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**1. Homework Statement**

A cord passing over a frictionless, massless pulley has a 4.0 kg object tied to one end and a 12 kg object tied to the other. Compute the tension and acceleration in the cord.

Right now, I'm just trying to calculate the acceleration. It is supposed to be 4.9 m/s^2

**2. Homework Equations**

F=ma

T1=T2=0

T1-mg=ma

T2-mg=ma

**3. The Attempt at a Solution**

For the 4kg block-

F=ma

T1-mg = ma

T1-39.2 = 4a

Solving in terms of T1 to get

T1= 4a+39.2

For the 12kg block-

F=ma

T2-mg = ma

T2-117.6= 12a

Solving in terms of T2-

T2= 12a+ 117.6

Now, because T1=T2, I can say

12a+ 117.6 = 4a+ 39.2

But then I end up with a=9.8.

The answer is 4.9, which, incidentally, is 9.8/2.

Do I have to divide by 2 because I have two cords? Or is there some other reason I am missing?