A block of material of volume 20cm³ and density of 2.5gcm³suspended from a
spring balance with half of the volume of the block immersed in water. What is the
reading of the spring balance?

0.4 N

Explanation:

Draw a free body diagram of the block.  There are three forces: weight force mg pulling down, buoyancy force ρVg pushing up, and tension force T pulling up.

Sum of forces in the y direction:

∑F = ma

T + ρVg − mg = 0

T = mg − ρVg

T = g (m − ρV)

Plugging in values:

T = (9.8 m/s) ((2.5 g/cm³ × 20 cm³) − (1 g/cm³) (10 cm³)) (1 kg / 1000 g)

T = 0.392 N

Rounded to one significant figure, the reading of the spring scale is 0.4 N.

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