Pressure in Solids

    OCR
    GCSE

    Pressure is defined physically as the force exerted per unit area acting perpendicular to a surface. Candidates must apply the relationship P = F/A to calculate pressure, force, or area in various mechanical contexts. The concept explains how force distribution affects surfaces, distinguishing between high-pressure applications, such as cutting tools, and low-pressure applications, such as caterpillar tracks on heavy machinery. Mastery requires precise unit conversion, particularly between N/cm² and Pa (N/m²), and a solid grasp of the inverse proportionality between pressure and area for a constant force.

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    Objectives
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    Exam Tips
    3
    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 correct recall of the equation P = F/A or Pressure = Force / Area
    • Award 1 mark for correct substitution of numerical values into the equation
    • Credit responses that correctly convert area from cm² to m² (dividing by 10,000) before calculation
    • Award 1 mark for the final answer with correct significant figures and unit (Pa or N/m²)
    • For explanation questions, candidates must state that for a constant force, increasing the surface area decreases the pressure

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the formula, but check your units—did you convert cm² to m² correctly?"
    • "Your explanation mentions 'force spreading out', but to gain the mark you must use the specific term 'surface area'"
    • "Remember that P=F/A requires Force in Newtons. If given mass in kg, what step must you do first?"
    • "Good calculation. To secure the final mark, ensure you include the correct unit (Pa or N/m²)"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for correct recall of the equation P = F/A or Pressure = Force / Area
    • Award 1 mark for correct substitution of numerical values into the equation
    • Credit responses that correctly convert area from cm² to m² (dividing by 10,000) before calculation
    • Award 1 mark for the final answer with correct significant figures and unit (Pa or N/m²)
    • For explanation questions, candidates must state that for a constant force, increasing the surface area decreases the pressure

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When calculating pressure, always check if the force is given in Newtons or if you are given a mass in kg; if mass, multiply by gravitational field strength (g) first
    • 💡Memorize that 1 Pascal (Pa) is exactly equal to 1 N/m²; this helps when checking unit consistency
    • 💡In 'Explain' questions involving snowshoes or sharp knives, explicitly mention the change in surface area and its resulting effect on pressure

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Incorrectly converting area units, typically dividing by 100 instead of 10,000 when converting cm² to m²
    • Substituting mass (kg) directly into the force variable without calculating weight (W=mg) first
    • Using vague language such as 'spreads the force' or 'impact is less' instead of 'distributes force over a larger area'

    Study Guide Available

    Comprehensive revision notes & examples

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    Key Terminology

    Essential terms to know

    Mathematical relationship P = F/A
    Inverse proportionality of Pressure and Area
    Unit conventions (Pascals, N/m², N/cm²)
    Applications of increasing and decreasing pressure

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    State
    Suggest
    Describe

    Practical Links

    Related required practicals

    • •{"code":"Suggested Practical","title":"Investigating pressure on surfaces","relevance":"Measuring the depression of a weight on sand using blocks of different surface areas"}

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