Question

•1 The floor of a railroad flatcar is loaded with loose crates hav- ing a coefficient of static friction of 0.25 with the floor. If the train is initially moving at a speed of 48 km/h, in how short a distance can the train be stopped at constant acceleration without causing the crates to slide over the floor?

Answer 1

The maximum friction force provides the most significant deceleration (of magnitude a)
(With F
n
​
=mg in this case).
Using Newton’s second law, we find
a=f
s, max
​
/m=μ
s
​
g
In this calculation, it is vital first to convert v = 48km/hr to 13m/s.
This gives the shortest distance to stop:△x = v
2
/2a=36m.