Question Details

A cable is replaced by another one of the same length and material but of twice the diameter. The maximum load that the new wire can support without exceeding the elastic limit, as compared to the load that the original wire could support, is

Options

A

Half

B

Double

C

Four times

D

One-fourth

Correct Answer :

Four times

Solution :

The correct option is Four times.

To understand why the maximum load increases by a factor of four, let us analyze the relation between stress, load, and the cross-sectional area of a cable.

The elastic limit of a material is defined in terms of the maximum stress (force per unit area) the material can withstand without undergoing permanent deformation. This maximum stress, known as the elastic limit stress (σmax), is a characteristic property of the material itself. Since the replacement cable is made of the same material as the original cable, both cables have the same elastic limit stress:

σmax, new=σmax, original

The stress σ in a cable supporting a load F is given by the formula:
σ=FA
where A is the cross-sectional area of the cable.

Therefore, the maximum load Fmax that a cable can support without exceeding its elastic limit is:
Fmax=σmaxA

For a cable with a circular cross-section of diameter d, the cross-sectional area is:
A=π4d2

Substituting this area into the maximum load equation gives:
Fmax=σmaxπd24

This shows that the maximum load is directly proportional to the square of the diameter (Fmaxd2).

Let the diameter of the original cable be d1 and the diameter of the new cable be d2=2d1. The ratio of the maximum load supported by the new cable (Fmax, new) to that of the original cable (Fmax, original) is:
Fmax, newFmax, original=(d2)2(d1)2=(2d1)2d12=4d12d12=4

Thus, the maximum load that the new wire can support without exceeding its elastic limit is four times the load that the original wire could support.

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