# Homework Help: Just getting into friction kinetic and static

1. Oct 22, 2008

### blakester

The floor of a railroad flatcar is loaded with loose crates having a coefficient of static friction of 0.30 with the floor. If the train is initially moving at a speed of 43 km/h, in how short a distance can the train be stopped at constant acceleration without causing the crates to slide over the floor?

I use weird variables i've noticed browsing these forums but here we go
fg = friction with ground
N=normal force

ok so from my free body diagram i got Force of x and y is:
x---- Fs = ma
y----N - mg = ma (which a is 0 since there is not vertical acceleration)

working it down I got N = mg, and using Fg=UN and combining the equations i get

ma=U*mg

divide by m on each side and plug in what i know i get

a=ug
a=.3 * 9.81
a= 2.943

Then using my constant acceleration equations i have Vf^2 - Vo^2=2ad

(0)^2 - (43)^2 = 2(2.943)d
d=314.135

This is the wrong answer apparently though according to the website and I can't for the life of me see what i did wrong. Any help is appreciated thanks! My midterms tomorrow so wish me luck too heh

2. Oct 23, 2008

### alphysicist

Hi blakester,

I think here is the error. Your acceleration is in m/s2, but your speed is in km/hour. You need to do a unit conversion for the initial speed.

3. Oct 23, 2008

### blakester

Oh man such a simple mistake haha! Thanks alot, I'll be sure to double check my units on my midterm in a few hours