# Force of friction help

1. Feb 14, 2012

### -Physician

1. The problem statement, all variables and given/known data
What's the friction's formula, and what do the formula pieces mean, thanks :)

2. Relevant equations

none

3. The attempt at a solution

none

2. Feb 14, 2012

### Shootertrex

You should be able to look this one up elsewhere on the internet, but:

$F_{friction}=N\mu$
N=normal force
$\mu$=coefficient of friction

Normal force is the force exerted on an object from the surface in contact. This is the force that prevents my hand from phasing through a wall when I push on it. In many cases, normal force is the force that pushes an object upward due to the force of gravity, though it is not limited to only this.

Coefficient of friction is the constant that defines the slickness between two surfaces. The higher the coefficient of friction, the more friction there is between two surfaces. The number is unique to the surfaces in contact, so the coefficient of friction will not be the same between a piece of paper and a plank of wood, and a piece of paper and concrete.

I hope I have provided an explanation that was in-depth as you hoped for.

3. Feb 14, 2012

### -Physician

I searched in google and you just completed the information but 1 more question, i heard about static/kinetic force and static/kinetic coefficient , what are those? And would the normal force be $N=G=mg$?

4. Feb 14, 2012

### Shootertrex

Static friction is the frictional force between a stationary object and a surface. Kinetic frictional force is between a moving object and a surface. The corresponding coefficients go with the respective type of friction. In most cases, the frictional force is referring to kinetic, such as an object sliding down a ramp or any other surface. Most of the time a friction question will specify which type of friction is acting upon an object. If it is not specified, you will need to figure out what type is acting on the object. (note: wheels use static friction)

5. Feb 14, 2012

### Shootertrex

Normal force is equal to the force pushing against a surface. If the surface is horizontal, then N=mg. If it is a ramp, N=mgcosθ. Like I had said before, normal force isn't always gravity, but in more cases than not it is.

6. Feb 14, 2012

### -Physician

thank you very much

7. Feb 14, 2012

### Shootertrex

Always a pleasure to help.

8. Feb 14, 2012

### -Physician

Sorry, 1 more question would Normal force would be static force divided by static coefficient ? and same thing with kinetic force and coefficient ?
$N=\frac{F_s}{μ_s}$

$N=\frac{F_k}{μ_k}$

9. Feb 14, 2012

### Shootertrex

If you know both Fs and $\mu$s, then I don't see why not. I find calculating normal force this way rare, but like I said, if you know both, then go for it.
You said you have other questions, send me a link to the thread in a pm and I will see how I can help, assuming you weren't talking about this one.

10. Feb 14, 2012

### -Physician

Okay this is the last question , is Kinetic Force equal to Force of friction ?$F_k = F_f$

11. Feb 14, 2012

### Shootertrex

I assume that you meant "kinetic friction" not "kinetic force". $F_{f}$ will equal what ever force of friction is acting on the object at that moment. If the object is moving, $F_{f}=F_{k}$, while if the object is stationary but a force is attempting to move it, $F_{f}=F_{s}$.

12. Feb 15, 2012

### -Physician

Oh, I got it now, thanks!