# How to Model a Tennis serve?

1. Feb 27, 2015

### MoAli

Hey Guys, I know its an Odd Question but i have an assignment where i should be doing a tennis serve and taking a video of it, importing it on laptop and draw graphs and make conclusions about its position with time and compare my results with theories, so any suggestions on how to do that, in fact, i have no clue how to measure speed and such things. Really appreciate any help. Thanks

2. Feb 27, 2015

### A.T.

Here is tutorial on tracking kinematics from video:

3. Feb 27, 2015

### sophiecentaur

Hi and welcome to PF
This could be an interesting project but I am not sure what you actually want to do.
With a digital video camera it is usually possible to look at one frame at a time and that will help you estimate speeds by looking at the changes between the sequence of frames. Speed is distance /time taken so, if you can decide the distances travelled and you have the frame rate (30frames per second, probably) you can estimate speeds as the times will be 1/30s. To measure the distances travelled by various bits of your arms it would be an idea to performs the action in front of a screen with a grid marked on it to act like graph paper. To avoid problems with perspective, you should make it a long shot (not wide angle) and stand very near the sceen. That way, a cm moved by your arm will correspond to a cm across the grid. Failing that, you could have a ruler with chunky markings on it, next to the server. Putting black dots on a white overall in various places on the body and having nice bright lighting and a consequent short shutter time should give you sharp dots (focus is vital, too). If you are trying to perfect your serve then you probably need to know more than just the position of the racket head.

4. Feb 27, 2015

### sophiecentaur

A high speed (slo mo) camera would be the icing on the cake. For human actions, it shouldn't be necessary. All that's necessary is to have a camera that has a fairly fast shutter setting.

5. Feb 27, 2015

### A.T.

Depends on MoAli's serve. But the main point of the tutorial is how to use that free tracking software.

6. Mar 2, 2015

### MoAli

Hey Guys, A few days ago I Asked for advice to help me model the serve and many were very useful in the experiment and i appreciate it. The result I got using the application tracker showed that the speed of the ball was decreasing along its path which is not what i expected, i mean the tennis ball after being hit with the racket should have speeded up as it proceeds in its trajectory or am i wrong? i need help in knowing what i should i expect in the speed(magnitude) time graph

7. Mar 2, 2015

### Staff: Mentor

Air resistance will slow down the ball.
In vacuum, the ball would speed up slightly (when moving downwards), but that effect is small for a typical serve.

8. Mar 2, 2015

### sophiecentaur

Once the source of energy (racket strings) is removed (and as long as the ball is not losing height), it will slow down in any case. Does it keep slowing down as it turns towards the ground? Upwards and horizontal, it will lose Kinetic energy.

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