Buoyant Force

Applet by Fu-Kwun Hwang

The java applet is divided into two areas. The cyan region represents the fluid while the gray block is the object. You can enter values into the text fields in order to change the density of the fluid or the object. There is a drag force Fr exerted on the object as it moves through the fluid. Fr = - kV where V is the object's velocity. k is a constant of proportionality.

Click the LEFT mouse button in gray block and drag the object to some place with in the fluid. The pair of numbers shown in the upper left are the X coordinate and the width of the object. The pair of numbers shown in the upper right pairs are the Y coordinate and the height of the object. Click the RIGHT mouse button in the gray block, and drag the mouse in order 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 submerge in the fluid. The black vector represents the weight of the object while the black circle is the center of gravity. The red vector represents the buoyant force. RIGHT CLICK the mouse button near the origin (green circle) in order 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 (which is the same as the center of gravity for the displaced liquid).

The animation starts when you release the object. The blue curve is the displacement in y-direction of the object verses time. The red curve shows the velocity of the object as a function of time.

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

A completely submerged body displaces a volume of liquid that is equal to its own volume. Experience also tell us that when an object is submerged, it appear lighter in weight because of the bouyant forces acting upward on the object. Archimedes' Buoyancy Principle asserts that an:

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. The 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. Imagine, without the object, the same immersed space that occupies the same volume of fluid.

The weight of the fluid is supported by other parts of the fluid. So the buoyant force is the weight of the displaced fluid. We hope that this java applet will help you learn more about buoyancy.