Aditional No me salen 1.9*. - An automovil heads North with a 20m/s speed; A minute after and 300 meters away from the first position, it started moving eastward, with a 20m/s speed. Then, the average acceleration modulus in that interval is:
                        a) 0                 b) 0,33 m/s²         c) 0,66 m/s²
                        d) 0,47 m/s²    e) 0,08 m/s²         f) 1 m/s²

This excercise is very simple, I chose it because it has some interesting points to analyze. Let´s see an schme where the mentioned speeds are shown. Here I added a reference system with two cartesian axes.

Between A and B we have the timelapse...

Δt_AB =  1 min = 60 s

We have everything to solve this problem (and we even have data to spare)

a_mAB = Δv_AB / Δt_AB =  v_B — v_A / Δt_AB

Let´s see what we are talking about when we mention Δv_AB.

There is another way to face this problem: obtain the cartesian expression of the average acceleration vector and then – the same way as before - calculate it´s modulus. Shall we do it?

a_mAB = ( 20 m/s /60 s) î — ( 20 m/s /60 s) ĵ

a_mAB = ( 0,33 m/s²) î — ( 0,33 m/s²) ĵ

Check this, we reached the same conclusion. That means I solved, I soo solved the excercise. But... now that you have reached this far, keep reading a little bit that the best part is coming: I´m going to talk to you about the spooky and malicious trap us teachers set.. guaaagh!!!

To begin, read again the statement and agree with me: the position B is not necessarily de one I portrayed in the first scheme. The statement says ``300 meters away from the first position´´... This can be 300m to the North or any other direction. The movil´s trajectory - even though the statement alludes to a straight line trajectory - It can be any, while it fulfills that the speeds are tangent to them. Here are a couple of possibilities.

I´ve drawn two trajectories in red, two of the infinite amount of possibilities there are. I´m going to work a little with the one at the right, since it is more usefull now that I can show you clearly the displacement vector, Δr_AB. There you have it, in green. It doesn´t matter where I point to, the displacement modulus is 300 m (the statement agrees)... but here you can crystañ-clear see that non nummerically, non vectorially, non nothing has anything to do with the speed variation, Δv_AB. The 300 m things is Olympian-out of place.

But it wouldn´t have been out of place to use it. I would, too. To know physics implicates to know which elements of the universe to use, and wich not. If the cutout was made by the teacher, and not you, the question would be a silly one, like asking wich color was San Martin´s white horse (well, not sooo silly) (btw, this is considered a really stupid question, the answer is supposed to be ``dark brown´´ but there are no solid sources about this, the paintings in this white horse are misinterpreted sometimes and some people claim there was not even a horse to begin with, so, please imagine a silly question). For the ``realistic´´ effect having some sense, between the options, here should be those that poorly choose the incorrect elements..