A rod 150 cm long and of diameter 2.0 cm is subjected to an axial pull of 20 kn. if the modulus of elasticity of the material of the rod is 2 x 105 n/mm2 ; determine: (i) the stress, (ii) the strain, and (iii) the elongation of the, rod

Respuesta :

Given:
rod of circular cross section is subjected to uniaxial tension.
Length, L=1500 mm
radius, r = 10 mm
E=2*10^5 N/mm^2
Force, F=20 kN = 20,000 N
[note: newton (unit) in abbreviation is written in upper case, as in  N ]

From given above, area of cross section = π r^2 = 100 π =314 mm^2
(i) Stress,
σ
=force/area
= 20000 N / 314 mm^2
= 6366.2 N/mm^2
= 6370 N/mm^2 (to 3 significant figures)

(ii) Strain
ε
= ratio of extension / original length
= σ / E
= 6366.2 /(2*10^5)
= 0.03183 
= 0.0318 (to three significant figures)

(iii) elongation
= ε * L
= 0.03183*1500 mm
= 47.746 mm
= 47.7 mm  (to three significant figures)

The stress, strain and modulus of elasticity are connected by an equation. Thus, the given question has the following answers:

(i) The stress is 2 KN/[tex]mm^{2}[/tex]

(ii) The strain is 0.01

(iii) The elongation is 15 mm

From the question, given: l = 1500 mm, D = 20 mm, F = 20 000 N, λ = 2 x [tex]10^{5}[/tex] N/[tex]mm^{2}[/tex].

Thus,

(i) Stress = [tex]\frac{Force}{Area}[/tex]

  Area of the rod = Area of a circle = [tex]\pi[/tex][tex]r^{2}[/tex]

where r is the radius.

radius, r = [tex]\frac{diameter}{2}[/tex]

            = [tex]\frac{20}{2}[/tex]

r = 10 mm

so that;

Stress = [tex]\frac{20000 }{10}[/tex]

          = 2000

Stress = 2 KN/[tex]mm^{2}[/tex]

(ii) To calculate the strain;

modulus of elasticity,  λ = [tex]\frac{stress}{strain}[/tex]

2 x [tex]10^{5}[/tex] = [tex]\frac{2000}{strain}[/tex]

strain = [tex]\frac{2000}{200000}[/tex]

         = 0.01

strain = 0.01

(iii) The elongation of the rod is also its extension, e.

So that;

strain = [tex]\frac{extension}{length}[/tex]

extension = strain x length

                = 0.01 x 1500

extension   = 15 mm

Therefore: the stress is 2 KN/[tex]mm^{2}[/tex], the strain is 0.01, and the elongation is 15 mm.

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