JEE Main 2021 · PhysicsMultiple choiceSingle correctMediumMulti-step

JEE Main 25 July 2021, Shift 1, Physics Q18

Question 18 of 90 in this shift, Physics question 18 of 30, Section A.

Two wires of same length and radius are joined end to end and loaded. The Young's modulii of the materials of the two wires are

Y_1

and

Y_2

. The combination behaves as a single wire then its Young's modulus is :

(1) $Y=\frac{Y_1Y_2}{Y_1+Y_2}$
(2) $Y=\frac{Y_1Y_2}{2(Y_1+Y_2)}$
(3) $Y=\frac{2Y_1Y_2}{Y_1+Y_2}$ Official answer
(4) $Y=\frac{2Y_1Y_2}{3(Y_1+Y_2)}$

Official answer

Option 3

NTA final key.

Chapter: Mechanical Properties of Solids
Topic: Elasticity
Idea tested: Series and Parallel Composite Wires

Same idea in other shifts

Asked 4× in all

4 Apr 2026, Shift 1 · Q34A string $A$ of length $0.314$ m and Young's modulus $2\times10^{10}\ \mathrm{N/m^2}$ is connected to another string $B$ of length and…MediumSingle correct
4 Apr 2026, Shift 2 · Q31A metal string $A$ is suspended from a rigid support and its free end is attached to a block of mass $M$ . Second block having mass $2M$ is…MediumSingle correct
6 Apr 2026, Shift 1 · Q34The two wires $A$ and $B$ of equal cross-section but of different materials are joined together. The ratio of Young's modulus of wire $A$ …EasySingle correct

Question text from the official JEE Main paper published by NTA; answer from the final answer key. Chapter, topic and idea tags, difficulty and skill are Lumi’s. Lumi analyses 42 of the 174 JEE Main shifts held from 2017 to 2026.