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Auteur:
Fu-Kwun Hwang

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Buoyant Force


A completely submerged body displaces a volume of liquid equal to its own volume. Experience also tell us that when an object is submerged, it appear lighter in weight; the water buoys it up, pushed upward, partially supporting it somehow. Archimedes' Buoyancy Principle asserts that an object immersed in a liquid will be lighter by an amount equal to the weight of the fluid it displaces. The upward force exerted by the fluid is known as buoyant force.

Buoyant force is caused by gravity acting on the fluid. It has its origin in the pressure difference occurring between the top and bottom of the immersed object, a difference that always exists when pressure varies with depth. Imaging without the object, the same immersed space will be occupied by the same volume of fluid.
The weight of those fluid is supported by other parts of the fluid. So the buoyant force is the weight of the displaced fluid. I hope this java applet will help you learn more about buoyancy.

The java applet is divided into two area.
The cyan region represents the fluid . The gray block is the object.
You can enter values into the text field to change the density of the fluid or the object.

There is a drag force Fr when the object is moving in the fluid.
    Fr = - k V where V is the object's velocity.
k is the propotional coefficient (the 2nd text field)

Click the LEFT mouse button in gray block and drag the object to some other place.
    Numbers shown in the upper left pairs are the X coordinate and the width of the object.
    Number shown in the upper right pairs are the Y coordinate and the height of the object.
Click the RIGHT mouse button in gray block and drag the mouse
    to change the volume of the object.
The mass of the object is shown in the textfield.

You can drag the object and let it submerged in the fluid.
    Black vector represents  the weight of the object. Black circle is the center of gravity .
    Red vector represent the buoyant force .
    RIGHT CLICK the mouse button near the origin (green circle)
        to clear the curves and reset time to zero t=0.0s.
    The buoyant force can be treated as acting at the circle in cyan
        (same as the center of gravity for the displaced liquid).

The animation starts when you release the object.
    (Gravitation force and buoyant force trigging the motion of the object)
Blue curve is the displacement in y-direction of the object verses time.
Red curve shows the velocity of the object as a function of time.

Click the Pause button to hold the animation, RIGHT CLICK mouse button to resume.