Triangular shaft change to circular shaft

[fonseh]

Homework Statement

for the circular cross section shaft , why we need to put an sin60 as to get the area ?

Homework Equations

The Attempt at a Solution

why the area shouldn't be pi(0.004) ^2 only ?

 

[BvU]

Homework Statement

for the circular cross section shaft , why we need to put an sin60 as to get the area ?

The author doesn't. This sine appears in the calculation of the area of the triangular shaft

Homework Equations

Nothing ?

The Attempt at a Solution

why the area shouldn't be pi(0.004) ^2 only ?

That's not an attempt at solution. Where do you see this value ? I don't even see it appearing anywhere. All I see is $\pi c^2$ which is a reasonable expression for the area of a circle with radius $c$.

 

[NascentOxygen]

why we need to put an sin60 as to get the area ?

The author is calculating the area of the equilateral[/color] triangle, using ½×base×height

He then equates this area to that of a circle of radius c and is able to calculate the value of c.