A particle body force can be set to act on particles when a specified condition is met, for example when particles are in certain positions or are traveling at a specified velocity.

Using the Interaction pulldown menu and + pick-list, you can build a list (stack) of particle body forces. The top element is applied first, then the next one down and so on. Use the up and down buttons to move the force up and down the list. To remove an item, click the x button. Click the preferences button to configure the selected force.