Masses Connected By Strings May Be Treated as Rigid Bodies?

[breez]

Say you have 2 masses connected by a massless string, and another massless string is connected to the rightmost mass. A tension force T acts along this rightmost string, resulting in the string connecting the 2 masses to become taunt.

In these cases, you can treat the 2 masses as one mass whose total mass is the sum of the 2 masses correct?

 

[Doc Al]

In these cases, you can treat the 2 masses as one mass whose total mass is the sum of the 2 masses correct?

For what purpose? (Describe the problem you're working on.)

 

[breez]

In the problem, 3 blocks are connected by massless strings, and another string pulls the rightmost block on a frictionless surface. You have to find the tension in the smaller strings in terms of the block masses, tension in the rightmost string, and the uniform acceleration.

 

[Doc Al]

Since the strings are taut, all three blocks have the same acceleration. You are free to choose your system as you see fit in order to solve the problem. For example, you can treat all three blocks as a single system or you can treat each block separately. (Or both!)

Just apply Newton's 2nd law to whatever system you choose and see what you can figure out. (Newton's 3rd law will help as well.)