# Magnitude of Force to move blocks

1. Jan 18, 2005

### ncrowley1818

I have a homework problem, that is giving me some trouble.
I have to move two blocks that weigh (1.2kg), connected by a rope. They give static friction coefficients for each block. I know that I must overcome the static frictions to make them move. I think I need to add the coefficients and then do something with the weight of the blocks?? Any comments or help???

Thanks,
Nick

2. Jan 18, 2005

### Staff: Mentor

Try considering the two blocks as a single system. What's the total friction force on the system just before it begins to move?

3. Jan 18, 2005

### ncrowley1818

I can add the static forces together .55+.4=.95 total static friction

4. Jan 18, 2005

### Staff: Mentor

I cannot comment on your answer since you did not provide the coefficients of static friction. Also: Be sure to use proper units.

5. Jan 18, 2005

### ncrowley1818

the values are the coefficients of static friction on the two blocks, I added them together to get the total static friction of the total system. I know I must over come the total to make it move. F=ma???

6. Jan 18, 2005

### Staff: Mentor

find the force

Are you saying that each block has a mass of 1.2 Kg and the coefficients of static friction are 0.55 and 0.45?

If so, find the frictional force on each block. What's the equation relating frictional force to the coefficient of friction?

7. Jan 18, 2005

### ncrowley1818

dont you take the

force of static friction/mass of object=coefficient static friction

8. Jan 18, 2005

### Staff: Mentor

No. The maximum force of static friction between two surfaces is given by:
$f = \mu N$, where N is the normal force pressing the surfaces together. For a block on a horizontal surface the normal force equals the weight of the block (mg), so $f = \mu mg$.