# Homework Help: Correct coordinate axis

1. Oct 10, 2011

### Miike012

I am confused how they picked the direction right of block M1 to be -x and the downward direction of block M2 to be +x..?

I didn't know that one could create two different coordinate axis.

Correct me if I am wrong but it seems that if you are working with two diff body's that are not in contact you can create diff axis for both body's.. but if two body's are in contact then you have to use the same axis for both body's?

2. Oct 10, 2011

### Staff: Mentor

What's block M1? What's block M2? What are they up to? Who are "they"? Without context, your statements are vague and your questions unanswerable.

3. Oct 10, 2011

### Miike012

sorry forgot to add the picture... here it is..

#### Attached Files:

• ###### phy.JPG
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4. Oct 10, 2011

### Staff: Mentor

Okay. In this situation there are two separate objects being considered. They are connected by a string (or rope, or cable, or however it's defined). Because they are so connected, their motions are coupled. That is, any change of displacement of one is going to be identical to the displacement of the other, in magnitude if not direction. This being so, it would make sense to choose the same variable name for both displacements (they will have equal values at all times). x is the value of the horizontal displacement of block M1, and it's also the vertical displacement of block M2. Does that work for you?