Floatation – Very Short Answer Questions
1. State the principle of floatation.
A body floats if the weight of the liquid displaced equals the weight of the body.
2. A piece of wood weighs 70 gf in air. It is lowered into water. What is the reading on spring balance?
The reading will be 0 because the wood floats and apparent weight becomes zero.
3. How does the density ρs of a substance determine whether it will float or sink?
If ρs < ρliquid, it floats; if ρs > ρliquid, it sinks.
4. A body floats with a part submerged. Is its weight more, equal to, or less than the upthrust?
The weight is equal to the upthrust when it floats.
5. What can you say about the average density of a ship that floats?
The average density of the ship is less than that of water.
6. Two identical pieces, one of ice and one of wood, float in water. Which displaces more water and experiences more upthrust?
Ice displaces more water and experiences more upthrust because it is denser than wood.
7. A piece of ice melts in water. Why doesn’t water level change?
Because the volume of water formed equals the volume of water displaced by the ice.
8. A buoy is held underwater by a string. Name the three forces acting and their directions.
1) Weight (downward), 2) Tension in thread (downward), 3) Upthrust (upward).
9. What is centre of buoyancy? How is it positioned?
Centre of buoyancy is the point where the buoyant force acts; it lies at the center of the displaced liquid volume.
10. What happens if helium balloon is placed in vacuum?
It falls because there’s no air to provide upthrust in a vacuum.
11. A loaded block just floats in water. What happens when salt is added or water is heated?
(a) Salt increases water’s density → block floats higher.
(b) Heating reduces density → block sinks more.
(b) Heating reduces density → block sinks more.
12. Why is floating ice less submerged in brine than pure water?
Because brine is denser than water, so less volume of ice is needed to displace enough liquid.
13. A man swims in sea and river. Where is it easier and why?
It is easier in sea because salt water is denser and gives more upthrust.
14. An iron nail floats with a cork on it in water. Remove the cork. What happens?
The nail sinks as cork was providing extra buoyancy.
15. A loaded cargo ship sails from sea to river. What happens to its level?
It sinks deeper in river because river water is less dense than sea water.
