# Statics Help again

Discussion in 'Off Topic [BG]' started by lamborghini98, Oct 23, 2005.

1. ### lamborghini98The Aristocrats

May 1, 2005
NYC; Portland, OR
Hey guys,
So Im doing my Statics HW, and we were assigned a problem using friction, a topic which we havent covered yet (in this class). If you have an object on a tilted surface, in which direction does the force of friction act? Parallel to the plain? Can you use F=uN?
Thanks!

2. ### lbparkSupporting Member

Apr 23, 2005
Mobile, Al.
From what I recently learned in physics, friction works parallel to the tiltled surface and in the opposite direction the object is sliding if the object slides at all. Don't know about the equation.

3. ### Brad Barker

Apr 13, 2001
berkeley, ca
yep, force due to friction is the coefficient of friction times the normal force.

now, that gets you the magnitude.

the direction is given, like said above, by the opposite of the direction the block would slide down if friction were not present.

4. ### Brad Barker

Apr 13, 2001
berkeley, ca
and keep in mind that the normal force is no longer mg if the plane is tilted.

you gotta do some geometry first!

5. ### lamborghini98The Aristocrats

May 1, 2005
NYC; Portland, OR
I have the unit vector for the normal force... can I just make a head to tail vector diagram with the weight, normal and force of friction? That might help.. .but how do I get the position vector (or the unit vector) of the force of friction?

6. ### Brad Barker

Apr 13, 2001
berkeley, ca
the frictional force will be perpindicular to the normal force.

and "position vector" is the wrong terminology. different chapter altogether!

7. ### lamborghini98The Aristocrats

May 1, 2005
NYC; Portland, OR
lol. good point. i guess i meant the unit vector.
sigh... its taking too long for all of these concepts to sink in. Thanks a bunch.

8. ### PunkerTrav

Jul 18, 2001
I just had a terrible flashback to physics class.

*shudder*

9. ### Jeff MooteSupporting Member

Oct 11, 2001
It's what they said But something you should always remember is that friction acts in the direction of motion, and always acts to oppose the direction of movement.

Plus nothing slides anywhere in statics, so this doesn't really seem like a fair question anyhow ...but apparently your mechanics class is just called statics, and my joke isn't nearly as funny.

10. ### Brad Barker

Apr 13, 2001
berkeley, ca

yeah, i jokingly say to my engineering friends, "statics just seems boring--it's the physics of stuff that doesn't move!"

(of course, you don't want bridges to move. but...then again, i don't think you get natural frequencies of materials in that course--that seems fun.)

and, yeah, friction is kind of confusing.

when you walk, the force due to friction acting on your shoe is in the direction of your motion. same with the wheels on your car.

it's because when you walk, you exert a force on the ground in the backward direction, so friction presents itself in the direction opposite of backwards--that's right, forwards!

11. ### Jeff MooteSupporting Member

Oct 11, 2001
I don't have to take a whole course on statics, just an introduction, but my civil eng. friends certainly do. We did do the resonant frequencies of physical bodies though. That was fun.

But friction is still acting in the direction opposite to the motion of the body it's acting on. You certainly agree thatin the case of the shoe or tire, you considder the sum of the forces on the shoe or tire, not your body or car, and thus friction points forwards while the object moves backwards.

You knew that, but I just wanted to clarify for any readers that didn't.

12. ### Geoff St. GermaineCommercial User

I have to tutor someone for a first year physics course on Thursday. I don't know how much I'm looking forward to it. From the sounds of it they're at about the same point.

13. ### AaronS

Not an engineer and it's been 45 years since I took physics, but it occured to me that, if I recall correctly, statics can involve rotational motion. If the plane is rotating, then I believe the force of friction would be tangent to the rotation. Did I put enough disqualifiers in that?

Oct 11, 2001

15. ### Brad Barker

Apr 13, 2001
berkeley, ca

yeah, the condition would be no net torque for a rotational process to be in static equilibrium.

(most generally, static equilibrium requires no net torque and no net force--there can be motion, either rectilinearly or rotationally, but no changes in motion!)

and i'm not sure what you mean by a rotating "plane," unless you switch to the wheel's frame...

but that's making me dizzy.

16. ### Milothicus

but remember, this is the MAXIMUM value. if the applied force (in this case, gravity) is less, the friction is less. otherwise friction looks like it's causing the block to accelerate uphill...

i got caught by this one once in high school....

17. ### Brad Barker

Apr 13, 2001
berkeley, ca
yeah, that's true. the "equation" is really an inequality.

the physics without calculus classes at my school (that i got most of my tutees from) really didn't care too much about this distinction, but my honors phys with calc class certainly did!