Ch5 section C - Floatation, short type

Short Answer Type Questions – Floatation

1. A solid iron ball of mass 500 g is dropped in mercury contained in a beaker.
(a) Will the ball float or sink? Give reason.
(b) What will be the apparent weight of ball? Give reason.

(a) The ball will float because the density of iron is less than that of mercury.
(b) Apparent weight = Actual weight − Upthrust. Since it floats, apparent weight = 0.

2. Explain why an iron nail floats on mercury, but it sinks in water.
Hint: Density of iron is less than that of mercury, but more than that of water.

The iron nail floats in mercury because its density is less than that of mercury. It sinks in water because its density is greater than that of water.

3. A homogeneous block floats on water:
(a) partly immersed
(b) completely immersed
In each case state the position of centre of buoyancy B with respect to the centre of gravity G of the block.

(a) When partly immersed, centre of buoyancy B lies below the centre of gravity G.
(b) When completely immersed, B and G coincide.

4. Fig. 5.15 shows the same block of wood floating in three different liquids A, B, and C of densities ρ_A, ρ_B, and ρ_C respectively. Which liquid has the highest density? Give reason.

The Upthrust on the body by each liquid is same and is equal to the weight of the body.Upthrust is equal to product of volume of submerged x density of liquid x acceleration due to gravity.For  liquid C in which the block is least submerged so  has the highest density.

5. What is centre of buoyancy? State its position in a floating body with respect to the centre of gravity of the body.

Centre of buoyancy is the point through which the buoyant force acts. In a floating body, it lies below the centre of gravity for stability.

6. A balloon filled with helium gas floats in a big closed jar which is connected to an evacuating pump. What will be your observation, if air from the jar is pumped out? Explain your answer.

The balloon will sink. As air is pumped out from jar,the density of air in jar decreases ,so the Upthrust on balloon decreases.As weight of balloon exceeds the Upthrust on it ,it's sinks.

7. A block of wood is so loaded that it just floats in water at room temperature. What change will occur in its state of floatation if
(a) some salt is added to water?
(b) water is heated?
Give reason.

(a) It will rise slightly as salt water is denser.
(b) It will sink slightly more as heated water is less dense.

8. Why is floating ice less submerged in brine than in water?

Brine is denser than water, so less volume of water  is required to balance its weight while floating,hence less submersion.

9. A man first swims in sea water and then in river water:
(i) Compare the weights of sea water and river water displaced by him.
(ii) Where does he find it easier to swim and why?

(i) Equal weight of water is displaced in both cases (equal to man's weight) ie 1:1.
(ii) It is easier for a man to swim in sea than in fresh (or river) water: The reason the density of sea water is more than that of river water so his weight is balanced in sea water with his less part submerged inside it.

10. An iron nail sinks in water while an iron ship floats on water. Explain the reason.

Density of iron is more than water ,it sinks .ships are also made of iron but they float. This is because the ship is hollow and the empty space in it contains air which makes its volume large and average density less than that of water. Therefore, even with a small portion of ship submerged in water, the weight of water displaced by the submerged part of ship becomes equal to the total weight of ship and therefore it floats.

11. A loaded cargo ship sails from sea water to river water. State and explain your observation.

when a ship sails from a sea of higher water density to a sea of lower water density (or from sea water to river water), it sinks further. The reason is that according to the law of floatation, to balance the weight of ship, a greater volume of water is required to be displaced in water of lower density in river.