# Frictional Forces and 1D Motion

INTRODUCTION:
This is a problem from my Introduction to Physical Science class using "Conceptual Physics" 10th Ed.by Paul G. Hewitt

EXACT PROBLEM:
"The coefficient of kinetic friction between a rubber tire and a wet concrete road is 0.5."

PROBLEMS FACED:
a) Find the minimum time in which a car whose initial speed is 30 mi/hr can come to a stop on such a road.
b) What distance will the car cover in this time?

MY THOUGHTS:
I know I have a coefficient of friction & V0 and Vf. Beyond that, I don't know how to look for the time it will take. What equation am I supposed to be using?

## Answers and Replies

Lets take this a step at a time. To stop, you need a negative acceleration. Assume the brakes are aplied full force as in panic stop.

I still find the fact that speed/velocity never enters the eqn curious: that is the braking force is constant. Whether from 100mph or 5mph the retarding force is the same. So its like a baseball thrown upwards at different speeds.:The stopping distance depends on velocity, but its predictable. This is key.

So in a horizontal position, the frictional force depends on the negative force/acceleration generated by the weight times the coeffiecient of friction. Any help?

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