# Homework Help: How to I find what needs to be the mass of an object in a friction problem?

1. Sep 30, 2011

### Mustachio

1. The problem statement, all variables and given/known data

In the figure the coefficient of static friction between mass mA and the table is 0.50, whereas the coefficient of kinetic friction is 0.42.
(a) What minimum value of mA will keep the system from starting to move?
(b) What value of mA will keep the system moving at constant speed?

2. Relevant equations
So I know that

Fs <= uN
Fk = uN

But I don't know what to do when I lack the Newtons.

Can anyone give me a hint on where to start?

3. The attempt at a solution

2. Sep 30, 2011

### Staff: Mentor

If the system isn't moving, what must be the tension in the rope?

3. Sep 30, 2011

### Mustachio

Would it be 0 because it'd be at equilibrium when not moving?

4. Sep 30, 2011

### Staff: Mentor

If someone asked you to support that hanging mass mB by holding the upper end of the rope, what force would your arm have to exert? It would not be zero.

5. Sep 30, 2011

### Mustachio

A force greater or equal to that of the force that is pulling down mb?

6. Sep 30, 2011

### Staff: Mentor

A force of how many newtons?

7. Sep 30, 2011

### Mustachio

Can I multiply 2 * 9.8(m/s^2) to get the mkg/s^2 = N unit?

8. Sep 30, 2011

Yes.

9. Oct 1, 2011

### Mustachio

................... ^ N
...................|
...................|
mSn...mKn.....|............. T
<---<--------|---------->
...................|
...................|
...................| Ma(g)
................... v

T
|
|
o
|
|
2g

And I derived 2 sets of equations.

For the first FBD I got
X: T - mkN - msN = (Ma)a
Y: N - (Ma)g = 0

Second one I got

Y: T - 2g = 2a.

Do I have the problem set it up right?

10. Oct 1, 2011

### Staff: Mentor

What do mkN and msN stand for? If those are supposed to be the friction forces, realize that only one acts at any given time; it's either static or kinetic friction.

OK.

OK. (What's a?)
If the system isn't moving--or is moving at constant speed--what must be the tension in the rope?

11. Oct 1, 2011

### Mustachio

I don't understand the question. :\

12. Oct 1, 2011

### Staff: Mentor

What does your equation for MB tell you? (What's the acceleration?)