# Homework Help: Understanding an instantanious velocity lab

1. Sep 13, 2007

### B-80

Hey, I did a lab yesterday, and I am working on the report right now, my problem is my teacher confused the hell out of me with what it is I need to have in the report.

We took a car and placed it on a track with a photo strip at the top, and sent it through a photo gate, this gave us the instantaneous velocity. Then we had to set it up so that the car went through two photo gates from 180cm until 10 cm, reducing the distance by 10cm each time.

So basically I have a value for what the instantaneous velocity should be from part A of the lab. From part B, I have a chart of 18 different runs, which gives change in time, distance and all that good stuff, and a graph of Velocity/Time. I did this in excel. What I need to find is the instantaneous velocity of the graph i think? I think that means I need to find the limit as t goes to 0 of my data, anyone know how to do that, or can point me in the right direction?

2. Sep 13, 2007

### bel

I think you already have the velocity-time graph, just read the y-co-ordinate of the point on that curve that corresponds to the time t for which you want the instantaneous velocity. If you don't have the velocity-time graph, find the slope of the tangent of the position time graph. Use the method of least squares to fit a second degree curve (I suppose the car is accelerating, else you wouldn't need instantaneous velocity) and use that curve to find derivatives.

3. Sep 13, 2007

### B-80

well I figured it would be constant a, but its not. but I assume thats human error, and the point of taking so many calculations was to show that Vbar would get closer to the number we wanted which was .6m/s as we approached 0m. I did forget to mention that in part A, we just took the measurement from 90cm away, and then we got to 95cm-85cm from the car as our last measurement