Physics · Mechanical Properties Of Solids · NEET
It is LATERAL strain on top, LONGITUDINAL strain on the bottom: sigma = lateral strain / longitudinal strain = (Delta d / d) / (Delta L / L). Students often flip it. Memory trick: the wire's length change (longitudinal) is the CAUSE you apply, and the width change (lateral) is the RESULT. Poisson's ratio measures how much result you get per unit cause, so the result (lateral) goes on top.
Both lateral strain (Delta d / d) and longitudinal strain (Delta L / L) are ratios of two lengths. metre / metre cancels, so each strain is a pure number with no unit. Dividing one pure number by another still gives a pure number. So Poisson's ratio is dimensionless. This is a very common one-line NEET fact.
For almost all normal materials it is treated as a POSITIVE number (typically 0 to 0.5). The minus sign in sigma = -(Delta d / d) / (Delta L / L) is put there because when length increases (Delta L positive) the diameter decreases (Delta d negative). The minus sign cancels that negative, making sigma come out positive. So the value you quote is positive; the sign only accounts for the opposite directions of the two changes.
No. Young's modulus Y = stress / longitudinal strain and it has a unit (N/m^2 or pascal) because stress has a unit. Poisson's ratio compares TWO strains (sideways vs lengthwise) and has NO unit. Young's modulus tells you how stiff a material is; Poisson's ratio tells you how much a material squeezes sideways when you stretch it lengthwise.
The most common symbol in NCERT and NEET is the Greek letter sigma. Some books use nu (the Greek 'nu'). If a question uses either symbol for a stretched-wire lateral-to-longitudinal strain ratio, it means Poisson's ratio. Do not confuse this sigma with the sigma used for stress in some other books.
Match List I with List II. List I: A. Young's Modulus, B. Compressibility, C. Bulk Modulus, D. Poisson's Ratio. List II: I. (Delta d/d)/(Delta L/L), II. FL/(A.Delta L), III. -(1/V)(Delta V/P), IV. -V.P/Delta V. Choose the correct answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is lateral strain divided by longitudinal strain when a material is stretched: sigma = (Delta d/d)/(Delta L/L).
No. It is strain divided by strain, so it is a pure number with no unit and no dimension.
For real materials it lies between about 0 and 0.5. The theoretical limits are -1 to 0.5, but positive values are what you meet in NEET.
Lateral strain (the sideways/diameter change) goes on top; longitudinal strain (the length change) goes on the bottom.
Because stretching increases length but decreases diameter. The minus sign makes the ratio come out as a positive number for normal materials.
