General Synonyms:
* Crashing
* Bumping
* Impacting
* Meeting (in the sense of coming together forcefully)
* Clashing (suggesting conflict or disagreement)
* Running into
* Hitting
More Specific Synonyms:
* Smashing (for forceful collisions)
* Grinding (for collisions that produce friction)
* Battering (for repeated collisions)
* Intersecting (for objects crossing paths)
* Converging (for objects moving towards each other)
The best synonym to use will depend on the specific situation you're describing. For example, if you're talking about two cars hitting each other, "crashing" would be a good choice. But if you're talking about two ideas clashing, "clashing" would be more appropriate.