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 liquid in which the block is least submerged has the highest density, because less volume of liquid is required to balance the block’s weight.

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 start falling because air pressure and upthrust decrease as air is pumped out.

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 ice is required to displace its weight, 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).
(ii) Swimming is easier in sea water due to greater buoyant force (higher density).

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

A ship's shape allows it to displace a large volume of water, reducing its average density below water’s density. An iron nail cannot displace enough water, so it sinks.

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

The ship sinks slightly more in river water because it is less dense than sea water, and more volume is required to provide the same upthrust.