Two packages stacked on one another

[hamzaarfeen]

Homework Statement

A delivery van has two packages left that are stacked one on top of another. The upper packages
has a mass of 4.0 kg and is a poorly packaged glass vase, while the lower package has a mass
6.5 kg. The coefficient of static friction between the two packages is 0.211 and the lower box is
fixed to the floor of the van. Since the delivery person is late, he has been accelerating quickly
whenever he has stopped and started (at lights, stop sign etc.), however he never exceeds the
town's speed limit of 60 km/h. What is the minimum amount of time it takes for him to reach
his top speed while still keeping the vase from breaking (sliding off the top box).

Homework Equations

friction= coefficient x contact force
fnet=ma

The Attempt at a Solution

friction = 0.211x 4kg x 9.81
= 8.24N
what do i do next?

 

[_N3WTON_]

a hint: for the 4kg box not to slide off of the 6.5kg box it must have the same acceleration as the 6.5kg box, this acceleration is caused by the static friction between the two boxes

 

[hamzaarfeen]

so how can i solve it? if i make two equations, then one can be made for the upper box using f=ma, but what about the other equation?

 

[_N3WTON_]

so how can i solve it? if i make two equations, then one can be made for the upper box using f=ma, but what about the other equation?

find the acceleration using the hint, and then using this acceleration find the time to reach 60 km/h...I hope I'm not giving away too much but to find the acceleration know that:
$F_{fr top} = (m_{top})(a) = (u_s)(F_{N top})$