Does Anyone Know how to find the force of friction?

Question

I'm in a pre-physics class and We have been building roller coasters out of popsicle sticks (craft sticks) that has a marble as the coaster cart. But now we have to do a bunch of questions like the actual speed, speed without friction, but now this question that says "What is the Force of Friction on the Marble?" This question is hard and i have been trying to find how to get the force of fiction on the marble for about one week and i need some help!

Answer 1

remember friction is fun and you will get far in life...
f=uN