ICSE-IX-Physics
05: Upthrust in Fluids , Archimedes' Principle and Floatation
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- #Section : A
- Qstn #1What do you understand by the term upthrust of a fluid? Describe an experiment to show its existence.Ans : When a body is partially or wholly immersed in a liquid, an upward force acts on it. This upward force is known as an upthrust.
Upthrust can be demonstrated by the following experiment:
Take an empty can and close its mouth with an airtight stopper. Put it in a tub filled with water. It floats with a large part of it above the surface of water and only a small part of it below the surface of water. Push the can into the water. You can feel an upward force and you find it difficult to push the can further into water.
It is noticed that as the can is pushed more and more into the water, more and more force is needed to push the can further into water, until it is completely immersed. When the can is fully inside the water, a definite force is still needed to keep it at rest in that position. Again, if the can is released in this position, it is noticed that the can bounces back to the surface and starts floating again.
- Qstn #2In what direction and at what point does the buoyant force on a body due to a liquid act?Ans : Buoyant force on a body due to a liquid acts upwards at the centre of buoyancy.
- Qstn #3What is meant by the term buoyancy?Ans : The property of a liquid to exert an upward force on a body immersed in it is called buoyancy.
- Qstn #4Define upthrust and state its S.I. unit.Ans : The upward force exerted on a body by the fluid in which it is submerged is called the upthrust. Its S.I. unit is ‘newton’.
- Qstn #5What is the cause of upthrust? At which point can it be considered to act?Ans : A liquid contained in a vessel exerts pressure at all points and in all directions. The pressure at a point in a liquid is the same in all directions - upwards, downwards and sideways. It increases with the depth inside the liquid.
When a body is immersed in a liquid, the thrusts acting on the side walls of the body are neutralized as they are equal in magnitude and opposite in direction. However, the magnitudes of pressure on the upper and lower faces are not equal. The difference in pressure on the upper and lower faces cause a net upward force (= pressure × area) or upthrust on the body.
It acts at the center of buoyancy.
- Qstn #6Why is a force needed to keep a block of wood inside water?Ans : Upthrust due to water on block when fully submerged is more than its weight. Density of water is more than the density of cork; hence, upthrust due to water on the block of cork when fully submerged in water is more than its weight.
- Qstn #7A piece of wood if left under water comes to the surface. Explain the reason.Ans : A piece of wood if left under water comes to the surface of water because the upthrust on body due to its submerged part is equal to its own weight.
- Qstn #8Describe an experiment to show that a body immersed in a liquid appears lighter than it really is.Ans : Experiment to show that a body immersed in a liquid appears lighter:
Take a solid body and suspend it by a thin thread from the hook of a spring balance as shown in the above figure
(a). Note its weight. Above figure
(a) shows the weight as 0.67 N.
Then, take a can filled with water. Immerse the solid gently into the water while hanging from the hook of the spring balance as shown in figure
(b). Note its weight. Above figure
(b) shows the weight as 0.40 N.
The reading in this case
(b) shall be less than the reading in the case
(a), which proves that a body immersed in a liquid appears to be lighter.
- Qstn #9Will a body weigh more in air or vacuum when weighed with a spring balance? Give a reason for your answer.Ans : A body shall weigh more in vacuum because in vacuum, i.e. in absence of air, no upthrust will act on the body.
- Qstn #10A metal solid cylinder tied to a thread is hanging from the hook of a spring balance. The cylinder is gradually immersed into the water contained in a jar. What changes do you expect in the readings of the spring balance? Explain your answer.Ans : The readings in the spring balance decreases.
As the cylinder is immersed in the jar of water, an upward force acts on it, which is in opposition to the weight component of the cylinder. Hence the cylinder appears to be lighter.
- Qstn #11A body dipped into a liquid experiences an upthrust. State two factors on which upthrust on the body depends.Ans : Upthrust on a body depends on the following factors:
- Volume of the body submerged in the liquid or fluid.
- Density of liquid or fluid in which the body is submerged.
- Qstn #12How is the upthrust related to the volume of the body submerged in a liquid?Ans : Larger the volume of body submerged in liquid, greater is the upthrust acting on it.
- Qstn #13A bunch of feathers and a stone of the same mass are released simultaneously in air. Which will fall faster and why? How will your observation be different if they are released simultaneously in vacuum?Ans : When a bunch of feathers and a stone of the same mass are released simultaneously in air, the feathers will fall after the stone falls due to air friction. In vacuum, as there is no air friction, the acceleration due to gravity of both bodies will be the same, and therefore, the feathers and the stone will fall at the same time.
- Qstn #14A body experiences an upthrust F1 in river water and F2 in sea water when dipped up to the same level. Which is more, F1 or F2? Give reason.Ans : F2 > F1; Sea water is denser than river water; therefore, the upthrust due to sea water will be greater than that due to river water at the same level. This shall make the body to appear lighter in the sea water.