A train approaching a station with speed $v = 60 km/h$ starts to slow down. What is the minimum time to stop the train so that the suitcases lying on the shelves do not move? Consider that during braking the train moves with constant acceleration, and the coefficient of friction $\mu$ of the suitcase against the shelf is 0.2.
Decision:
The suitcases do not slip if $a \leq \mu g$. Thus, the maximum possible acceleration $a_{max} = \mu g$. This is matched by the minimum time required to stop
$t_{min}=\frac{v}{a_{max}}=\frac{v}{\mu g}=8.4 c$.
Decision:
The suitcases do not slip if $a \leq \mu g$. Thus, the maximum possible acceleration $a_{max} = \mu g$. This is matched by the minimum time required to stop
$t_{min}=\frac{v}{a_{max}}=\frac{v}{\mu g}=8.4 c$.