In my previous analysis of Fruit Ninja (the popular iPhone game), I found that if the acceleration of these 'fruits' is to be 9.8 m/s^{2}, the fruit would have to be HUGE. We are talking a grapefruit over 2 feet in diameter.

But what if I take a different approach? What if I assume normal fruit with normal gravity? How fast would this look. This is what I will make for you. Key points:

- I want to assume reasonable sized fruit. Let me just assume the grape-fruity thing has a diameter of 15 cm.
- The acceleration of the objects will be -9.8 m/s
^{2}. - I will assume the current frame rate is wrong.
- The current (wrong) frame rate is 30 frames per second.

Using Tracker Video Analysis, I can look at the vertical motion of a fruit. This is the vertical velocity assuming normal sized fruit.

From this, it has a vertical acceleration of:

Here I am calling 'fs' a 'fruit-second'. I am assuming this is some different time scale. Yet, at the same time I will assume that the acceleration in units of real seconds would be:

Simply algebraically solving for the relationship between fruit and real seconds gives:

This means that a frame rate of 30 frames per fruit-second should be 69.8 frames per real second. For a quick check and Tracker tip: in Tracker Video, there is a button to set the video properties. It looks like this:

That brings up this window where you can manually change the frame rate for the analysis:

Tracker does not actually change the rate the video plays - just the analysis time step.

Here is what you waiting for (probably). This video shows the Fruit Ninja gameplay at normal speed and then 'correct' speed. Notice how fast and ninja-like my hands are.

http://www.youtube.com/watch?v=0OvRio7ze2I