Homework Help: Quick question about a FBDs involving two masses held vertically by strings.

1. Oct 9, 2011

shawli

I just wanted to double check my logic on something! If two masses are held vertically by two strings like so:

[ceiling]
[string #1]
[mass #1]
[string #2]
[mass #2]

...then what forces are acting on mass #1?

My free body diagrams are below, could someone double check if they're correct? (You'll need pretend they are rotated 90degrees clockwise!).

for mass 1:
Tension in string 1 <------ [mass#1]------>Tension in string 2 AND mass1*gravity

for mass 2:
Tension in string 2 <------ [mass#2]------>(mass1+mass2)*gravity

for both masses:
Tension in string 1 <------ [mass#1 and mass#2]------>(mass1+mass2)*gravity

Do these make sense?
Feedback is appreciated (:

2. Oct 9, 2011

Staff: Mentor

Good!

The weight of mass 1 acts only on mass 1, so it doesn't belong here.

Good!

3. Oct 9, 2011

shawli

Ah, thank you!

So, just conceptually, the weight of mass 1 acts transmits as a force to mass 2 only through the the string between them? And this ends up translating into the "force of tension in string 2"? (mass 1 just pulls up on mass 2)

4. Oct 9, 2011

Staff: Mentor

I think you have things a bit backwards.

The weight of mass 1 has no effect (even indirectly) on mass 2. You could take string 2 and attach it directly to the ceiling (without mass 1 or string 1) and the the tension will be the same.

Now the weight of mass 2 is transmitted to mass 1 via the tension in string 2, which in turn adds to the tension in string 1. But yes, the only way that the weight of mass 2 can affect mass 1 is via the string between them.

5. Oct 9, 2011

shawli

Thank you Doc Al! I'm glad I asked because I see the flaw in my logic now -- there is no way for mass 1 to act on mass 2.

Thanks again!

6. Oct 9, 2011

shawli

By the way, is there any way for me to mark this thread as "solved"?

7. Oct 9, 2011

Staff: Mentor

You are most welcome.
Unfortunately not. (But anyone reading this will know that it's 'solved'.)