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Grade 5 · Science · Unit 5 of 6

Float or Sink?

Buoyant force in water and air

What your child practises

Buoyant force as the upward push of a fluid, comparing it with weight to explain floating and sinking, and why shape matters for ships and boats.

Alberta Curriculum learning outcome: 5E 1.2 · buoyant force, weight, and why things float or sink in a fluid

Sample questions

A taste of this unit. In the app, questions change every time and get easier or harder to match your child.

  1. Question 1

    Air is a…

    • force
    • liquid
    • fluid, because gases are fluids too
    • solid
    Show answer

    fluid, because gases are fluids too

    Hint: Fluids are liquids and gases.

  2. Question 2

    A huge steel ship floats. Why?

    🚢
    • The ship has no weight
    • Its hollow shape gives it a large buoyant force
    • Steel is lighter than water
    • Ships are never affected by gravity
    Show answer

    Its hollow shape gives it a large buoyant force

    Hint: The ship's shape pushes aside a lot of water, so the water pushes up a lot.

  3. Question 3

    A student wants a fair test of whether different objects float. What should stay the same?

    • the kind of liquid and the amount of water
    • the colours of the objects
    • the objects
    • the number of tries only
    Show answer

    the kind of liquid and the amount of water

    Hint: In a controlled experiment, only one thing changes. The liquid stays the same.

  4. Question 4

    A life jacket helps a swimmer float because it…

    • removes gravity
    • adds buoyant force without adding much weight
    • adds a lot of weight
    • makes the water disappear
    Show answer

    adds buoyant force without adding much weight

    Hint: It is filled with light material that displaces water.

  5. Question 5

    Fresh water or salt water: in which does a boat float higher?

    • neither, it makes no difference
    • fresh water, because it is lighter
    • only in a lake
    • salt water, because it pushes up more
    Show answer

    salt water, because it pushes up more

    Hint: Salt water is denser, so the buoyant force is greater.

  6. Question 6

    A hot air balloon rises. Which statement is true?

    • Water pushes it up
    • Gravity stops working on it
    • The balloon has no weight
    • The buoyant force from the cooler air around it is greater than its weight
    Show answer

    The buoyant force from the cooler air around it is greater than its weight

    Hint: Air is a fluid, so it can push objects up too.

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