Question

5. two spheres A and B , each of volume 100cm3 are placed on water (density = 1 g cm-3).the sphere A is made of wood of density 0.3 g cm-3 and sphere B is made of iron of density 8.9 g cm-3.

find - upthrust on each sphere.

Answer 1

Given:
Sphere A: [made up of wood]
Volume =100cm³
density of sphere A=0.3gm/cm³
mass of sphere A:
m=volume x density
⇒0.3x100⇒30 gms.
So weight of Sphere A is =W=mg=30x 980=29400N
For sphere B:[ Made up of iron]
volume=100cm³
density of sphere :8.9 gm/cm³
mass=volume x density
⇒100 x 8.9⇒890 grams
weight of sphere B is =W=mg=890 x 980=872200N
Since sphere A is made up of wood, its density is less than water , it will float.,
since sphere B is made up of iron, its density is more than water it will sink.

UPTHRUST ON SPHERE A;
for sphere A to float, by laws of flotation
upthrust= weight of sphere A
∴upthrust on sphere A is 29400N

UPTHRUST ON SPHERE B;
upthrust=Volume of water displaced x density of water x g
=100x 1x 980
98000N