Mechanics at 400 frames/s.

© ULB- PhysGenLabs
with the collaboration of
Jean-Marie Frère
Paul Duhamel
Anastase Karusho

Some examples

3rd image

description :
I was curious to try the "fast videos" offered by recent cameras, in particular 400 frames per second for some specific experiments
Why ?
Tracker is an excellent pedagogical tool for the study of mechanics. Using relatively fast video allows to study different situations, without using "didactical" equipment, like air tracks.
While "start at rest" is easily covered by normal video, there are several circumstances where a faster take proves useful.
I just chose 3
  • Parabolic fall with a non-negligeable initial speed (here, the speed can be determined independently from the set-up, a ball rolling ON a U-shaped rail)
  • Collision (balistic pendulum) : the advantage here is not so much in the speed, but in having an abundance of data to calculate the speeds at impact time, and check conservation of momentum -- simple pendulums are used, without the need of air tracks or tables
  • Air friction on a sphere: this is a notoriously difficult part. To measure a significant drag, a high speed is needed (even to differentiate a ping-pong ball from a steel bearing!) -- here about 5m/s
Software  used
The most important tool is the "Tracker" software.
I have used version 4.62 on Windows64
and thank its author, Douglas Brown for friendly assistance
YOU NEED IT (if only to see the data from this page),
Is is freely available from:

Conservation of momentum

reminder: Download BOTH the video clip AND the Tracker file in the SAME directory, and open the tracker file from the Tracker program

Ball A (at rest) is hit by ball B,
With respective masses 95.7g and 57.1g, we measure the initial (horizontal) speed of B to be  64cm/s, and its (recoil, counted negatively) speed -16.6cm/s, while the recoil speed of A is 47cm/s.

The balance of initial (3654 g cm/s) and final ( 3650 g cm/s ) momenta is well verified (accuracy of the measurement is a few percent, so the last digits should not be taken seriously in any case)
Here is the "Tracker file"
And the Video Clip      

(this is the full clip, the best is to save both tracker file and video in the same folder, and open the tracker file in Tracker)

Free Fall with Air resistance 

reminder: Download BOTH the video clip AND the Tracker file in the SAME directory, and open the tracker file from the Tracker program
2 balls (one ball bearing, the other an (old norms) table tennis ball , of diameter 37.8mm and mass 2.3g) are dropped. We did not try to synchronize the departure of the 2 balls.
The video is taken where a speed of  approx. 5m/s is reached.
It is easy to see that the table tennis ball "loses ground" (the entrance time is not significant, but the increase in distance between the balls is).
We did not try to fit the full trajectory to standard formula, as the trajectory is still very well described with a parabolic motion over the interval.
While the friction appears negligeable for the steel ball (acceleration = 9.8 m/s2), the friction is very visible on the table tennis ball with the (average over the path measured) a=5.42m/s2
(again, accuracy is only  a few percent, but I did not round up the last digits).

This allows to measure the ratio of friction force (Ff) to mass for the second projectile :  Ff/m = 4.36 m/s2
 The friction force is found to be in good agree
ment with the usual formula for friction in air (proportional to the square of the speed) , which is found to be of order 4m/s2 based on the inital speed of 4.1m/s  (the friction is of course increasing along the path)

(see for instance :

The Tracker File is found here

The Video File is found here :  

Parabolic trajectory

reminder: Download BOTH the video clip AND the Tracker file in the SAME directory, and open the tracker file from the Tracker program
This is a more classical case, simply a parabolic fall. The ball bearing has significant initial speed (which is easy to calculate form the set-up: a sphere rolling ON a U-shaped rail .. gives the students a bit to figure out...). The advantage here is to have direct access (by the slope) to the initial parameters of the fall, thanks to the large number of points (thks for the auto-tracking facility in the software! It is essential in the present context)

The Tracker File

The Video file 

Equipment and Hint
This page is a quick check of possibilities based on a small NIKON V1 camera on short-term loan, using the 10-30mm zoom.
(other data were taken at 60fps with higher resolution but are not presented here; the price to pay for the 400 fps is a smaller frame - letter-box shaped)

Hint: don't forget to adjust the video speed in Tracker.
We found the 400 fps specification to be  quite accurate

(filming a digital chronometer!)