballast tank(floatation tank)...i also will have to discuss it with my teammate.

 

[foggyeyes]

We're not pulling with strings.
The motor is mounted on the submersible.
The pumps make a good second option. Procuring tanks for them is a bit hard.

Also, we don't need the submersible to stay up/. We only need part of it to resurface.
After that , it can sink back down, if it wants too.

 

[sophiecentaur]

The baking soda idea was a bit of nonsense really but if a motor could just tip over a container of the stuff into some lemon juice you could achieve the appropriate fizz. It would be a one-off though.

For a small system like yours, I would image that you could do better with a swim bladder approach. You could pump air (from the internal cavity - and I assume it's sealed) out into four balloons arranged low down around the sides. That would bring the craft up to the surface in a level attitude. There are small 12V air pumps available for aquariums that should do the job and they're quite cheap. You could incorporate a valve which could let the bladders deflate and sink the craft when you wanted to.
Having just read your last post, the bladder system seems perfect - and just like the 'real thing'. It just relies on having a fixed mass of air inside the box (plus bladders), so that the pump just increases the volume of water displaced.

 

[foggyeyes]

We have height of 30cm to work with.
Also, your idea requires a fair bit of engineering and technical skill. We don't possesses that.
Securing and sealing the balloons is quite a task. Pressure is another issue.
Also the mechanism has to be time activated. We've got that in place (hopefully), but it adds a dimension of complexity to the problem.

 

[sophiecentaur]

You wouldn't be talking 'high' pressure. Just enough to part-inflate these balloons. You would need the thing to be water-tight. in any case, I would have thought. The only sort of seal you would need would be achievable with a rubber bung in a round hole, with a tube through the middle (like a Chemistry delivery tube). The balloons could be tied over the ends of the tubes.
I couldn't quite see how your fan / propeller arrangement would operate. Would the fans be fixed to the sides? Can you actually get hold of waterproof versions?
I can appreciate your worry about "engineering skill" but it wouldn't involve anything more skilled than the existing design, I think.

 

[foggyeyes]

Tieing is a quick and dirty way of doing it. It might work, but its prone to failiure. I feel that a ballast tank filled with water, in which the water gets replaced by ai from the main body, through a simple pipe.

 

[ak47_hardik]

yes,we are attaching them to the sides using l-brackets ... we are using smaller versions of exhaust fans(sort-of) ... we are using 4 of them that will work in tandom ...they will generate enough torque to lift it up ... that is how we planned it out to work

 

[sophiecentaur]

And are they waterproof?

 

[foggyeyes]

I should hope so.
They look like they are.

 

[sophiecentaur]

Tieing is a quick and dirty way of doing it. It might work, but its prone to failiure. I feel that a ballast tank filled with water, in which the water gets replaced by ai from the main body, through a simple pipe.

Bladder or ballast tank would both be quite workable. In both cases, the body of the container must be sealed - not difficult to achieve but I can appreciate that it may seem so.
I just thought that bladders around the sides would be nice and stable.

 

[sophiecentaur]

I should hope so.
They look like they are.

You need to test them if you have them already or enquire. What is their intended use?

 

[Buckleymanor]

I'm also working with him . we are trying to make something that goes down 40cm in water and comes up on its own after 2-3 mins . we figured out the going down and automation part but we still need help with the coming up ... so we thought of using small waterproof fans as rotors or propellors to get our robot to come up.

Could you adapt this.http://en.wikipedia.org/wiki/Cartesian_diver
Think the link works now.

 

[foggyeyes]

Your link isn't working.
Please post again.

 

[ak47_hardik]

Bladder or ballast tank would both be quite workable. In both cases, the body of the container must be sealed - not difficult to achieve but I can appreciate that it may seem so.
I just thought that bladders around the sides would be nice and stable.

i don't think the competition we are participating allows for the size of the body to change once the body is fully in water.Also it does not sound so stable.I like the tank idea better.The problem that I thought of was is not technical but after reading the challenge most teams would have gone on the internet and searched for the working of a submarine.Hence,the rotor idea that we initially came up is more original and might fetch us more points.But anyways your idea sounds great and i we will definitely use it incase our idea doesn't work

 

[foggyeyes]

@Buckleymanor : the link miraculously works now.

 

[sophiecentaur]

Could you adapt this.http://en.wikipedia.org/wiki/Cartesian_diver
Think the link works now.

That is, effectively, a swim bladder but controlled by squeezing the bottle, which is not autonomous and very difficult to achieve for a large tank.

 

[sophiecentaur]

i don't think the competition we are participating allows for the size of the body to change once the body is fully in water.Also it does not sound so stable.I like the tank idea better.The problem that I thought of was is not technical but after reading the challenge most teams would have gone on the internet and searched for the working of a submarine.Hence,the rotor idea that we initially came up is more original and might fetch us more points.But anyways your idea sounds great and i we will definitely use it incase our idea doesn't work

I don't think stability would need to be a problem - the flotation could be right near the top if you only need the very top to get above the surface. The bladders could, of course, be inside. I have been thinking some more. You are obviously making a watertight compartment to contain the electrics. If the compartment were to be cylindrical, one end (the lid) could be a diaphragm of, say, 1mm neoprene (held on with a large 'jubilee clip', around the whole thing) A motor could then push a ram against the inside of the diaphragm to increase the volume of the air and displace some extra water.
Many deep sea exploration craft use your proposed 'fan' method because buoyancy is a serious issue at great depth. I'm still worried about the suitability of your fans as I know that waterproof propeller drives on boats are not a trivial matter. Unless they are sold as really waterproof then you won't be able to rely on it.

Whatever happens, please let me know. A PM would do if the results are embarrassing!

 

[betel]

If your process does not have to be reversible, bladders or a ballast tank would be the easiest way I think.

You could glue the ballast tank to your robot with some water soluble glue (probl. starch works). In the water the glue will start to dissolve in the water and at some point be to waek to hold the ballast to the body.

A funny idea for bladders would be to put a small amount of dry ice in a large cavity inside the body. At the start the cavity should be partially filled with water to allow the body to sink to the bottom. The water should not touch the dry ice, but at the bottom, some mechanism (lever, etc) pushes the ice into the water. The ice will melt, emitting loads of CO2 and pushing the water in the body out via some hole at the bottom. This should be enough to lift your body. Question is if you can get your hands on dry ice and whether you could construct a clever mechanism.

Funny contest though.

 

[ak47_hardik]

we are not alllowed to use any kind of remote control or mechanical control once the robot has entered water .The robot has to come up on its own once it has entered the water .Also can you suggest us how to make a circuit that will activate after 2.5 mins .The circuit that we planned didnt work out too well.soo can you suggest us something