I have a question regarding a friction experiment, so here's what I have so far: 1. The problem statement, all variables and given/known data Calculate the approximate force of friction between the wheels of a cart and a ramp it is rolling on by only using the cart's mass and different values of acceleration up and down the ramp. Here was the general procedure used to collect the acceleration data: 1. Hold cart at rest directly before the sensor on the ramp that will calculate acceleration, then give it a little push so it rolls up the ramp past the sensor. Do this five times and average the value of deceleration. 2. Hold cart at rest directly above the sensor on the ramp that will calculate acceleration, then let it go so it begins rolling down the ramp and past the sensor. Do this five times and average the value of acceleration. Cart's mass = .265kg Cart's acceleration on the way down the ramp = .369m/s^2 Cart's acceleration on the way up the ramp = -.435m/s^2 2. Relevant equations By choosing the acceleration of the cart down the ramp as the positive direction: ƩFx as the cart rolls down the ramp = .265(9.8)sinθ - Ff = .265(.365) ƩFx as the cart rolls up the ramp = .265(9.8)sinθ + Ff = .265(-.435) 3. The attempt at a solution Subtracting the bottom equation from the top equation yields: -2Ff = .265(.804) Ff = -.107N (Rounding to significant figures) Now my question deals with that resultant friction force. I was confused as to why the force came out negative, but unless I made an error somewhere, I think I have an idea as to why. Because the wheels of the cart and the ramp never changed, the coefficient of friction between them stayed constant. And because the mass of the cart never changed, the normal force exerted on the cart stayed the same, and thus there is a constant normal force and coefficient of friction during both runs of the experiment, so the friction force in both runs of the experiment was the same. Because of this fact, must I consider Ff between the ramp and cart as |Ff|, i.e. the sign doesn't matter in this context? This would make sense to me, because I would expect an very small force of friction to act on small, smooth wheels of a cart. If anyone can confirm or help me understand this better, that would be greatly appreciated, thanks.