Not sure about my solution (fictionless Pulley)

[phalanx123]

Masses m and 2m hang from opposite ends of a string passing over a light,
frictionless pulley. What is their acceleration?



I said that the total unblalance force on the two objects is 2mg-mg=mg and the total mass is 2m+m=3m

So the acceleration a is a=mg/3m=g/3

Is this right? could somebody check it for me please? thanks

 

[Doc Al]

You are correct. But why not check it yourself, by using a more general method: Set up force equations (Newton's 2nd law) for each mass separately and combine the two equations. That will allow you to solve for both the acceleration and the string tension.

 

[phalanx123]

Ok that makes sense ^_^ I'll try it. thanks again