
newer measurement made on the path of travel suggests 140 +- 30 km/h
thanks to D wong for stimulating discussions.
Step1: use any YouTube down-loader such as this
http://downloadyoutubevideo.org/download.php?url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DS80bDrxrCiI%26feature%3DrelatedStep 2: save the video into the computer
Step 3: use tracker
http://www.cabrillo.edu/~dbrown/tracker/ to open the video file
 |
new estimates of length of car travel in video is now 13.0 ± 1 m , still the cab bumper can be fitted by 1.81 m length |
reason for readjusting the length of car traveled, results from a discussion with physics education expert D, Wong who suggested using nokia maps
to estimate the length of car in the video ( we measure from 2 white line of the
pedestrian walking to the center of the junction box. )
 |
to estimate the length of car in the video ( we measure from 2 white line of the pedestrian walking to the center of the junction box. ) show about 13 ± 1 m, against the map scale on the bottom right 10 m
|
 |
using tracker, the measurement of length is done easily using calibration tape (length to be measured) and stick (10m) |
 |
distance traveled by car is roughly 10 m and bumper of taxi is roughly 1.8 m (see diagram) |
Step 4 :state assumptions made:
- taxi camera is stationary for the duration of the 0.3 seconds the Ferrari sped across
the height of the traffic light post is approximately 2.0 m from base to green light center
- distance traveled by car is roughly
13 ± 1 10 m and bumper of taxi is roughly 1.8 m (see diagram)
- the video is properly time-frame as 30 frame/seconds as default in the video data, and read correctly by tracker.
- the Ferrari is traveling at constant acceleration = 0 m/s^2track the front of the Ferrari.
Step 5: calibrate pixels in video with known lengths, improved strategy after discussion on Facebook distance traveled by car is roughly
13 ± 1 10 m and bumper of taxi is roughly 1.8 m.
Step 6: Make roughly 8 to 9 points as shown
 |
Make roughly 8 to 9 points as shown
|
 |
using tracker to trace the motion of the car |
Step 7: .Analyze the speed
7.1 s vs t graph, which is roughly
39.23 m/s from as evidence from the
 |
linear fit x = 39.231*t + -0.352 where a = 39.231 which means the average velocity assuming a constant velocity motion |
 |
linear fit x = 30.187*t + -0.271 where a = 30.187 which means the average velocity assuming a constant velocity motion |
 |
linear fit x = 60.178*t + -13.821 where a = 60.178 which means the average velocity assuming a constant velocity motion |
linear fit x = 60.178*t + -13.821by comparing coefficient to constant acceleration equation x = u*t + x0roughly u = 60 m/s7.2 v vs t graph statistic mean v average =
39.75 30(wrong estimate) 60 (wrong estimate) m/s
 |
data tool notice the mean for x is 3.975E1 which means 39.75, another evidence of average velocity
|
 |
data tool notice the mean for x is 3.059E1 which means 30.059, another evidence of average velocity |
 |
data tool notice the mean for x is 6.097E1 which means 60.97, another evidence of average velocity |
average it out = (39.231+39.75)/2 = 39.5 m/s
Error Analysis:assume average v = d/terror analysis suggests,
Δv/v = Δd/d + Δt/t
using estimates of
Δd = 1 m (estimate from nokia map), d = 13 (nokia map)
Δt = 1/30 (from framerate) , t = 0.30 (from tracker)
v = 140 m
therefore, Δv = 0.18*v = 26 ≈
30 m/s (1 sign figure)Conclusion:
when converted to km/hr, v = 39.5*3600/1000 =
142 km/hr ≈ 140 ± 30 km/hr ( follow the order of magnitude of the error)
Claim: The vehicle is likely to be traveling at speed of 140± 30 (current best estimate, as of 17May2012)
Question:What do you think ??Source of errors:
- moving source camera and some parallax error taxi camera was not stationary for the duration of the 0.3 seconds the Ferrari sped across.
- the distance traveled by the car may not be corrected measure, thus, a nokia map was used to check.
the height of the traffic light post is not 2.0 m from base to green light center
- the source of the video could not be verified in the time frame used to capture the video, and the time-frame may not be 30 frame/seconds, though it is likely to be correct.
Physics & Science lesson learn could be:
- Using current affairs to educate the children on everyday physics and making physics relevant.
- using evidences to justify claims.
- science knowledge is socially constructed with newer evidences suggesting better ways of explaining the phenomena.
Morale lessons caught could be:
- Don't drive recklessly.
- Don't beat red lights.
- Be responsible
Open Content Sharing of Educational resources creative commons attribution:the video, trk files are downloadable at NTNU java Forum
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=2447.0 , available to registered users of the forum, registration is FREE.
trackerF04.png (324.91 KB, 1280x994 )
videoplaybackwee.trk (14.38 KB )
videoplaybackwee.zip (2382.25 KB ) based on incorrect light post height
videoplaybackwee01.zip (2594.63 KB )
based on incorrect length of car traveled in video evidences and measurements
videoplaybackwee02.zip (2996.85 KB) based on better estimates of length of car travel
Disclaimer: This post is made with the best intentions to share good active learning practices in physics education, any offenses taken please leave a comment, i will amend the post or/and to remove offensive content if any.
The mass media media corp 5 news also reported on the accident.