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Author: Morten Barslund (GotCha) | Website: http://www.hhbk.subnet.dk/GotCha.html

Friction and Elasticity

When we look at our method, we then have to position each movie clip dependent on the distance to the previous movie clip. In this context, we have to take a look at friction. Normally, this means a physical friction, but in this context simply that we want to slow down a movie clip until it reaches the position, which it's moving towards. If you picture a person on a bike, biking along a road and suddenly decides to stop biking, then at some point the person will stop. After this, we mark the place, where the person stopped biking and the place, where the person stopped. Furthermore, we notice that the person moved faster the longer the distance to the stopping point was.

In our case, we know both the current position (equivalent to the place, where the person stopped biking) and the place, where we want to go (equivalent to the place, where the person stopped). It seems that the speed can be described by calculating the distance between these two and divide this with a constant (the greater the distance, the greater speed). We store this in a variable, where our next position is our current position plus this variable. To illustrate this, we look at a movie clip, which moves towards a certain position (here, we are only interested in the horizontal plane since the same logic applies to the vertical plane). If we say that our current position is 0 and we want to move towards 100, we have the following values, where the constant is the earlier mentioned myFriction with a value of 2, the current position is x, the distance a, the speed v and the next position y:

As the values indicate, we start out by moving fast, which gradually decreases (v is smaller). This confirms that we take the distance (100-x), divide it by a constant, store this in a variable (in this case v) and calculate the next position (x+v). We also notice that the greater the constant, the slower we move (the speed v becomes smaller). If we divide by something greater, the result becomes smaller, when the numerator is the same. You might wonder about, when x is greater than the position, we want to move towards. This lies within the expression itself, since the speed v becomes negative (the distance is negative); we are still approaching the position.

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Added: 2000-12-30
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