Short Answer Questions – Buoyancy & Upthrust
1. What is meant by the term buoyancy?
Buoyancy is the upward force exerted by a liquid on a body immersed in it.
2. What is the cause of upthrust? At which point can it be considered to act?
Upthrust is caused by the difference in liquid pressure at different depths. It acts at the centre of buoyancy.
3. State three characteristic properties of upthrust.
(i) It always acts vertically upward.
(ii) It increases with the volume of liquid displaced.
(iii) It depends on the density of the liquid.
(ii) It increases with the volume of liquid displaced.
(iii) It depends on the density of the liquid.
4. A piece of wood if left under water, comes to the surface. Explain the reason.
Wood is less dense than water, so the upthrust is more than its weight, and it rises to the surface.
5. Will a body weigh more in air or in vacuum when weighed with a spring balance? Give a reason.
It will weigh more in vacuum as there is no upthrust from the air acting upward on the body.
6. A metal solid cylinder tied to a thread is hanging from a spring balance and gradually immersed in water. What change do you expect in the reading?
The reading on the spring balance decreases as the upthrust from water partially supports the cylinder's weight.
7. A body dipped into a liquid experiences an upthrust. State two factors on which upthrust depends.
Upthrust depends on (i) volume of liquid displaced and (ii) density of the liquid.
8. How is the upthrust related to the volume of the body submerged in a liquid?
Upthrust is directly proportional to the volume of the body submerged in the liquid.
9. A bunch of feathers and a stone of the same mass are released in air. Which will fall faster and why? What if released in vacuum?
The stone falls faster in air ,the reason is that upthrust due to air on pebble is less than that on the bunch of feathers because the volume of pebble is less than that of the bunch of feathers of same mass. However in vacuum, both the bunch of feathers and pebble will fall together because there will be no upthrust..
10. A block of wood is released in (i) water and (ii) glycerine. What do you observe and why?
In both cases, the block rises. It rises faster in glycerine due to greater density and more upthrust.
11. A sphere of iron and one of wood (same radius) are placed on water. Which will sink and why?
The iron sphere will sink because its density is greater than that of water, while wood floats due to lower density.
12. How does the density of a body affect its floating or sinking in water?
If the density of the body is less than water, it floats. If more, it sinks.
13. Why is it easier to lift a heavy stone under water than in air?
Because water exerts upthrust on the stone, reducing its apparent weight.
