Harry Donovan book question

[Dagger]

page 16- 9 on LOADS

From the Example ( Cantilevers and weight)

Shoulde F2 not be -765 lbs (negative reaction) ?

 

[egilson1]

It’s going to take me a while to answer as I don’t have a copy of the book with me while on the Holiday break.

 

[Amiers]

page 16- 9 on LOADS

From the Example ( Cantilevers and weight)

Shoulde F2 not be -765 lbs (negative reaction) ?

take a picture of the figure you are referring to.

 

[Dagger]

See image please (Example Problem)

 

[egilson1]

Ok, thanks for the image. No, the total F2 force is not a negative number. There is 880lbs of force generated by the truss self weight. There is -115lbs of force generated by the cantilevered point load. When you add these together (the equation used simplifies this to strait subtraction) you get the TOTAL force of 765lbs on F2. The only way for this number to be negative is if the force generated by the cantilevered point load was to be greater than the force of generated by the truss self weight. If it’s a negative number, then the truss end would be moving upwards.

 

[Dagger]

I tried calculating that problem using Jay' s way.

I have attached my work - didnt get correct answer

( highlighted red is what i think the issue is)

 

[egilson1]

Step three you add 33000 and 4312.5 instead of subtracting them. If you subtract it works correctly.

 

[Dagger]

I dont quite underdtand F1 is PLUS and F 2 is MINUS? both are cantilevered point loads , no?

 

[Dagger]

Step three you add 33000 and 4312.5 instead of subtracting them. If you subtract it works correctly.

I dont quite underdtand F1 is PLUS and F 2 is MINUS? both are cantilevered point loads , no?

Looked up some info and found
clockwise torque is Minus -
Counterclockwise is Plus +

 

[Dagger]

Jay Glerum

Clockwise is positive
Counter negative.

I have been watching a video about forces - here is a screenshoot. ( it s the opposite? )

 

[Dagger]

Akso for example ( see attached )

Should it not be

M= ( D1x400lb)+ ( - 175lbx 16) ???

 

[DrewE]

Torque sign conventions are just that: conventions, not laws, and I would not be the least bit surprised if different authors chose different ones. If you were to look at the truss from the backside rather than the front, your clockwise torques would "magically" become counterclockwise torques and vice-versa. The important thing in terms of solving a statics problem is to use the same sign convention for torques throughout (and, for that matter, the same sign convention for force directions). So long as you stay consistent and don't make any arithmetic or other mistakes, you should arrive at the same final solution regardless.