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Upthrust and Floatation
- A liquid pushes up on an object in it
- This upward force is upthrust
- Also called the buoyant force
- If gravity beats upthrust, it sinks
- If upthrust is enough, it floats
- push a capped bottle into water
- feel an upward force
- it rises when released
- a mug in water feels lighter
- upthrust pushes it up
- so it seems to weigh less
- Upthrust always acts in the upward direction.
- If gravity is more than upthrust the object sinks; if upthrust is equal or more, it floats.
- Archimedes' principle: an immersed object feels an upward force equal to the weight of liquid it displaces.
Release the bottle.
- Pushing in: feel upward force.
- Released: bottle rises.
- Upthrust beats gravity.
Name the following: The force which increases the speed of a ball when dropped from a height.
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Explanation
- The Earth's gravity pulls the falling ball down, increasing its speed.
Name the following: The upward force which acts on a body partially or fully submerged in a liquid.
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Explanation
- The liquid pushes the immersed object upwards with the buoyant force.
Fill in the blank: The property of any liquid to exert upward force on an object immersed in it is called ________.
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Explanation
- This upward force is the buoyant force or upthrust.
On what factors does buoyant force depend? Explain.
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Explanation
- By Archimedes' principle, an immersed object feels an upward force equal to the weight of the liquid it displaces.
- So it depends on the volume of the object immersed and the density of the liquid.
- More liquid displaced means a greater buoyant force.
When you drop a coin in a glass of water, it sinks, but when you place a bigger wooden block in water, it floats. Explain. (NCERT Book Question)
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Explanation
- An object sinks if its weight is greater than the buoyant force.
- The coin's weight is more than the upthrust of water, so it sinks.
- The wooden block's weight is less than the upthrust, so it floats — its size lets it displace enough water.
(a) Is any force being applied to an object in a non-uniform motion? (b) If a ball is thrown upwards, it slows down, stops momentarily, and then falls back to the ground. Name the forces acting on the ball and specify their directions: (i) during its upward motion, (ii) during its downward motion, (iii) at its topmost position. (NCERT Book Question)
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Explanation
- (a) Yes — a force is applied, because the speed of the ball keeps changing.
- (i) During upward motion, the gravitational force acts downward, slowing the ball.
- (ii) During downward motion, the gravitational force acts downward, speeding it up.
- (iii) At the topmost position, the gravitational force still acts downward.
Seema observes that heavy logs of wood from a forest near her village are transported by water. She wonders how heavy wood floats on the surface of water instead of sinking. Help Seema to resolve her query.
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Explanation
- An object floats if the upthrust of water is equal to or greater than its weight.
- A wooden log displaces a large amount of water, giving a large upthrust.
- Since this upthrust is enough to balance the log's weight, the log floats even though it is heavy.
Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.
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Explanation
- The gravitational force pulls the bucket downward.
- The muscular force of the hand pushes the bucket upward.
- These two forces are equal and opposite, so the net force is zero and the bucket does not move.
Three objects 1, 2, and 3 of the same size and shape but made of different materials are placed in water. They dip to different depths as shown in the Figure. If the weights of the three objects 1, 2, and 3 are w₁, w₂, and w₃, respectively, then: (NCERT Book Question)
- (a) w₁ = w₂ = w₃
- (b) w₁ > w₂ > w₃
- (c) w₂ > w₃ > w₁
- (d) w₃ > w₁ > w₂
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Explanation
- The object that dips deepest is the heaviest because its weight is largest compared to the upthrust.
- Object 1 dips most and object 3 least, so w₁ > w₂ > w₃.
