# Enabling gravity in the model

**URL:** <https://forum.itascainternational.com/t/enabling-gravity-in-the-model/1920>\
**Category:** PFC\
**Created:** [January 31, 2025, 6:30am UTC](https://forum.itascainternational.com/t/enabling-gravity-in-the-model/1920 "2025-01-31T06:30:25Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![sowmyaramanis](https://avatars.discourse-cdn.com/v4/letter/s/839c29/32.png) [@sowmyaramanis](https://forum.itascainternational.com/u/sowmyaramanis)\
**Post date:** [January 31, 2025, 6:30am UTC](https://forum.itascainternational.com/t/enabling-gravity-in-the-model/1920/1 "2025-01-31T06:30:25Z")

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In my model, initially the balls were confined in a box and made attain equilibrium. Later, the top wall is removed. As the system is disturbed, I have allowed it to attain equilibrium again. Despite model gravity being enabled, now the balls move upward showing gravity has not been enabled. What could be the possible reason for this? And how top overcome this?

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**Author:** ![dblanksma](https://avatars.discourse-cdn.com/v4/letter/d/858c86/32.png) [@dblanksma](https://forum.itascainternational.com/u/dblanksma)\
**Post date:** [January 31, 2025, 8:09pm UTC](https://forum.itascainternational.com/t/enabling-gravity-in-the-model/1920/2 "2025-01-31T20:09:57Z")

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Hello @sowmyaramanis,  
This is most likely due to high positive contact forces between the balls prior to removing the top of the box. Some options to mitigate this would be to zero out the contact forces (see this example - [Generating a Bonded Assembly — Itasca Software 9.0 documentation](https://docs.itascacg.com/itasca900/pfc/docproject/source/manual/examples/tutorials/generating_a_bonded_assembly/generating_a_bonded_assembly.html?node5782)). Or increasing the contact bond strength if your contact model allows for this.

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**Author:** ![sowmyaramanis](https://avatars.discourse-cdn.com/v4/letter/s/839c29/32.png) [@sowmyaramanis](https://forum.itascainternational.com/u/sowmyaramanis)\
**Post date:** [February 1, 2025, 7:25am UTC](https://forum.itascainternational.com/t/enabling-gravity-in-the-model/1920/3 "2025-02-01T07:25:12Z")

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In my model, after creation of the model setup, I am moving the walls to confine the balls closer. After that how can the contact forces be nulled only for the top wall?