Elastic and Inelastic Deformation

    OCR
    GCSE

    Deformation occurs when forces stretch, compress, or bend an object, requiring a minimum of two acting forces to alter shape rather than cause acceleration. Elastic deformation is characterized by the object returning to its original dimensions upon force removal, whereas inelastic (plastic) deformation results in permanent structural distortion. Hooke's Law defines the linear relationship between force and extension ($F=ke$) up to the limit of proportionality. The gradient of a force-extension graph represents the spring constant (stiffness), and the area under the curve represents the work done, stored as elastic potential energy.

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    Objectives
    3
    Exam Tips
    4
    Pitfalls
    5
    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that elastic deformation is reversible and the object returns to its original shape when the force is removed
    • Award 1 mark for identifying inelastic deformation as a permanent change in shape where the object does not return to original dimensions
    • Award 1 mark for calculating the spring constant ($k$) as the gradient of the linear section of the force-extension graph
    • Award 1 mark for linking the area under the force-extension graph to the work done or elastic potential energy stored
    • Credit responses that identify the limit of proportionality as the point where the graph ceases to be linear

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the linear region. Now, explain what the gradient of this line represents in terms of physical quantities"
    • "Check your units for extension — the formula requires metres, but the question gave centimetres"
    • "Good definition of elastic deformation. To secure higher marks, contrast this explicitly with inelastic deformation using the concept of energy transfer"
    • "You mentioned measuring the length. Be more precise: describe how you calculate 'extension' from the original and new lengths"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that elastic deformation is reversible and the object returns to its original shape when the force is removed
    • Award 1 mark for identifying inelastic deformation as a permanent change in shape where the object does not return to original dimensions
    • Award 1 mark for calculating the spring constant ($k$) as the gradient of the linear section of the force-extension graph
    • Award 1 mark for linking the area under the force-extension graph to the work done or elastic potential energy stored
    • Credit responses that identify the limit of proportionality as the point where the graph ceases to be linear

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When calculating the gradient (spring constant), select two points on the line that are as far apart as possible to minimize percentage error
    • 💡In 6-mark practical questions, explicitly mention the use of a fiducial marker (e.g., a pin) to reduce parallax error when measuring extension
    • 💡Remember that the area under the graph represents work done for both elastic and inelastic deformation, but the formula $E=0.5kx^2$ applies ONLY to the linear (elastic) region

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing the 'limit of proportionality' with the 'elastic limit' — while related, the former refers specifically to the linear graph relationship, while the latter refers to the point of permanent deformation
    • Calculating the spring constant using data points from the non-linear (inelastic) region of the graph
    • Failing to convert extension from centimetres (cm) to metres (m) before calculating Elastic Potential Energy, leading to orders of magnitude errors
    • Defining extension incorrectly as the total length of the spring rather than 'new length minus original length'

    Key Terminology

    Essential terms to know

    Elastic vs. Inelastic Deformation
    Hooke's Law and Spring Constant ($k$)
    Limit of Proportionality vs. Elastic Limit
    Force-Extension Graphs (Linear and Non-linear)
    Elastic Potential Energy ($E_e = 0.5ke^2$)

    Likely Command Words

    How questions on this topic are typically asked

    Define
    Calculate
    Describe
    Explain
    Plot
    Estimate

    Practical Links

    Related required practicals

    • •{"code":"PAG P2","title":"Investigation of the force-extension relationship for a spring","relevance":"Direct assessment of Hooke's Law and experimental techniques"}

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