how does the coefficient of friction depend upon the normal force between the surfaces in contact?

Question

i'm doing a physic experiment to investigate about the coefficient of friction by using wooden block

Answer 1

You've had some good answers, so I won't bother answering your question directly. Instead let me give you some advice on your experiment. Friction is a very difficult force to measure reliably. Now this may seem counter-intuitive; many people will say that it can be measured as simply as pulling a weight across a surface with a spring scale. The answer to that is yes and no. First a little background; I am an aerospace engineer for the US Navy. I perform ground loads analysis for our aircraft. Therefore the coefficient of friction of our ship decks is very important to me. Unfortunately, determining the friction is very complicated. NAVSEA, the Naval Sea Warfare Center, that controls the ships, is responsible for telling me what the CoF is. The problem is: They won't give a definitive answer. Why? Because there are many methods of determining friction, and they will all give different answers. Even the same test, repeated minutes later, will give different answers. Change anything in a test- test patch location, temperature, humidity, time, etc. and you will get widely varying answers. It is really impossible tell what exactly is the proper value to use. That's where we engineers have to use our judgement. We have to decide a good value to use. NAVSEA has told us their tests have produced coefficient values above 1. That's right- the anti-skid deck paint is practically grabbing the aircraft wheels. Anyone who has been on a new painted Aircraft Carrier can believe this- you trip and fall and you will lose every bit of skin on your palm or any other body part that skids.

The basis of my pointing this out is that when you have trouble with repeatability in your experiment, don't worry about it. It's to be expected. It is also something good to bring up in your report.

Answer 2

The frictional force is directly proportional to the normal force between the surfaces in contact .

The constant of proportionality is called coefficient of friction.

HENCE IT DOES NOT DEPEND UPON THE NORMAL FORCE.

The coefficient of friction depends upon the nature ( rough or smooth) of the two surfaces in contact.

It does not depend upon the magnitude of area of contact.

Therefore by using different wooden blocks which vary in their weights you can find that the coeffiecient of friction is a constant

Answer 3

The heavier the object, the greater the co-efficient of friction.

Think about it, a 5kg block on a pice of snadpaper is easier to push than a 10kg block on the same piece of sand paper.

Also take into account the amount of area touching the surface - a smaller area of contact will reduce the co-efficient of friction

Answer 4

the equation for force of friction is given by
f=uR
where u-coefficient of friction
R-normal reaction(normal force)

the coefficient of friction,u,is generally less than 1.
for a body lying on the surface of earth normal reaction is equal to
mg where m is mass and g-acceleration due to gravity.
i hope that i have answered ur question.

Answer 5

When there z no relative motion:
friction force = max(u*N , F)
where,
u=coeff of static friction
N = normal force
F = External applied forve

If relative motion z there:
friction force = u*N
u=coeff of kinetic friction