Calculating Pull Force on Weights Connected by Cords

[Motorboar]

Homework Statement

A series of weights connected by very light cords are given an upward acceleration of 4.00 m/s2 by a pull P, as shown in Fig. 5.3.( a vertical strand of 4 balls with mass 5,10,15,and 20 kg with the pull at the top) A, B, and C are the tensions in the connecting cords.
the pull P is closest to:

290 N
690 N
50 N
200 N
490 N

Homework Equations

F= ma

The Attempt at a Solution

treating the weights as one mass
Pull-mg=ma
pull=m(a+g)
= 552
Which is not one of the answers. What is the error in my logic?

 

[collinsmark]

The Attempt at a Solution

treating the weights as one mass
Pull-mg=ma
pull=m(a+g)
= 552
Which is not one of the answers. What is the error in my logic?

I'm don't see an error in your logic (if I'm interpreting your description of the figure correctly). Maybe there's an error in your calculations? What did you use for m and g?

[Edit: In other words, your equations seem fine. But I don't know why you ended up with 552]

 

[Motorboar]

added the masses wrong xD Thank you.