Two wires are made of the same material and have the same volume. The first wire has cross-sectional area A and the second wire has cross-sectional area 3A. If the length of the first wire is increased by l on applying a force F, how much force is needed to stretch the second wire by the same amount?
- 6F
- F
- 9F
- 4F
Answer
(3) 9F
Solution
Same volume: A l1 = 3A l2 ⇒ l2 = l13
Young's modulus: Y = F lA l ⇒ F = YA ll
First wire: F = YA ll1
Second wire: F' = Y(3A) ll1/3
= 9 YA ll1
∴ F' = 9F
Answer: (3)