a question was given to us in assignment which was as follows

A person moves 20 metres towards north then 30 metres towards east and then moves 30 x 1.414 metres towards south west. The whole trip takes 5 seconds. What is the person's average velocity for this whole journey?

i just squared every magnitude of velocity in all the three directions and then I took square root of that sum

doing so what i got is this number 55.735267112 so this will be the total displacement of that person and dividing this with the given time [5in this case] we get the answer as 11.1 but the answer is 2 m/s

can any one suggest me where is my concept going wrong


closed as off-topic by John Rennie, BMS, Ali, Brandon Enright, David Z Jul 6 '14 at 22:25

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  • $\begingroup$ The problem as written isn't very clear. You just gives directions and distance and no time so no velocities can be found. $\endgroup$ – jkeuhlen Jul 6 '14 at 16:18
  • $\begingroup$ but this is the only information given in the question $\endgroup$ – agha rehan abbas Jul 6 '14 at 16:20
  • 1
    $\begingroup$ @jkeuhlen i think distances and time would be enough for us to calculate the average velocity $\endgroup$ – agha rehan abbas Jul 6 '14 at 16:21
  • $\begingroup$ You have distance and then you have a time to stop. That time would be used to find acceleration. You would need time to cover the distance to get a speed. $\endgroup$ – jkeuhlen Jul 6 '14 at 16:23
  • $\begingroup$ @jkeuhlen yes time is given and it is 5secs in this case $\endgroup$ – agha rehan abbas Jul 6 '14 at 16:24

If you draw the path, it looks like this:
enter image description here

The person moved 10 m in 5 seconds.

  • $\begingroup$ how can you say that he moved 10 metre in 5 seconds as there is no evidence for it $\endgroup$ – agha rehan abbas Jul 6 '14 at 16:25
  • $\begingroup$ Look at the vectors. The distance from the endpoint to the starting point is 10 m. $\endgroup$ – LDC3 Jul 6 '14 at 16:25
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    $\begingroup$ +1 for the drawing getting the point across better than any calculation $\endgroup$ – ACuriousMind Jul 6 '14 at 16:26
  • $\begingroup$ @LDC3 ok now i can see thanks a lot for your answer $\endgroup$ – agha rehan abbas Jul 6 '14 at 16:27
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    $\begingroup$ @agharehanabbas The person has a total displacement of 10 metres south. Velocity is the displacement per time. $\endgroup$ – jkeuhlen Jul 6 '14 at 16:27

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