# Relative Velocities/Accelerations

I seem to just not understand how relative velocities and accelerations work.

Here is a problem that I will use to ask my question:

When the box lands, its moving 3m/s to the right, and the belt is moving 1m/s to the right. Therefore, the velocity of the box is 4m/s from an absolute point of view at the instant it lands on the belt. From the view of the belt, the velocity is 3m/s. Is this right?

So to determine the time to bring the package to rest, can't I just ignore the belt and do my work with a "ground" and a box with a velocity of 3m/s?

Moving at 2 m/s with a $\mu$ of 0.2, the force experienced is $\mu\cdot m \cdot g$ and the time take to come to rest is $\frac{mv}{\mu m g} = 1 \mathrm{\;s}$. In that time the belt moved 1 m, and the parcel moved $\frac12 a t^2 = 0.5\cdot 2 \cdot 1^2= 1 \mathrm{m}$ relative to the belt, for a total distance of 2 m.