How do you work out the coefficient of friction

Is there an equation?, and if so how do you get that equation.

Use Newton's Second Law:
F=ma
gravity and friction are acting on the particle:
Fg-Ff=ma
sin40mg-uk*N = ma
sin40mg-uk*N=m*3
Now you need to find what the (N)ormal force is:
Fy=ma (forces in the 'y' direction):
cos40mg - N = ma
since there is no acceleration in the 'y' direction:
N=cos40mg (there is a component of gravity in the'x' as well as the 'y' direction)
now substitute your value for N into the forces in the 'x' direction:
sin40mg-uk*cos40mg=m*3
the masses cancel out because you have an 'm' in each of your terms, so the mass is not relevant:
sin40g-uk*cos40g=3
knowing that g=9.8 m/s^2 which is the acceleration due to gravity:
6.30-uk*7.51=3
7.51*uk=3.30
solve for uk to get:
.44 as your coefficient of friction..

There is one figure for sliding and one for not sliding (called static). Weigh the object. Measure the horizontal force needed to make the object start to move. Measure the force needed to keep it moving at a steady speed. The first measurement divided by weight is the coefficient of static friction, the second is the coefficient of sliding friction.

If you are working a problem about tires, remember that "moving" is not the same as "sliding". When tires roll you use the static number because they are not sliding.

The Coefficient of Friction often denoted as 'u' is equal to the Frictional force (F) over the normal reaction force (R) acting on the body.
In Summery : u =F / R .

Generally you would use grease, ice or lard.
lard a-mighty, move dammit.