Determine static coefficient of friction

[werson tan]

Homework Statement

why the µ for block is 0.3(between B and A) ? why can't be µ= 0.4( between B and C) ??

Homework Equations

The Attempt at a Solution

 

[PhanthomJay]

The problem statement is worded incorrectly. It apparently means to consider friction between blocks and the inclined surface, not between blocks. Move on to the next problem, and study the formulas for static and kinetic friction, and when each applies.

 

[werson tan]

The problem statement is worded incorrectly. It apparently means to consider friction between blocks and the inclined surface, not between blocks. Move on to the next problem, and study the formulas for static and kinetic friction, and when each applies.

Can you explain further ??

 

[haruspex]

Can you explain further ??

The first difficulty is that there is no actual question. It describes a set-up, but doesn't say what is to be determined.

Following the calculation, we see that the first step shows that block C is not about to slide down, nor does it require any support from block B. So the conclusion is that there is no force between those two blocks. However, the calculation makes use of a coefficient of static friction between block C and the ramp, whereas the description gave this as the coefficient between blocks B and C.

Next, we perform the same calculation for block B. This time we find that it will slide, if not held in place by block A. Again, it uses the stated coefficients between A and B as though they are between B and the ramp.

The calculation for block A completely ignores the force that would be exerted by block B, so disagrees with the diagram. Again, it uses the stated coefficients between A and B as though they are between A and the ramp.

In short, the question and solution are nonsense from start to finish.

 

[werson tan]

The first difficulty is that there is no actual question. It describes a set-up, but doesn't say what is to be determined.

Following the calculation, we see that the first step shows that block C is not about to slide down, nor does it require any support from block B. So the conclusion is that there is no force between those two blocks. However, the calculation makes use of a coefficient of static friction between block C and the ramp, whereas the description gave this as the coefficient between blocks B and C.

Next, we perform the same calculation for block B. This time we find that it will slide, if not held in place by block A. Again, it uses the stated coefficients between A and B as though they are between B and the ramp.

The calculation for block A completely ignores the force that would be exerted by block B, so disagrees with the diagram. Again, it uses the stated coefficients between A and B as though they are between A and the ramp.

In short, the question and solution are nonsense from start to finish.

well , i still don't understand why 0.3 is used for B , why not 0.4 is used for block B ?

 

[haruspex]

well , i still don't understand why 0.3 is used for B , why not 0.4 is used for block B ?

if you take the question statement as correct, there is no reason for using either. There simply is no information on the coefficients of friction between the blocks and the ramp.
If you accept that the question statement is garbled, the only way we can deduce what it should have said is by looking at the solution; what it actually said becomes irrelevant.

 

[werson tan]

if you take the question statement as correct, there is no reason for using either. There simply is no information on the coefficients of friction between the blocks and the ramp.
If you accept that the question statement is garbled, the only way we can deduce what it should have said is by looking at the solution; what it actually said becomes irrelevant.

Ok, I know my mistakes already. Thank you for your help.

 

[werson tan]

The problem statement is worded incorrectly. It apparently means to consider friction between blocks and the inclined surface, not between blocks. Move on to the next problem, and study the formulas for static and kinetic friction, and when each applies.

why the µ for block is 0.3(between B and A) ? why can't be µ= 0.4( between B and C) ??
can you explain on that ?

 

[PhanthomJay]

Stop trying to make sense out of a problem that makes no sense, and move on.

 

[werson tan]

Stop trying to make sense out of a problem that makes no sense, and move on.

i still didnt get you . can you explain further ?

 

[haruspex]

i still didnt get you . can you explain further ?

I see no way to make it any clearer than in my post #6.

 

[werson tan]

I see no way to make it any clearer than in my post #6.

tat means there is something wrong with the question ?

 

[haruspex]

tat means there is something wrong with the question ?

Yes!