Question

A glass plate of length 10 cm, breadth 4 cm and thickness 0.4 cm weighs 20 grams in air. It is held vertically with longest and shortest side horizontal and half the plate immersed in water. ( Surface tension of water = 70 dyne/cm) (gf represent gram-force)

	Column 1		Column 2
1	Buoyant Force	a	20 gf
2	Force due to surface tension	b	8 gf
3	Apparent weight of plate	c	1.49 gf
		d	13.49 gf
		e	11.51 gf

• A. 1$\to$b, 2$\to$c, 3$\to$d
• B. 1$\to$c, 2$\to$a, 3$\to$d
• C. 1$\to$b, 2$\to$a, 3$\to$d
• D. 1$\to$b, 2$\to$c, 3$\to$e

Answer 1

The correct option is A 1$\to$b, 2$\to$c, 3$\to$d

W = mg = 20 gf
Buoyant force (G) = $\frac{10\times 4\times 0.4}{2}$ gf = 8 gf
Buoyant force will act upward on plate.

Force due to surface tension
(F) = T $\times$perimeter of plate in contact with water = $\frac{70}{980}\times 2(10+0.4)$gf = 1.49 gf (Since, 1 dyne = 980 gf)
Surface tension will act downward on plate.

Apparent weight = W + F - G = 20 +1.49 - 8 = 13.49 gf