力的迷思...............自然界係咪得4種力?

Out of the four forces of nature, strong and weak nuclear interactions ( 強作用力及弱作用力) are short range forces within distances in nucleus. The other two, gravitation (重力) and electromagentic forces (電磁力) are of much longer ranges.

Floatation arises because of difference in pressure in a fluid. But pressure is due to interaction of fluid molecules by intermolecular force, which is electromagentic in nature.

For "tension", I suppose you refer to string tension. If this is the case, tension is clearly electromagentic in nature, as it is also due to intermolecular force that holds molecules together.

自然界基本的4種力、重力、電磁力、強作用力、及弱作用力等.
浮力同重力:
http://www.ananedu.com/a/5/61/a.htm

表面張力同電磁力

表面張力源於分子引力，分子引力源於電磁力.

http://cnx.org/content/m14045/latest/