Very Short Answer Questions – Buoyancy and Upthrust
1. In what direction and at what point does the buoyant force on a body due to a liquid act?
It acts vertically upward through the centre of buoyancy (i.e., the centre of the displaced liquid volume).
2. Define upthrust and state its S.I. unit.
Upthrust is the upward force exerted by a liquid on a body immersed in it. S.I. unit: newton (N).
3. Why is a force needed to keep a block of wood inside water?
Because upthrust acts on it and tends to push it upward as wood is less dense than water.
4. A body experiences upthrust F1 in river water and F2 in sea water when dipped to the same depth. Which is more and why?
F2 is more because sea water is denser than river water, so it provides more upthrust.
5. A body of volume V and density ρ is kept completely immersed in a liquid of density ρL. If g is acceleration due to gravity, write expressions for:
(i) Weight of the body
(ii) Upthrust
(iii) Apparent weight
(iv) Loss in weight
(i) Weight of the body
(ii) Upthrust
(iii) Apparent weight
(iv) Loss in weight
(i) Weight = ρ × V × g
(ii) Upthrust = ρL × V × g
(iii) Apparent weight = (ρ - ρL) × V × g
(iv) Loss in weight = Upthrust = ρL × V × g
(ii) Upthrust = ρL × V × g
(iii) Apparent weight = (ρ - ρL) × V × g
(iv) Loss in weight = Upthrust = ρL × V × g
6. Complete the sentences:
(a) Two balls, one of iron and the other of aluminium experience the same upthrust when dipped completely in water if …
(b) An empty tin container with its mouth closed has an average density equal to that of a liquid. The container is taken 2 m below the surface of that liquid and is left there. Then the container will …
(c) A piece of wood is held under water. The upthrust on it will be …
(a) Two balls, one of iron and the other of aluminium experience the same upthrust when dipped completely in water if …
(b) An empty tin container with its mouth closed has an average density equal to that of a liquid. The container is taken 2 m below the surface of that liquid and is left there. Then the container will …
(c) A piece of wood is held under water. The upthrust on it will be …
(a) … they have equal volume.
(b) … stay at that depth as its average density equals that of the liquid.
(c) … equal to the weight of the wood piece.
(b) … stay at that depth as its average density equals that of the liquid.
(c) … equal to the weight of the wood piece.
7. A sphere of iron and another of wood of the same radius are held under water. Compare the upthrust on the two spheres.
Both experience the same upthrust because both displace the same volume of water.
8. A body of density ρ is immersed in a liquid of density ρL. State the condition when the body will:
(i) Float
(ii) Sink
(i) Float
(ii) Sink
(i) Float: when ρ < or = ρL
(ii) Sink: when ρ > ρL
(ii) Sink: when ρ > ρL
9. State Archimedes' principle.
When a body is wholly or partially immersed in a fluid, it experiences an upthrust equal to the weight of fluid displaced by it.
