Determining Coefficient Of Friction

[YeeHaa]

Hi,

I'm doing research on the Coefficient Of Friction using a forceplate. I'm trying to determine the coefficient by pulling several objects over the plate, and using the frictional force and normal force to determine the coefficient of friction (measured by the force plate).

When I pull the object in a different angle in the vertical plane, does this effect the coefficient of friction? I know the coefficient is a constant, but how else would i go by determining it when the angle changes constantly?

[PLAIN]http://www.roymech.co.uk/images11/friction.gif

In other words, when Fapp changes angle (upwards), how do i determine the coefficient of friction?

 

[grzz]

I am assuming that the surface on which the object is moving is horizontal.

As the angle of the pull on the object with the horizontal is increased, the normal reaction of the surface will decrease. But the frictional force will also decrease so that the coefficient of friction will (hopefully!) remain constant.

 

[YeeHaa]

Well I thought so too, and came up with this:

P = applied force
Fw = friction force
N = normal force

N = G - Px

and

Fw = Py

Py = P.cos(α)
Px = P.sin(α)

µ = Fw/N

So this means that:

µ = P.cos(α)/(G-P.sin(α))

Which has very weird results (doesn't remain constant for a given applied force and weight).

What am I missing here?

 

[grzz]

What I had in mind was exactly as in your last post.Even the calculation.

Note that the pulling force P is not any force you choose. P must be that force which JUST makes the object tend to slide.

I think that as angle of P with the horizontal changes, the value of P will also have to change.

Anyway, the topic of friction is not easy. You may read about it in THE FEYNMANN LECTURES vol 1.

 

[YeeHaa]

I know for static friction, the friction force and normal force are the forces on the moment when the object JUST tends to slide. I made all my calculations based on this.

For kinetic friction i averaged the values of the normal force and friction force during the time the object is moving.

So the explanation i give would be correct, if i say the applied force must change too in order to keep balance?

I'll look up the book you referenced me to.

Thanks.

 

[grzz]

Yes I think that the applied force just to make the body slide changes with angle of applied force.