Force Diagrams

[mikewill53]

Hi,
Im a little bit lost on this question thats in my engineering degree. I had a go at drawing the force diagram myself and came up with the top drawing. When I checked againt the universities, they had done it a completley different way. First of all I thought you needed an acute angle which is why i created a line straight down from the centre as opposed to across. As far as I know 210 degrees isnt an acute angle, also if your using 210 degrees how can you have a right angle in the triangle? Obviously theres a reason for doing it like this, Im just wondering what it is and where Ive gone wrong
Regards
Mike

 

[wjm11]

The angles you have labeled as 40 and 70 are incorrect. They should be 30 and 60.

Break the F and G vectors into their component i and j vectors (x and y directions).

The sum of the forces in the x direction (i vectors) must equal zero; therefore the i component of F must be equal and opposite to the i component of G.

Likewise, the sum of the forces in the y direction (j vectors) equals zero, so the sum of the j components of F and G must be equal and opposite to H.

The book is using 210 degrees to keep track of the proper signs on the vectors (up and right -- positive; down and left -- negative) instead of using only acute angles which would all give positive results.

Hope that helps.