# Dynamics Pushing Blocks

1. May 21, 2014

### alingy1

Weirdly, my textbook does not give the answer to this question.

Can you check my answer? Sorry for the really bad writing. If you ask, I can more clearly write it.

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Last edited: May 21, 2014
2. May 21, 2014

### haruspex

The handwriting is decipherable, but it's hard to follow what you are doing because it's not clear which equations relate to which FBD.
Please post your working by typing - it's not hard - making it clear which forces relate to which body.
Click on the Go Advanced button. You can pick ∑ off the quick symbols on the right and superscript and subscript from the X2, X2 icons along the top.

3. May 21, 2014

### alingy1

Here goes nothing!
For m1:
∑Fy=N1-m1g-sin(30)F=0
∑Fx=-N21-fk+cos(30)F=m1a

For m2:
∑Fx=-μkm2g+N12=m2a
∑Fy=N2-m2g=0

km1g-μkm2g+cos(30)F=(m1+m2)a

a=1.41 m/s^2
N12=N21=11.52N

That looks great! How about the physics?

4. May 21, 2014

### haruspex

You've missed something out. Write out the equations relating N1, F, fk.

5. May 21, 2014

### alingy1

I do not understand. Can you be more specific? What is it I am missing? I've been looking at the problem long enough that everything seems good.

6. May 21, 2014

### alingy1

Got it. I mixed up the fk equation. I'm fixing it now and posting the new solution.

7. May 21, 2014

### alingy1

Here is my new equation:

μk(sin(30)F+m1g)-μkm2g+cos30F=(m1+m2)a

a=1.16 m/s^2
N12=N21=10.52 N

8. May 21, 2014

### Nathanael

Those answers are correct (and if they're not, we made the same mistakes hahah)

Except for the rounding, if you rounded a to 2 decimals it would actually go up to 1.17 but that's pedantic, your equation is correct.

edit: Actually, there should be a negative sign in very the beginning of your first equation, but you must have just forgotten to type that, because your answer is correct.

9. May 21, 2014

### alingy1

Yes, I did forget the minus! I hope someone else confirms! At least we are two ;)

10. May 22, 2014

### haruspex

Make that three.