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Forces and Matter

Upthrust, floating and sinking

  • 3 things to know
  • Paper 2

Overview

Upthrust from the pressure difference across an immersed object, stones, helium balloons and ships, upthrust as the weight of fluid displaced with a calculation, the conditions for floating and sinking, and how fluid density affects floating.

What you need to know

Answer each one before you reveal it.

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Key term

State the condition for floating, and explain how fluid density affects floating and why a ship sits lower in fresh water than in the sea.

Key term

Explain why an object immersed in a liquid experiences an upward force.

Key term

Explain why a stone feels lighter to hold when it is under water.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Explain why an object immersed in a liquid experiences an upward force.

    Model answer

    The pressure on its lower surface is greater than the pressure on its upper surface, giving a resultant upward force.

  2. 2

    Name the upward force that a fluid exerts on an object placed in it.

    Model answer

    Upthrust.

  3. 3

    Explain why a stone feels lighter to hold when it is under water.

    Model answer

    Upthrust from the water acts upwards on the stone, reducing the force needed to support it.

  4. 4

    Explain why a helium balloon rises through the air.

    Model answer

    The upthrust from the surrounding air is greater than the weight of the balloon.

  5. 5

    Explain why upthrust still acts on a ship whose hull is only partly below the water surface.

    Model answer

    The submerged part of the hull still has water pressure pushing up on it from below.

  6. 6

    Explain why the upthrust on an object grows as it is pushed further into a liquid until it is fully immersed.

    Model answer

    More of the object is surrounded by liquid, so a greater volume of liquid is pushed aside.

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