Pulley and friction question

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  • #1
Coco12
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Homework Statement



m1 has a mass of 47 kg and m2 has a mass of 35 kg. the coefficient of friction between m1 and the surface of the inclined plane are us=.42 and uk=.19 m1 is on an inclined plane angled 25 degrees

A) if the masses are held in place and then released, will they start to move?
B) what mass would you have to add to m2 to cause the masses to begin to move?
C) what would be the acceleration of the masses after adding the additional mass?

Homework Equations


For b- made fnet equal to 0 and solved for m2. (37.8) subtracted 37.8 from 35 to get 2.8kg of additional mass

For c- I used fnet=ma fnet= -Ff-fparallel+m2g/m1+m2
However I didn't get the right answer??

The Attempt at a Solution


I know the answer for a is no.. But how do u determine that it's a no? Is it if Ff + f parallel is more than Ft?
For c.. The answer is 5.7m/s2, how do u get that??
 
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Answers and Replies

  • #2
arildno
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"I know the answer for a is no.. But how do u determine that it's a no?"
1. ASSUME that they are at rest.
2. From this, calculate what the magnitude of the frictional force must be.
3. Compare this value with the MAXIMAL value static friction can have. If the force from 2. exceeds that, it means the objects must move instead. Otherwise, if the solution in 2. is less than the maximal value the static friction can have, then the objects are at rest.
 
  • #3
Coco12
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"I know the answer for a is no.. But how do u determine that it's a no?"
1. ASSUME that they are at rest.
2. From this, calculate what the magnitude of the frictional force must be.
3. Compare this value with the MAXIMAL value static friction can have. If the force from 2. exceeds that, it means the objects must move instead. Otherwise, if the solution in 2. is less than the maximal value the static friction can have, then the objects are at rest.


Do u mean take the kinetic friction and compare it to static?
 
  • #4
arildno
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Do u mean take the kinetic friction and compare it to static?
No.
Call your friction "f", and see what the system at rest implies that the magnitude of "f" must be.
THEN compare it to the maximal value for static friction.
 
  • #5
Coco12
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No.
Call your friction "f", and see what the system at rest implies that the magnitude of "f" must be.
THEN compare it to the maximal value for static friction.

U mean set fnet = 0 and solve for Ff?



Also do u understand how to do c?? I'm stuck on it. Thanks
 
  • #6
arildno
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"fnet = 0 and solve for Ff"
Quite so! :smile:

Then, you'll get a value for F.f
If that value exceeds the maximum static friction can have, your solution is INCONSISTENT with the premises, meaning the system had to be moving after all.
If, however, F-f's value is LESS than maximum value, system at rest is the correct solution.
 
  • #7
Coco12
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Thank you :).. As for question c?
 

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