PRESSURE IN LIQUIDS 27 



In lifting an anchor or a stone from under water, it 

 seems to be heavier the moment it rises above the sur- 

 face. It may be said, in general, that all bodies seem to 

 weigh less when held in a liquid. This is not because 

 the thing really does weigh less, but because it is then 

 acted upon by some other force which acts in the opposite 

 direction to the force of gravity. The force is exerted 

 by the liquid body and is called buoyant force. 



Experiment 24. Hang several heavy bodies (e.g. a stone or 

 a scrap of iron), one at a time, to a sensitive spring balance. 

 Note the weight of each. Then, without removing it from the 

 balance, lower each over water till it dips wholly below the sur- 

 face, and again note its weight. Does it pull the pointer down 

 more or less when in the water ? How do you account for this ? 

 Does each body really change in weight or only seem to? Can 

 you measure the buoyant force in each case? 



38. Buoyant Force explained The molecules of any 



liquid at rest will, of course, be as low as gravity can 

 pull them. If, now, any body be lowered into that liquid, 

 some of the particles will be displaced (pushed out of 

 their places) and the surface of the liquid will be raised 

 by an amount just equal in volume to the size of the 

 body which displaced it. These particles will, of course, 

 be still pulled downward by gravity, and in tending to 

 return to their places they will exert force upon the 

 body. Since this force will cause greater pressure upon 

 the bottom (see 34), the body will, of course, be pushed 

 upward. 



It is clear that any body held entirely in a liquid will 

 displace its own volume (size) of the liquid. And as the 

 body is buoyed upward by the force that these displaced 



