# Homework Help: Newton's Laws/ Tension Forces Problem

1. Apr 14, 2012

### BEANTOWN

1. The problem statement, all variables and given/known data

Two Objects are hung from strings. The top object (m1) has a mass of 18 kg and the bottopm object (m2) has a mass of 12 kg.

Looks like this:

(a). Calculate tension in each string.

(b) Calculate the tension in each string if you pull down on m1 with a force of 45 N [down]
2. Relevant equations

FNet = ma
Fg = mg
g = 9.8

3. The attempt at a solution
I knew how to do (a).
I creted a free body diagram for each weight first.
Then for m1:
FNet = T1 - Fg
0 = T1 - Fg ----> FNet = 0 because it is at rest
T1 = Fg
T1 = (18)(9.8)
T1= 176.4 N
And then i did the same for the second one, and came up with T2 = 117.6

Then I got to (b). But im lost.
I attemted to find acceleration by doing:
FNet = Fa ----> Fa = the 45 N
FNet = 45 N
ma = 45 N
(m1 + m2)a= 45 ---- a= 45/(18 + 12)
a = 1.5
So:
FNet = T1 - Fg1
(18)(1.5) + (9.8)(18) = T1
T1 = 203.4 N
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Apr 14, 2012

### PeterO

Red line above.

T1 is supporting BOTH masses so the mass used to calculate T1 should be 30kg.

With a free body diagram:-

for M2: