# Homework Help: Block and friction

1. Oct 3, 2008

### physics_learn

I need help with this question:

Please see attachement. I tried to solve by making a free body diagram of all the blocks but i don't seem to get to a solution

Any sugestions on how to solve are greatly appreciated.

Thanks

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2. Oct 4, 2008

### Hootenanny

Staff Emeritus
Welcome to Physics Forums,

Firstly, I'm assuming that since no 'stiffness parameter' given, the question means that the string does not undergoe any compression or extension, would that be correct?

3. Oct 4, 2008

### physics_learn

That is correct, the string compression or extension is neglected. We don't know the material of the string either so we assume the string only applies tension.

Any idea on how to solve this question. Thank you?

4. Oct 4, 2008

### Hootenanny

Staff Emeritus
Start by analysing the forces on each block.

5. Oct 4, 2008

### physics_learn

I tried to do the free body diagram for all blocks but I'm confused about the part that says "the blocks do not move relative to each other". Does this mean that the system is moving with constant velocity? and how do you calculate the normal force between block 3 and block 2?

Thanks for the suggestions

6. Oct 4, 2008

### Staff: Mentor

No, it means that all three blocks move with the same velocity.
Start by analyzing the forces on each block. If you don't know a given force, label it and continue. (For example, call the normal force N.) Apply Newton's 2nd law.

Hint: Do the blocks accelerate?

7. Oct 4, 2008

### physics_learn

Thank you Doc Al. Since there is a force applied to m3 there is acceleration, but inside the system the three blocks are stationary.
I am still not sure how to calculate the normal force. Do i have to consider a contact force between blocks 2 and 3? I am having difficulties picturing this.