Friction Lab Troubles: Finding Static & Kinetic Friction

[ariana0923]

My problem is with this lab on friction...it is a virtual lab, as I attend a virtual high school. I've put in numerous hours and can't seem to figure out what is wrong. You see, we need to determine max static friction and kinetic friction using this program. For all the objects EXCEPT the dog and crate, I did this. But for those 2, there was never a point where the friction dipped...in another words, kinetic never got lower than static, which I KNOW is incorrect.

My teacher tried to show me, but now I can;t seem to replicate what she did...she said that kinetic is when the net force=0, both the applied and frictional forces are equal, BUT the object is still moving. Is that correct?

If you could please explain to me how to use this program to find static and kinetic friction, I would GREATLY appreciate it.


http://phet.colorado.edu/simulations/sims.php?sim=Forces_in_1_Dimension



I do have screen shots to show what I am getting...they explain better:

http://i38.tinypic.com/16kchsp.jpg



Where am I going wrong??

Thank you so much!

 

[LowlyPion]

Welcome to PF.

With the dog - the coefficient of static friction for all that shaggy hair is the same as for the coefficient of kinetic friction and is only .1. Once you apply 25N to the dog, that puppy will be in motion. You won't see any sudden changes in force from friction as it switches from static to kinetic.

The filing cabinet is heavier and the coefficient of static friction is greater (.3), kinetic is .2, so it takes more force to get it started 200*.3*9.8 = 600 and then once moving it drops back to 200*.2*9.8 = 400.