# Finding mass within a pulley system. Finding coefficient of friction

1. Oct 11, 2011

### Serendipitydo

Ok so I'm in college upgrading, and doing my physics 20. I have a hand in assignment due Wednesday and I can't for the life of me figure out these two questions.

6. The 4.0kg block shown accelerates across a frictionless horizontal table at 1.5m/s2.
Find the mass of object m1

A. 0.61kg B. 0.72kg C. 6.0kg D. 26kg

For m1 I have: FN=Fg. Fg=mg, so 39N. FT-0(no Ff)=ma . So FT is 6.0N. And then I'm lost.

8. A 45kg toboggan and rider decelerate on level show at 0.53m/s2. What is the coefficient of friction between the toboggan and the snow?

A. 0.012 B. 0.054 C. 0.22 D. 0.53

I got that FN=Fg. Fg=mg so 441.45N(rounded to 2SD). Fnet=ma so 24N. And then I'm lost. I know that μ=Ff/FN

Last edited: Oct 11, 2011
2. Oct 11, 2011

### Staff: Mentor

You have a combined mass of M1 + M2 being accelerated (together). Write an equation for the force that is needed to accelerate such a mass.

3. Oct 11, 2011

### Serendipitydo

I got the mass...I think. FT=ma. 6.0N=m1(9.81m/s2) So m1=0.6112kg

Now for the other one...

4. Oct 11, 2011

### Serendipitydo

---
Ok I think I solved it. Ff is Fnet in the direction of acceleration? So since it's decelerating, wouldn't it accelerate in the direction of the friction?

So Ff=ma
Ff=24N

μ=Ff/FN
μ=24N/441N
μ=0.05442

5. Oct 11, 2011

### Staff: Mentor

Not so fast. You haven't got this one right, yet. You must determine the force needed to accelerate the combined masses, since they accelerate together.

6. Oct 11, 2011

### Serendipitydo

But I only need the mass of the one.

7. Oct 11, 2011

### SammyS

Staff Emeritus

8. Oct 11, 2011

### Serendipitydo

I'm not having problems with the others, just when I need to find a mass with minimal info.
Fg1-FT=ma
Use the total mass of the system. The difference of force of gravity and tension on the rope would be the net force acting on the system.

Last edited: Oct 11, 2011
9. Oct 12, 2011

### Staff: Mentor

The question supplies all the information you need. You have a mass (M1 + M2) being accelerated. The force that is causing the acceleration is the weight of M1.

One equation, one unknown.

10. Oct 12, 2011

### Staff: Mentor

That looks right.

11. Oct 12, 2011

### Serendipitydo

Thanks for your help. I tend to overthink these things