No solution can be attempted without the mass of the block.

In summary: Then solve for r.In summary, the question asks for the optimal placement of a brass block on a revolving turntable with a coefficient of friction of 0.18. The problem can be solved by setting up equations and using the angular frequency of the turntable to find the velocity. By equating two equations for v, the value for r can be solved for.
  • #1
mujadeo
103
0

Homework Statement



The coefficient of friction between a certain brass block and a large revolving turntable is µ = 0.18. How far from the axis of rotation can the block be placed before it slides off the turntable if it is rotating at 33 1/3 rev/min?

Homework Equations


how can i do anything with this question without the mass of the block?


The Attempt at a Solution

 
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  • #2
You can still set up the equations describing the situation. Try setting up the equations and just use "m" as a placeholder for mass. The fact that you don't know the mass shouldn't be a problem if you have this problem set up right.
 
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  • #3
how can i get the velocity (to get centrip accel) if i don't know r?
how can i find the right r if i can't get the velocity?
Please help
 
  • #4
You may not know v directly, but you do know that the angular fequency is 33 and 1/3 rev/min. Can you use this to find the velocity? You must have some work for this problem. At least show me how you set up the problem or something. I'm really not supposed to give you this much help if you don't show work.
 
  • #5
yes i know that v=2pier/T
i have T as 1.8s

But then I need r to get v. \
But the questions is asking me for the optimal r (so it won't slide off).
??
 
  • #6
so it seems like a contradiction.
i need r to calcui;llate v, but r is not known
 
  • #7
also i realize that F= un
so i will likely do something like equate the un = mv^2/r
and then cncel the n and m somehow to find r
 
  • #8
mujadeo said:
also i realize that F= un
so i will likely do something like equate the un = mv^2/r
and then cncel the n and m somehow to find r

Your on the right track. What force will the normal force be equal to?
 
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  • #9
normal will be same as the mass
 
  • #10
in the opposing direction i mean
 
  • #11
ok so
.18n = mv^2/r
cancel the n and m
.18 = v^2/r

but this is same problem. --i need a value for r in order to calculate v?
 
  • #12
mujadeo said:
normal will be same as the mass

Mass is not a force. I think you mean the normal force will be the same as the weight:

n=mg

So, your line above should read:

.18g=v^2/r

Now, once you know this, you have gotten rid of the m. Now the only problem is that pesky v! If only you had another equation for v in terms of r to plug in there...hint hint.:wink:
 
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  • #13
well this is tangetial vel right
so vt = wr
please tell me what I'm doing wrong?
you can see by now that i know basically what's going on but I'm just tripping over something really stupid.
 
  • #14
You said the formula you need in post #5.

You can plug in v_t=rw for v, but what about this?

(1)[tex]v = \frac{2\pi r}{T}[/tex]

Since you have already calculated T, why not use this?

If [tex]\mu g = \frac{v^2}{r}[/tex]

(2)Then, [tex]v=\sqrt{r\mu g}[/tex]

So if v = (1) and v = (2)...
 
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  • #15
yes but what value for r then??
 
  • #16
see this is the same problem iv been having all along
they ask what's the optimal r, but to find r i need v, and to find v i need a value for r??
 
  • #17
mujadeo said:
yes but what value for r then??

I have edited my above post to be clearer.

You basically have two equations for v. The trick is to equate the two equations, so you are left with just r as an unknown.
 

1. What does the phrase "no solution can be attempted without the mass of the block" mean?

The phrase means that the problem or situation cannot be solved or addressed unless the mass of the block is known or considered.

2. Why is the mass of the block important in finding a solution?

The mass of the block is important because it affects the behavior and interactions of the block with other objects or forces in the problem or situation.

3. Can't we just estimate the mass of the block?

Estimating the mass of the block may be possible, but it may not provide an accurate solution. It is important to have an exact or close estimate of the mass in order to obtain a reliable solution.

4. How do we determine the mass of the block?

The mass of the block can be determined by using a scale or by using the density and volume of the block. It can also be measured directly if the block has a known density and volume.

5. Is the mass of the block the only factor to consider in finding a solution?

No, the mass of the block is not the only factor to consider. Other factors such as friction, gravity, and other external forces may also affect the solution. However, the mass of the block is an important factor that cannot be ignored in most situations.

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