Is My Physics Teacher Wrong About Teaching Force?

[godvilla]

I believe my physics teacher to be incorrect in his methods for teaching force. I drew up a quick example of the type of problem we're covering, and I'd like to know whether you believe the statements to be true or false.

[PLAIN]http://img263.imageshack.us/img263/9185/forcesproblem.jpg

 

[Doc Al]

I believe my physics teacher to be incorrect in his methods for teaching force. I drew up a quick example of the type of problem we're covering, and I'd like to know whether you believe the statements to be true or false.

Looks like some basic errors were made:
Dx was found correctly.
Gx was found correctly.
Fx was not found correctly.

Applying ΣF = ma to box 2 gives the net force, not Fx. And why was the acceleration made negative?

Newton's 3rd law will tell you that Gx and Fx must be equal and opposite.

(I'm making the assumption that no other forces are involved. The blocks are sliding along a frictionless surface, for example.)

 

[godvilla]

Exactly! That was my point too. Sounds like I'll be going to see the dean.