Combining Forces

When more than one force is acting on an object they are combined by adding them together. To do this with a set of forces Force1, Force2, Force3, etc, the vertical components are summed, and the horizontal components are summed:

xAccel = xForce1 + xForce2 + xForce3 + ...
yAccel = yForce1 + yForce2 + yForce3 + ...
xVelo = xVelo + xAccel
yVelo = yVelo + yAccel
x = x + xVelo
y = y + yVelo

In the friction and spring demos, those forces are combined with gravity and operate on the vertical axis. In the general gravity demo, there are many sources of gravity acting on each other at different angles so it is a good example of combining many forces that have both horizontal and vertical components.