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Dynamics Pushing Blocks

  • Thread starter alingy1
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  • #1
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Please look at pictures.
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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  • #2
haruspex
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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
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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

Add both ∑Fx equations:
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
haruspex
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You've missed something out. Write out the equations relating N1, F, fk.
 
  • #5
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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
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Got it. I mixed up the fk equation. I'm fixing it now and posting the new solution.
 
  • #7
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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

Add both ∑Fx equations:
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
 
  • #8
Nathanael
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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.
 
  • #9
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Yes, I did forget the minus! I hope someone else confirms! At least we are two ;)
 
  • #10
haruspex
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I hope someone else confirms! At least we are two ;)
Make that three.
 

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