Pulley Questions: 15kg, 12kg, Fnet, ma, Fg, mg

[magico24]

Homework Statement

Two weights are attached to a frictionless pulley. 15kg and 12kg each in opposite directions
a) find the acceleration of the masses
b) the tension in the rope connecting the masses

Homework Equations

Fnet=ma
Fg=mg

The Attempt at a Solution

I have found the the answer a) but can't figure out how to find b)

 

[rock.freak667]

Homework Statement

Two weights are attached to a frictionless pulley. 15kg and 12kg each in opposite directions
a) find the acceleration of the masses
b) the tension in the rope connecting the masses

Homework Equations

Fnet=ma
Fg=mg

The Attempt at a Solution

I have found the the answer a) but can't figure out how to find b)

How did you find part a?

I would like to know how exactly you did it as that might help for part b

 

[magico24]

I multiplied gravity times both weight fg=mg, fg=15(9.80) and fg=12(9.80) giving me 147 newtows and 117.6 Newtons than i did Fnet=ma Fnet=147n-117.6n Fnet=29.4 than i did 29.4divided by 27 giving me 1.088889 and i rounded that too 1.1m/s squared. which was the right answer in my answer key

 

[rock.freak667]

ok well if that is the answer for a)

There are only two forces acting on either mass, its weight and the tension. What would be the resultant force on either one? (and you have got the resultant acceleration from part a)

 

[magico24]

the answer the teacher gives is 130 Newtons. your reply really does not make a whole lot of sense to me.

 

[magico24]

i just want to know how he gets this answer. The steps he does

 

[magico24]

The 15kg mass has a force of 147 Newtons downwards. I am just confused on the tension of the rope is it something i am just not seeing but i already have the answer to or is is something i need to solve for now using a different equation?

 

[rock.freak667]

Well if you considered the forces acting on the 15kg weight. The weight moves down right? So the resultant is down. Tension (T) acts up..weight acts down.

making ma=T-mg (m=15,g=9.8) and you found a.

 

[dmcay1]

The entire thing can be simplified by leaving gravity equal to g. This would make your equations for a the following
15g-t=15a
t-12g=12a (sim equation)
----------
3g=27a
so

3g 1g
-- = a ---
27 or 9

Having found a you can simply sub it into an original equation i.e.
15g-T=15g
---
9


and solve to find T