Newton's law problem have attempted it

[thugg]

Newton's law problem...have attempted it

http://img128.imageshack.us/img128/2941/asd6rh.jpg

rope is of negligible mass and no friction...now this is what i did

the bc and ab are subscripts for cords between each block

9.8(10)- Tbc = 10a
9.8(40)-Tab = 40a
Tab=30a

so by substituting i get a =5.6

then i plug it into get Tbc...which gives me 42N.

but the answer in the book says 36.8N...dunno what I am doing wrong, any ideas?

 

[quantumdude]

9.8(10)- Tbc = 10a

This is OK.

9.8(40)-Tab = 40a

This is not OK.

You've got the weight of the 40 kg block and the tension pulling upwards, which is good. But Tbc is also acting on the 40 kg block, and you've left that out.

That's the only mistake I see, so that should fix the problem.

 

[quantumdude]

And I've deleted your identical thread in the K-12 HW Forum. Please do not post multiple threads for the same topic.

 

[thugg]

wow can't believe i didnt think of that...weird