Use Inertial Relief

If a model is not adequately restrained, a small force can cause rigid body motion. Even in cases when the applied forces are balanced, a small unbalanced force can result from numerical approximations.

When this option is checked, the program automatically applies forces to counteract unbalanced external loading. This option is particularly useful when you import loads from SolidWorks Motion where external loads can be slightly unbalanced. When you check this option, the program does not complain if the restraints are not adequate.

Do not check this flag if the external loads are not approximately balanced.