2 blocks, each on an inclined plane

  • Thread starter RonakPat
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  • #1
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Homework Statement


Two 2kg blocks are joined by a massless cord and pulley, ignoring friction, determine each block's acceleration using a free body diagram for each.

Picture for visual aid: http://img27.imageshack.us/img27/2653/86460048.png [Broken]
or a link: http://img27.imageshack.us/img27/2653/86460048.png [Broken]


The Attempt at a Solution


I did the free body diagrams and solved for the tension on the string to try to equate the two sides

for the first block i got: T - mgsin30 = ma(1) so T = ma(1) + mgsin30
for the second block i got: mgsin60 - T = ma(2) so T = mgsin60 - ma(2)

The problem is the fact that i have two variables that i am solving for. I can't figure it out
 
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Answers and Replies

  • #2
tiny-tim
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Welcome to PF!

Hi RonakPat! Welcome to PF! :smile:

You're missing the obvious …

the length of the string is constant, so a1 + a2 = 0. :wink:

(some people call that a geometric constraint)
 
  • #3
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Hi RonakPat! Welcome to PF! :smile:

You're missing the obvious …

the length of the string is constant, so a1 + a2 = 0. :wink:

(some people call that a geometric constraint)

So should I still set the two things equal then substitute one of the accelerations(a2) out to find the other(a1) then use what you gave me to find the other(a2)?
 
  • #4
tiny-tim
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Equal and opposite, yes. :smile:
 
  • #5
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Well i solved it and it came out to be 1.79m/s2. Does it matter which of them has the negative acceleration or do both have equal positive accelerations?
 
  • #6
tiny-tim
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Does it matter which of them has the negative acceleration or do both have equal positive accelerations?

hmm … well, you'd better say that one goes upward with acceleration 1.79m/s2, and the other goes downward …

you'll have to make it clear you know which way they're going! :wink: :biggrin:
 

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