Optional Practical: Video analysis of a projectile

In this practical the motion of a projectile will be recorded with a video camera and analysed with loggerPro. You will need to have a webcam on your laptop or a camera / mobile. Your computer will also need to have Quicktime installed.

If using a camera take a video of a ball thrown a distance of about 3m at an angle of approximately 45°. Make sure you get the complete flight in the frame but don’t move the camera. You will need to scale the video so place something of known length in the video at the same distance from the camera as the motion. Save the video as mov, mpeg, avi or wav. Then open LoggerPro and insert > movie.

If using your webcam then open LoggerPro and insert > video capture, you can then record your action directly into LoggerPro.

Your video will now be displayed in the video analysis window in LoggerPro, if you don’t see the tools on the left of the video click the button bottom right of the video window.

Setting the scale

  • Click the set scale button
  • Click on one side of the “object of known size” (a book in my case) and drag a line to the other side. Enter the length of the object in the pop up window that appears.

Tracking the motion

  • Advance the video to the start of the motion and click “add point”. The image of the ball will probably be a bit blurred (unless you have a good camera), decide where you are going to mark (e.g. the front back or middle) and place a dot at that point.
  • The video should advance one frame (if it doesn’t, do it manually) click on the ball again, make sure you use the same point as before.
  • Keep doing this until the whole path is marked.

Analysing the data

  • Behind the video window you will see graphs of X and Y displacement against time. Click on the labels and select X against time. Is the X velocity constant?
  • Now select Y against time and try a quadratic best fit. Determine g from the parabola. (if you can't remember how to do this the refer to this page Loggerpro curves )
  • Plot Y velocity against time and plot a linear best fit. Determine g from the gradient.
  • Plot Y vs X and plot a quadratic best fit. Was the trajectory parabolic?

Repeat the experiment with a balloon instead of a ball. Comment on the results.

To help with using LoggerPro here is a screencast from another experiment:

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