# Thread: Average velocity Vs. velocity

1. I am teaching high school physics for the first time and I was challenged by a student about a practice problem.

The problem is:
You walk 10 m forward, 5 m backward, and 10 m forward all in 15 seconds. What is your overall forward velocity?

I explained that we would use average velocity = change in position/change in time and I drew a vector diagram and used vector addition to show them that the forward displacement would be 15 m and therefore the forward velocity would be 1m/s

He is arguing that overall does not mean we use average velocity and he wants to use instantaneous velocity = distance/time and says it should be 10m forward + 10 m forward = 20m and claims the answer should be 1.3 m/s. He is very persistent and I cannot convince him. I wanted to reach out and get other's opinon to 1. make sure I am right and 2. what is a good way to further explain this if I am right? I feel like if we would have to use average velocity because we do not know how long it took to cover the various segments of the walk. We are only given the overall time of 15 s.

Thoughts?  2. Originally Posted by skj351 I am teaching high school physics for the first time and I was challenged by a student about a practice problem.

The problem is:
You walk 10 m forward, 5 m backward, and 10 m forward all in 15 seconds. What is your overall forward velocity?

I explained that we would use average velocity = change in position/change in time and I drew a vector diagram and used vector addition to show them that the forward displacement would be 15 m and therefore the forward velocity would be 1m/s

He is arguing that overall does not mean we use average velocity and he wants to use instantaneous velocity = distance/time and says it should be 10m forward + 10 m forward = 20m and claims the answer should be 1.3 m/s. He is very persistent and I cannot convince him. I wanted to reach out and get other's opinon to 1. make sure I am right and 2. what is a good way to further explain this if I am right? I feel like if we would have to use average velocity because we do not know how long it took to cover the various segments of the walk. We are only given the overall time of 15 s.

Thoughts?
Here is a way to prove your student wrong: assume that the speed was constant, that means that each 10m segment is covered in 6s (5m is covered in 3s) for a total of 6+6+3=15. Therefore, the instantaneous speed is 1.666m/s, not 1.3m/s.  Posting Permissions
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