- #1

Altairs

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I had impression that there is just no difference between projection of a vector and its components until I took the Statics course. We are following The Engineering Mechanics : Statics book by Meriam and Krage. I got stuck up in the very beginning. the problem is that before this book, in elementary courses the trick of using costheta for projection and component worked very well. now it seems to be giving way.

In Sample problem 2/4 there are two forces at an agle of 50 to each other acting on a hinge. The question asks to find the projection of resultant on the line of force of the force making the lower side of parallelogram. Here, I took

**R**costheta and the anser came wrong. the solution given is the Force(downward) + Force(upward)costheta(other). why didn't taking simply cos theta work when it would give the component in the direction of that force? Does it mean that components and projections aren't same always? How will I know what to do in a particular situation?

In contrast to above ,in another problem there is a 800lb force at 60 from an axis 'a' and the axis 'b' is at 135 from axis 'a'. In this case the projection is simply 800cos 60 while for component you have to make that parallelogram and that stuff. Why can't I simply find the component also by taking 800cos60 and 800cos75?

Why are the above two cases at 180 of each other?

I am very confused. And Statics is my Primary Course.