# Dynamics Pushing Blocks

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.

#### Attachments

• 27.2 KB Views: 337
• 38.8 KB Views: 382
Last edited:

Related Introductory Physics Homework Help News on Phys.org
haruspex
Homework Helper
Gold Member
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.

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?

haruspex
Homework Helper
Gold Member
You've missed something out. Write out the equations relating N1, F, fk.

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.

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

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?

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

Nathanael
Homework Helper
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
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.

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

haruspex