Solving Physics Problem: Find Coefficient of Friction

[Eiano]

Hello all,
I'm new to this site and think it's a great idea to have a Physics site to help with, b/c we all know physics is the devil... (j/k) :)

Anyway I have this problem which I think i figured out but want to be sure.



Suppose a block of mass 25 kg rests on a horizontal surface and the coefficient of static friction is .22

a). What is the maximum possible Fs that could act on the block.

This is what I got, Fs=m*g*Us
x=(25)(10)(.22) = 55N

b). What is the avtual Fs that acts on the block if an external force of 25N acts horizontally on the block.

ME:
55-25= 30M


And this problem:

2). A 5kg block rests on a horizontal plane a force of 10N applied horizontally causes the block to more horizontally at a constant velocity. What is the coefficient of friction b/w the block and the plane, assume G is 9.8 m/s^2.

My answer doesn't make sense because the coefficient is not between 0 and 1.0

Anyway:

ME:
(9.8)(5)/10= about 5

Any ideas?

Thanks in advance!

 

[OlderDan]

For 1b) the block is not moving. What must be true of the sum of the forces acting on the block?

#2 is upside down.

 

[Eiano]

Okay, so for 1b.)

since it's at equilibrium, the sum of the forces must be equal. (err, must equal 0)?

Fs= (uS)(N)?
Fs= (.22)(25)
Fs=5.5?

Is this good or am I still missing something.

and thanks for correcting my error on 2.

 

[OlderDan]

Okay, so for 1b.)

since it's at equilibrium, the sum of the forces must be equal. (err, must equal 0)?

Fs= (uS)(N)?
Fs= (.22)(25)
Fs=5.5?

Is this good or am I still missing something.

and thanks for correcting my error on 2.

The coefficient of static friction tells you the maximum possible force of friction acting on a stationary object. The actual frictional force can be any value between zero and the maximum. It is often less than the maximum, and that is true in this case.

 

[Eiano]

The coefficient of static friction tells you the maximum possible force of friction acting on a stationary object. The actual frictional force can be any value between zero and the maximum. It is often less than the maximum, and that is true in this case.

thanks, i read the book and didn't get this concept, thanks again for clearing it up.