Lab 1: Measuring speed from a video
Our goal is to measure speed from an iPad video...
1.) Take two videos of someone / something moving
With your lab/discussion partners, you'll make two videos of someone walking or biking or longboarding or ... moving. The two videos should be of people moving at *different* speeds.
- One video should be recorded by the partner(s) in class and one video should be recorded by the partner(s) who are remote... (unless the remote partner(s) can not reasonably do it, e.g. because they're isolating and shouldn't be interacting with other people.)
Pay attention to the conditions we discussed for making measurements from images.
As much as possible, all the points of the object to be measured (in this case, the length of sidewalk) should be the same distance from the camera.
How to measure speed of someone / something on a video
Speed, or rather average speed is $$\langle\text{speed}\rangle=\frac{\Delta d}{\Delta t}.$$ where
- $\Delta d$ means the distance travelled (change of distance measured from some common reference point), and
- $\Delta t$ means the time it took to travel that distance (the final time minus the initial time).
On the video screen, we can pick out two points, let's call them $A$ and $B$, and then we can measure the on-screen distance between them:
Choose 2 points on the sidewalk that your subject will pass over. I choose point just below the 2 trees shown.
Consult these notes on how to convert screen units to real-life units using a reference object, for example, if you know the height of the person walking, or some part of a moving bicycle that you can measure before or after.
Lab assignment
On a separate piece of paper, write down your responses and calculations to the numbered questions below. Hand in (on Moodle) a scan of your answers, and also upload the videos you took. Do this with your assigned lab/discussion partners and hand in one "lab" per team. Just make sure you include the names of everybody on your team on your response sheet.
1.) Write a short description of the reference points you used on the video, and then show your calculations to find the distance moved. Include both your original length measured on the screen and the distance, $\Delta d$ in "real life" and show how you calculated.
To measure the time, $\Delta t$, I used the stopwatch on my *phone* while I ran the video a few times on my *iPad*, timing the woman several times as she walks from $A$ to $B$. I decided that $\Delta t=5.7$ seconds was about right. You could also formally take the average of a couple of measurements.
2.) Write a short description of how you measured the time it took to pass through the two points, and show what you got for $\Delta t$.
Now we can calculate someone's speed, once we know $\Delta d$ and $\Delta t$, since $$\text{speed}=\frac{\Delta d}{\Delta t}.
But the answer may be in some units that you aren't familiar with. For example, if she moves 240 inches in 4 seconds, that would be a speed of $$\frac{240\,\text{in}}{4\,s}=60 \text{ inches / second}.$$
We *could* do the whole unit conversion ourselve, but this time, let's take the easy way out and ask Google to do it. For example if you type this into a Google search:
60 inches per second in miles per hour
the response will be
that is, 3.4 mph.
3.) Report the speed you found. Report both the first speed you calculated, using whatever units you used for $\Delta d$ and $\Delta t$, as well as the speed you get in miles per hour after you've used Google to convert speeds.
4.) As we did in class, compare the speed you calculated to some speed that you are familiar with from your own experience, to see if your answer sounds believable.
5.) This is a challenging question: Distance runners do not talk about how fast they're running in miles per hour. Instead, they talk about 'mile times': They figure out how many minutes they run per mile. Can you see what to do to get minutes per mile from the speed you calculated? [Hint: Another way of talking about a speed of 30 miles per hour, is that it takes you 1 hour per 30 miles, which is the same saying: It took you 60 minutes per 30 miles travelled.]