Upthrust

    OCR
    GCSE

    Upthrust is the resultant upward force exerted by a fluid on a submerged or partially submerged object, arising directly from the pressure difference between the object's upper and lower surfaces. According to Archimedes' Principle, the magnitude of this upthrust is equal to the weight of the fluid displaced by the object. Candidates must analyze the interaction between upthrust and weight to determine equilibrium conditions for floating, sinking, or rising. Mastery of this topic requires integrating concepts of density, hydrostatic pressure ($p = h \rho g$), and free-body force diagrams.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that pressure in a fluid increases with depth due to the weight of the column of liquid above
    • Credit responses that explain upthrust arises because the force on the bottom surface is greater than the force on the top surface
    • Award 1 mark for stating that an object floats when the upthrust is equal to the object's weight
    • Candidates must link sinking to the condition where the object's weight exceeds the maximum upthrust (weight of fluid displaced)
    • Award 1 mark for calculating pressure difference using P = h × ρ × g and relating this to Force = Pressure × Area

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified that pressure increases with depth; now explain how this difference creates a resultant upward force."
    • "Your calculation is correct, but check your units—density must be in kg/m³ when using g = 10 N/kg."
    • "You stated the object sinks because it is heavy; refine this to compare the object's density to the fluid's density."
    • "Excellent link between weight and upthrust. To secure full marks, specify that the upthrust equals the weight of the *displaced fluid*."

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that pressure in a fluid increases with depth due to the weight of the column of liquid above
    • Credit responses that explain upthrust arises because the force on the bottom surface is greater than the force on the top surface
    • Award 1 mark for stating that an object floats when the upthrust is equal to the object's weight
    • Candidates must link sinking to the condition where the object's weight exceeds the maximum upthrust (weight of fluid displaced)
    • Award 1 mark for calculating pressure difference using P = h × ρ × g and relating this to Force = Pressure × Area

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When asked to explain the cause of upthrust, you must explicitly compare the forces on the top and bottom surfaces, not just state pressure varies.
    • 💡For floating objects, start your explanation with the equilibrium condition: 'Upthrust equals Weight'.
    • 💡In calculation questions involving P = hρg, ensure you use the density of the *fluid*, not the object, to find the pressure exerted by the fluid.

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Believing that upthrust increases as a fully submerged object sinks deeper (it remains constant as volume displaced is constant)
    • Stating that 'pressure pushes the object up' without specifying that it is the *difference* in pressure between top and bottom that creates the resultant force
    • Confusing density with weight, arguing that 'heavy' objects sink rather than objects denser than the fluid
    • Failing to convert units correctly when calculating pressure (e.g., using cm instead of m for depth)

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Archimedes' Principle and displacement
    Pressure difference as the origin of upthrust
    Conditions for floating and sinking (equilibrium)
    Relative density relationships

    Likely Command Words

    How questions on this topic are typically asked

    Explain
    Calculate
    Describe
    Compare
    Suggest

    Practical Links

    Related required practicals

    • •{"code":"PAG P1","title":"Determination of Density","relevance":"Understanding density is prerequisite for predicting floating/sinking behavior"}

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