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## Homework Statement

A train slows down as it rounds a sharp horizontal turn, slowing from 90 km/h to 50 km/h in the 15 s that it takes to round the bend. The radius of the curve is 150 m. Compute the total acceleration at the moment the train speed reaches 50 km/h

## Homework Equations

a = sqrt[a_t^2 + a_r^2]

## The Attempt at a Solution

I have the solution for the total acceleration, I'm just wondering what I am supposed to give the angle relative to in these kinds of problems in general. The answer key gives the angle relative to the final velocity vector, is that a good reference point in general?