Newton's Laws/ Tension Forces Problem

[BEANTOWN]

Homework Statement

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:
http://www.google.ca/imgres?q=physi...w=158&start=0&ndsp=15&ved=1t:429,r:6,s:0,i:77

(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]

Homework Equations

FNet = ma
Fg = mg
g = 9.8

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 I am 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
According to my answer key, this is not right. Please help.

 

[PeterO]

Homework Statement

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:
http://www.google.ca/imgres?q=physi...w=158&start=0&ndsp=15&ved=1t:429,r:6,s:0,i:77

(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]

Homework Equations

FNet = ma
Fg = mg
g = 9.8

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 I am 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
According to my answer key, this is not right. Please help.

Red line above.

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

T₂ is supporting only M₂ so your answer is probably correct.

With a free body diagram:-

for M₂:

M₂g down, T₂ up so your answer should be correct

for M₁:

M₁g down + T₂ down, T₁ up

For part b, there is no acceleration; merely an EXTRA 45N down in the free body diagram for M₁