A rectangular glass slab measures `0.1 m xx0.0154 m xx0.002m` and its weight in air is `80.36xx10^-3N`. The slab is immersed half in water keeping its length and thickness horizontal. What will be the apparent weight of the slab ? The surface tension of water is `72xx10^-3 N. m^-1`.

Explanation

While it is immersed, the following forces act on the glass slab (1) weight of the slab acting downwards, (2) upward buoyant force due to the weight of displaced water and (3) downward force due to surface tension.
Now, weight of the slab`=80.36xx10^-3 N`
Buoyant force = weight of displaced water
`=0.1xx(0.0154)/(2)xx0.002xx1000xx9.8`
`=15.092xx10^-3 N`
Force due to surface tension
`=2xx(0.1+0.002)xx72xx10^-3 N`
`=14.688xx10^-3 N`
`therefore` The apparent weight of the slab
`=80.36xx10^-3-15.092xx10^-3+14.688xx10^-3`
`=79.956xx10^-3 N`.