Average acceleration using vectors

[jperk980]

At one instant a bicyclist is 30 m due east of a park's flagpole, going due south with a speed of 18 m/s. Then, 14 s later, the cyclist is 45 m due north of the flagpole, going due east with a speed of 9 m/s. For the cyclist in this 14 s interval, find each of the following.
A)average acceleration

What i did was, i used the fact that a=v/t. So i pluged in a=(9+(-18))/14. The acceleration I got was 1.93. that is the wrong answer. Can someone please tell what i did wrong and help me figure out a way to get the angle. thank You

 

[learningphysics]

At one instant a bicyclist is 30 m due east of a park's flagpole, going due south with a speed of 18 m/s. Then, 14 s later, the cyclist is 45 m due north of the flagpole, going due east with a speed of 9 m/s. For the cyclist in this 14 s interval, find each of the following.
A)average acceleration

What i did was, i used the fact that a=v/t. So i pluged in a=(9+(-18))/14. The acceleration I got was 1.93. that is the wrong answer. Can someone please tell what i did wrong and help me figure out a way to get the angle. thank You

velocity and acceleration are vectors... can you write the two velocities in vector form... (use i and j vectors). Then calculate average acceleration just as you did, but you'll be subtracting two vectors...

 

[jperk980]

sorry i missed type what i did i did subtract i did a=9-(-18)/14 and i got 1.93. i know the answer is 1.37 but i don't know how to get it

 

[learningphysics]

sorry i missed type what i did i did subtract i did a=9-(-18)/14 and i got 1.93. i know the answer is 1.37 but i don't know how to get it

You can't subtract like that because east and south aren't along the same line... write the two velocities as vectors (using i for east west... j for north south).

For example: 20m/s west is

$$\vec{v} = -20\vec{i}$$

20m/s north is:
$$\vec{v} = 20\vec{j}$$

Does this make sense?

If the notation doesn't make sense... then try it like this... what is the average acceleration in the east west direction (taking east as positive) ?

what is the average acceleration in the north south direction (taking north as positive) ?

What is the magnitude of the net acceleration?