A car is initially moving at 20 m/s east, and a little while later it is moving at 10 m/s north. Which of the following best describes the orientation of the average acceleration during this time interval?

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Conceptually
speaking the direction of change in velocity, which will also be
direction of average acceleration must have component along West to
neutralize its motion along east and must also have component along
North in order to finally move in that direction. Mathematically draw
the initial, V1 and final vector, V2. Apply Triangle law of
addition/subtraction to get the direction of the vector, V2 - V1 and see
that it has direction of tan^-1( 2) = 63.4 degree,.
Please note that average acceleration = (V2 - V1)/t, where t is time
interval.

something has to stop it going east, so the a is west and to go north you need an a value north

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