When two negatively charged objects are close to each other, they exert a repulsive force on each other. This means that they push away from one another due to their like charges.
To increase the force between the objects, you can:
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Decrease the distance: Bringing the two negatively charged objects closer together will increase the force between them, as electric force is stronger when charges are nearer to each other.
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Increase the charge: Increasing the amount of negative charge on either or both of the objects will also increase the repulsive force between them, since the force is directly proportional to the amount of charge.