# Kinetic friction

1. Nov 4, 2008

### Jackpontikka

My teacher gave us a homework. We must count the coefficient of kinetic friction between the object and the surface. We can only use the object, ruler and book or table etc. as the surface of motion. We are not allowed to count the acceleration with clock.

I decided to make the object slide by increasing the angle between book(the surface) and table. Finally object started sliding and I measured the angle($$\alpha$$). And finally I got: $$\mu$$= tan$$\alpha$$.

Gx = sin$$\alpha$$*G

Then I realised that the angle $$\alpha$$ is the angle when Gx>Ff0(read:the maximum of static friction)
But isn't Ff< Ff0.

So is this task even possible(without measuring the mass of the object or/and the acceleration) or what am I doing wrong?

P.S English isn't my first language! :)

2. Nov 4, 2008

### Stovebolt

The task is possible with the items you have. You will not need the specific mass of the object as you are not looking for the specific frictional force.

Good job on finding the coefficient of static friction - you found the point by figuring out how to start motion. For kinetic friction, it may work better if you find out how to stop motion.

Depending on how you set things up, it may also help to consider the energy lost (or work done) in your experiment. Remember that potential energy = mass * acceleration (gravity) * height and that work = force * distance.

Keep working on it, you'll find a way to get the answer.