Buoyant force — Science definition

The net upward force a fluid exerts on an immersed object.

Where it is taught

In use

An object floating steadily has equal upward buoyant force and downward weight.

Pressure & Buoyant Forces

Questions using buoyant force

Problem 1

A toy floats steadily with weight 3 N. What is its buoyant force?
Answer and explanation

3 N

The upward and downward forces balance.

Problem 2

A floating object weighs 18 N. What is its buoyant force when stationary with no other vertical support? Show the reasoning, not just the conclusion.
Hints

Use displaced fluid volume, not the container's total water volume.

Answer and explanation

18 N upward, balancing its weight.

Problem 3

Floating, sinking and being held are different casesFloating, sinking and being held are different casesFloating steadilyReleased heavy blockHeld by a stringWeight 3 N downBuoyancy 3 N upNet force = 0 NWeight 4 N downBuoyancy 2 N upNet = 2 N downNet force = 0 NThese are separate cases. Arrows use 25 drawing units per newton.Predict acceleration from the net force; an object can still have velocity.Weight:4 N downBuoyancy:2 N upString:2 N upBlue horizontal lines mark the water surface.
Interpret the different support conditions in this diagram, then apply the same method to the changed force values in the question. The labels are the illustrated examples, not the numerical answer to the new case.
A submerged object is held still by a string. Its weight is 9 N and buoyant force 6 N. What upward string force balances it, and why would drawing only equal buoyancy and weight misdescribe the case?
Answer and explanation

The string supplies 3 N upward. Buoyancy alone is smaller than weight; the third force is needed to make the held object’s net force zero.

List all forces on the object.

Use 6+T−9 = 0 without calling the held state unsupported floating.

The string supplies 3 N upward. Buoyancy alone is smaller than weight; the third force is needed to make the held object’s net force zero.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 4

Floating, sinking and being held are different casesFloating, sinking and being held are different casesFloating steadilyReleased heavy blockHeld by a stringWeight 3 N downBuoyancy 3 N upNet force = 0 NWeight 4 N downBuoyancy 2 N upNet = 2 N downNet force = 0 NThese are separate cases. Arrows use 25 drawing units per newton.Predict acceleration from the net force; an object can still have velocity.Weight:4 N downBuoyancy:2 N upString:2 N upBlue horizontal lines mark the water surface.
Interpret the different support conditions in this diagram, then apply the same method to the changed force values in the question. The labels are the illustrated examples, not the numerical answer to the new case.
Independent chapter check: A submerged object is held still by a string. Its weight is 9 N and buoyant force 6 N. What upward string force balances it, and why would drawing only equal buoyancy and weight misdescribe the case?
Answer and explanation

The string supplies 3 N upward. Buoyancy alone is smaller than weight; the third force is needed to make the held object’s net force zero.

List all forces on the object.

Use 6+T−9 = 0 without calling the held state unsupported floating.

The string supplies 3 N upward. Buoyancy alone is smaller than weight; the third force is needed to make the held object’s net force zero.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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