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.

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.

Some balls bounce better than others.

A particular ball can be characterized by its

coefficient of restitution:

when its bounces off a hard, stationary surface that can't move.

coefficient of restitution r = V_f / V_i

remains constant over a wide range of collision speeds.

the collision energy (become thermal energy).

different coefficient of restitution.

This is a home work problem shown in many Fundamental Physics textbooks .

• How to stack four uniform blocks on top of a table,
so that they extend as far right as possible and still remain stable.
• How should each be positioned?
• Can the top block have its entire length beyond the edge of the table.
• Would you like to play! 

1. You can drag and move blocks horizontal with your mouse.
2. The stability of the sub-system is color coded
1. Green: the sub-system is in stable equilibrium
2. yellow: the center of gravity is right above the edge of the supporting block.
3. red: the sub-system is unstable, it will fall in real life.
3. The center of gravity for each block is shown as a small blue dot.
4. If you press "Show c.g." button
1. The center of gravity for the blocks being moved will be shown as a small circle.
2. The length of the arrow is proportional to the gravitational force for each balanced sub-system.
3. Label of this button change to "Hide c.g", and you know what it means.
5. Current mouse position is shown in the "Text Field" (relative to top left edge of the table)
6. The percentage to the max. distance is shown on right edge of top block. It will smile when you get 100%
7. All the other numbers are coordinates measured from the left edge of the current window and they are all color coded.

The left edge of each block under the number

The center of gravity of each block the number is in

The center of gravity for all the blocks above the number