Calculate objects position in water affected by buoyancy

Harris von Hassell · 2012-05-15T11:52:58

Hello there, im currently working on a framework to create my own games and wanted to add buoyancy physics to it. I understand the fundamentals, but still have problems to implement it the right way into my framework. I calculated the resulting force on the object by subtracing the gravity force from the buoyancy and worked out the resultgin acceleration. I know that the density of the water gets bigger as the object rises in the water, but i dont know when it gets lower...but these are only some problems...anyways ill post my code here(written in C#) and hope somebody will be able to help me creating realistic buoyancy physics. In the moment the object just accelerates up and never returns down -,-... using System; using System.Collections.Generic; using System.Linq; using System.Text; using DextroEngine; using DextroEngine.Physics; using Tao.OpenGl; namespace Shooter { class buoyancy_test:IGameObject { Renderer _renderer = new Renderer(); Sprite s = new Sprite(); Gravitation g = new Gravitation(9.81f); double pK = 0.75f;//object density double pFl = 1;//water density double watervolume = 0;//amount of object in water double YIncrement = 0;//amount to increase/decrease objects posítion double buoyancy = 0; double bodyforce = 0; double _totaltimePassed; float PercentInWater = 9f; bool moveup = false; bool movedown = false; public buoyancy_test() { s.SetWidth(10); s.SetHeight(10); s.rigidbody.Density = pFl; s.rigidbody.SetVolume(s.GetWidth(), s.GetHeight(),10); CalculateMass(); Get_pFl(); } public double CalculateMass() { bodyforce = pK* g.gravitation * s.rigidbody.GetVolume(); s.rigidbody.Mass = bodyforce / g.gravitation; return s.rigidbody.Mass; } public double InWaterVolume() { watervolume = s.GetWidth() * s.GetHeight() * PercentInWater; return watervolume; } public double Get_pFl() { double v_inWater = InWaterVolume(); pFl = (s.rigidbody.Volume * pK)/v_inWater; return pFl; } public void UpdateMovement(double time) { _totaltimePassed += time; buoyancy = pFl * g.gravitation * s.rigidbody.GetVolume(); double result_force = buoyancy - bodyforce; double a = result_force / s.rigidbody.Mass; if ((pFl > pK)&&(pFl < 1)) { moveup = true; movedown = false; } if ((pFl >1)) { moveup = false; movedown = true; } if (moveup) { YIncrement += (a * 0.5 * (_totaltimePassed * _totaltimePassed)); PercentInWater -= 0.1f; if (PercentInWater > 7.5f) { Get_pFl(); } } if (movedown) { YIncrement -= (a * 0.5 * (_totaltimePassed * _totaltimePassed)); PercentInWater += 0.1f; if (PercentInWater < 9.0f) { Get_pFl(); } } } public void Update(double elapsedTime) { UpdateMovement(elapsedTime); s.SetPosition(0, 0+YIncrement); } public void Render() { Gl.glClearColor(0.0f, 0.0f, 0.0f, 1.0f); Gl.glClear(Gl.GL_COLOR_BUFFER_BIT); _renderer.DrawSprite(s); } } }

Math and Physics Programming

Started by Hairies May 13, 2012 04:30 PM

25 comments, last by FishFace 11 years, 11 months ago

FishFace

109

May 14, 2012 10:46 PM

Okas, I get that...but how is it, if the bouyancy still is less then the gravity force on the object? How does that affect the resulting acceleration? Because it already slows down, before its buoyancy is bigger than its gravity force ...or not?

Bacterius

13,181

May 14, 2012 10:50 PM

The acceleration can never decrease beyond g (=-9.8 m/s) since the buoyancy force is *always* upwards however the velocity will of course. Does it make sense?

“If I understand the standard right it is legal and safe to do this but the resulting value could be anything.”

FishFace

109

May 14, 2012 10:58 PM

Yea it makes sense, but you said that if the bouyancy is bigger than its gravityforce only then it will begin to deaccelerate, but what is, if bouyancy is smaller than its gravityforce? Then it should also deaccelerate or not?

jefferytitan

2,525

May 14, 2012 11:12 PM

[background=rgb(250, 251, 252)][font="helvetica, arial, verdana, tahoma, sans-serif"][size="2"][color="#282828"]buoyancy is smaller, it will decelerate still. Think about a ship sinking. It sinks slower than it would fall through air, even though it still sinks.[/font][/background]

[background=rgb(250, 251, 252)][font="helvetica, arial, verdana, tahoma, sans-serif"][size="2"][color="#282828"]My advice is just to implement it and see how it goes. Even for light objects [/font][/background]

[color=#282828][font=helvetica, arial, verdana, tahoma, sans-serif][size=1][background=rgb(250, 251, 252)]buoyancy[/background][/font][color=#282828][font=helvetica, arial, verdana, tahoma, sans-serif][size=1][background=rgb(250, 251, 252)] will be smaller than the gravity force *for a while*, e.g. until enough of it is submerged to provide a strong [/background][/font][color=#282828][font=helvetica, arial, verdana, tahoma, sans-serif][size=1][background=rgb(250, 251, 252)]buoyancy[/background][/font][color=#282828][font=helvetica, arial, verdana, tahoma, sans-serif][size=1][background=rgb(250, 251, 252)] force.[/background][/font]

FishFace

109

May 15, 2012 05:23 AM

Okay, but, if buyancy is bigger than its gravityforce it should actually start rising with the resulting accelreation and not decelrate by (g-resulting acceleration)? Because if so, it would decelerate even if it had its maximuim bouyancy force. For example if a body had the density 0.75 and a volume 0f 1000, its gravityforce would be 7357.5N.
Its maximum buoyancy would be 9819. The resulting force would be 2452.5 N and the resulting max acceleration would be 3.27 m/s², so it should rise by that acceleration and not sink by(g-resulting acceleration), right?

Bacterius

13,181

May 15, 2012 05:59 AM

I think you are confusing velocity and acceleration (if you are not, apologies). Acceleration is (in the least technical sense of the term) the tendency of an object to go in a particular direction. Velocity is the direction the object is actually going. Acceleration affects velocity which affects position (but acceleration never directly affects position).

Okay, but, if buyancy is bigger than its gravityforce it should actually start rising with the resulting accelreation and not decelrate by (g-resulting acceleration)? Because if so, it would decelerate even if it had its maximuim bouyancy force. For example if a body had the density 0.75 and a volume 0f 1000, its gravityforce would be 7357.5N.
Its maximum buoyancy would be 9819. The resulting force would be 2452.5 N and the resulting max acceleration would be 3.27 m/s², so it should rise by that acceleration and not sink by(g-resulting acceleration), right?[/quote]
It won't rise automatically. It will slowly lose downwards velocity, and then start rising back up when the velocity flips signs under the influence of the upwards acceleration.

Imagine a car going at constant speed on a road. Its acceleration is zero. When the car is braking, the brakes force the road to apply friction to the car, which creates a force pointing in the opposite direction of the car's motion. When the car starts losing speed, its acceleration is in fact negative. At some point, its velocity becomes zero, and the car stops. At this point, the car should start going backwards under the negative acceleration it is undergoing (in real life it doesn't happen because of how car brakes work although you can glimpse it by seeing how cars are kind of pushed backwards a bit after quickly decelerating, but are stopped by static friction from the road and brakes).

Short story long, object's don't move from sheer acceleration - acceleration influences velocity which then causes the object to move.

“If I understand the standard right it is legal and safe to do this but the resulting value could be anything.”

FishFace

109

May 15, 2012 11:52 AM

yes your right, even if the resulting acceleration is > 0, the velocity will still be negative and only shortly after velocity will become positive and the object will rise again

Calculate objects position in water affected by buoyancy

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