OCR · A-Level · Physics

    Course version: H556 · Version from Sept 2015

    (c) stress, strain and ultimate tensile strength

    Full specification wording

    Mechanical properties of matter: (c) stress, strain and ultimate tensile strength

    Stress is the force acting per unit cross-sectional area, σ = F/A, measured in pascals (Pa), where 1 Pa = 1 N m⁻².

    Checked against the OCR H556 specification (opens in a new tab) on .

    • Point 3 of 7
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Stress is the force acting per unit cross-sectional area, σ = F/A, measured in pascals (Pa), where 1 Pa = 1 N m⁻². Strain is the fractional change in length, ε = x/L, where x is the extension and L the original length; it has no unit. Stress and strain allow materials of different sizes and shapes to be compared. Ultimate tensile strength is the maximum stress a material can withstand before it breaks when being stretched. On a stress–strain graph, the ultimate tensile strength is the highest stress value reached.

    For example: A wire of cross-sectional area 2.0 × 10⁻⁶ m² carrying a force of 10 N experiences a stress of 5.0 × 10⁶ Pa.

    What earns the marks

    • Stress is force per unit cross-sectional area, σ = F/A, measured in pascals.
    • Strain is extension divided by original length, ε = x/L, and is dimensionless.
    • Ultimate tensile strength is the maximum stress a material can withstand before breaking.
    • Stress and strain allow comparison of materials independently of sample dimensions.

    Common mistakes and how to put them right

    • Using the total length instead of the original length in the strain calculation.

      Put it right: Divide the extension by the original length.

    • Confusing stress with force.

      Put it right: Stress is force divided by cross-sectional area and has units of pascals.

    • Treating ultimate tensile strength as the breaking stress on a force–extension graph.

      Put it right: It is the maximum stress on a stress–strain graph.

    Exam tips

    • Convert area to square metres and length to metres before substituting into stress and strain equations.
    • Check that strain has no unit and stress is in pascals when reporting answers.
    • On a stress–strain graph, identify the highest point as the ultimate tensile strength.

    Try the lesson

    The start of the Mechanical properties of matter: (c) stress, strain and ultimate tensile strength lesson, made from checked teaching notes for this specification point.

    Listen

    Stress is the force acting per unit cross-sectional area, σ = F/A, measured in pascals (Pa), where 1 Pa = 1 N m⁻². Strain is the fractional change in length, ε = x/L, where x is the extension and L the original length; it has no unit. Stress and strain allow materials of different sizes and shapes to be compared. Ultimate tensile strength is the maximum stress a material can withstand before it breaks when being stretched. On a stress–strain graph, the ultimate tensile strength is the highest stress value reached. For example, a wire of cross-sectional area 2.0 × 10⁻⁶ m² carrying a force of 10 N experiences a stress of 5.0 × 10⁶ Pa. A strong answer earns marks for these points. Stress is force per unit cross-sectional area, σ = F/A, measured in pascals. Strain is extension divided by original length, ε = x/L, and is dimensionless. Ultimate tensile strength is the maximum stress a material can withstand before breaking. Stress and strain allow comparison of materials independently of sample dimensions. A common slip: Using the total length instead of the original length in the strain calculation. Instead, divide the extension by the original length. Exam tip: Convert area to square metres and length to metres before substituting into stress and strain equations.

    See it

    A top-down flowchart. On the left, a node reading 'Force F and cross-sectional area A' points to a node reading 'Stress σ = F/A, in pascals (Pa)'. On the right, a node reading 'Extension x and original length L' points to a node reading 'Strain ε = x/L, no unit'. Both of these nodes point to a node reading 'Stress and strain allow comparison of materials of different sizes and shapes'. That node points to a node reading 'Stress–strain graph', which points to a final node reading 'Ultimate tensile strength: highest stress value reached, maximum stress before breaking'.
    It shows how stress and strain are calculated from force, area, extension and original length, how together they allow materials to be compared, and how the ultimate tensile strength is read as the highest stress on a stress–strain graph.

    Key words

    Say what each word means before you turn the card.

    Stress

    Try answering aloud or in your head before turning the card.

    Strain

    Try answering aloud or in your head before turning the card.

    Ultimate tensile strength

    Try answering aloud or in your head before turning the card.

    See every definition
    Stress
    The force acting per unit cross-sectional area, σ = F/A, measured in pascals (Pa), where 1 Pa = 1 N m⁻².
    Strain
    The fractional change in length, ε = x/L, where x is the extension and L the original length; it has no unit.
    Ultimate tensile strength
    The maximum stress a material can withstand before it breaks when being stretched.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. On the left, a node reading 'Force F and cross-sectional area A' points to a node reading 'Stress σ = F/A, in pascals (Pa)'. On the right, a node reading 'Extension x and original length L' points to a node reading 'Strain ε = x/L, no unit'. Both of these nodes point to a node reading 'Stress and strain allow comparison of materials of different sizes and shapes'. That node points to a node reading 'Stress–strain graph', which points to a final node reading 'Ultimate tensile strength: highest stress value reached, maximum stress before breaking'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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