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## Homework Statement

Boxes are placed on a slope at uniform intervals of time trelease and slide down the slope with uniform acceleration. Knowing that as a box B is released, the preceding box A has slid 6 meters down the slope and that 1 second later they are 10 meters apart, determine the following:

(a) The value of trelease

(b) The acceleration of the boxes.

## Homework Equations

ΔV = a⋅Δt

Δx=Vi⋅t+½⋅a⋅t

^{2}

2⋅a⋅Δx=Vf

^{2}-Vi

^{2}

## The Attempt at a Solution

This problem isn't actually homework, I'm just studying. I'm posting the solution to the problem because I simply don't understand how exactly they arrive at the equations that they do. Here it is:

http://i.imgur.com/oSyIFsm.png

For example, equation (2) doesn't make sense to me as t = tr + 1, yet in the problem t = 1. Also, how come in eqn. (3) they don't employ the 10 m @ t=tr+1? Why is Δx simply xA@tr+1?