I have two physics related questions for you guys.
1). On our last test, I was stumped on one question. The scenario is a car rolling down an incline at an initial velocity of 20 m/s. At t=0.0s the driver locks the brakes, and comes to a rest 30m down the slope.
For the last question related to this, I had to find the force of friction on the car tires, which sounded easy at first. After some thought, I figured I needed to solve for both the acceleration of the car and the time that it took for it to come to rest. So I figured:
s(t) = (1/2)at^2 + 20t
v(t) = at + 20
v(tf) = 0 = at + 20
s(tf) = 30 = (1/2)at^2 + 20t
Ok, I don't know how to solve that (if it's even possible), so I must be going about the problem wrong. Any thoughts?
2). This is an extra credit question, so if you don't want to help me earn extra credit, then don't answer
We're using the equation for kinetic friction f = un, where u is the coefficient of friction and n is the normal force.
Scenario: A mug sits at rest on a paper towel which extends 50cm past the mug. A force is applied to the paper towel so that it accelerates at 200 m/s/s until none of the towel is under the mug. How far does the mug move? (For simplicity, assume the paper towel contacts the mug at a point)
Also given: Mass of mug .5kg, coefficient of friction between mug and towel = .7, coef. of friction between towel and table = .5 (do we even need this?), and coef. of friction between mug and table = .4
I haven't given a whole lot of thought to the second problem yet, but it doesn't seem that complicated to me. It must be complicated, though, since my professor said he hadn't ever had a student completely solve the problem.
