# Can someone explain to me why the two models are not fully matched? **URL:** \ **Category:** World Creation\ **Tags:** model, models, unreal-engine, Physics, combine, question\ **Created:** [December 16, 2020, 9:50pm UTC](https://forums.unrealengine.com/t/can-someone-explain-to-me-why-the-two-models-are-not-fully-matched/476025 "2020-12-16T21:50:11Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Sophus10005](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Sophus10005](https://forums.unrealengine.com/u/Sophus10005)\ **Post date:** [December 16, 2020, 9:50pm UTC](https://forums.unrealengine.com/t/can-someone-explain-to-me-why-the-two-models-are-not-fully-matched/476025/1 "2020-12-16T21:50:11Z")
I’m trying to combine these two models using Physics Thrusters, and for some reason they don’t match completely, although their 3D shape should. Can someone help me fix it, please? I am relatively new to the engine so I don’t have much experience. Since the video is not light enough to be uploaded here, here’s a reddit link where I have also asked. There you can see the video: > **[r/unrealengine - Can anyone explain to me why the two models are not fully...](https://www.reddit.com/r/unrealengine/comments/keif79/can_anyone_explain_to_me_why_the_two_models_are/)** > > 2 votes and 3 comments so far on Reddit ---
**Author:** ![Kehel18](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Kehel18](https://forums.unrealengine.com/u/Kehel18)\ **Post date:** [December 17, 2020, 5:20pm UTC](https://forums.unrealengine.com/t/can-someone-explain-to-me-why-the-two-models-are-not-fully-matched/476025/2 "2020-12-17T17:20:19Z")
Hi! Are you using complex collision for this models? (ref [Simple versus Complex Collision | Unreal Engine Documentation](https://docs.unrealengine.com/en-US/InteractiveExperiences/Physics/SimpleVsComplex/index.html)) ---
**Author:** ![Sophus10005](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Sophus10005](https://forums.unrealengine.com/u/Sophus10005)\ **Post date:** [December 19, 2020, 7:19pm UTC](https://forums.unrealengine.com/t/can-someone-explain-to-me-why-the-two-models-are-not-fully-matched/476025/3 "2020-12-19T19:19:33Z")
Here’s the thing. For whatever reason, when I select the complex collision on the model and then press Play, the models just remain floating in the air. ---
**Author:** ![Kehel18](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Kehel18](https://forums.unrealengine.com/u/Kehel18)\ **Post date:** [December 20, 2020, 3:39pm UTC](https://forums.unrealengine.com/t/can-someone-explain-to-me-why-the-two-models-are-not-fully-matched/476025/4 "2020-12-20T15:39:39Z")
So, they dont fall? Do you change only collision usage for asset? Or smth else? ---
**Author:** ![Sophus10005](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Sophus10005](https://forums.unrealengine.com/u/Sophus10005)\ **Post date:** [December 21, 2020, 8:17pm UTC](https://forums.unrealengine.com/t/can-someone-explain-to-me-why-the-two-models-are-not-fully-matched/476025/5 "2020-12-21T20:17:41Z")
Nope, everything’s set as default. The only thing I did when the model was added was enable physics and select the movable option. Now, when I press Play, I get a warning saying “Trying to simulate physics on ‘’…’’ but it has ComplexAsSimple collision” ☹ ---
**Author:** ![Kehel18](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Kehel18](https://forums.unrealengine.com/u/Kehel18)\ **Post date:** [December 22, 2020, 4:27pm UTC](https://forums.unrealengine.com/t/can-someone-explain-to-me-why-the-two-models-are-not-fully-matched/476025/6 "2020-12-22T16:27:20Z")
Oh, that is because Unreal Physics can’t work with Complex collision. Ok, then one thing you can do - go back to simple collision and create that simple collision from several primitives (however, it can be a little bit tricky). Another way - to separate tooths from body and build Actor with several mesh components. ---
**Author:** ![Sophus10005](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Sophus10005](https://forums.unrealengine.com/u/Sophus10005)\ **Post date:** [December 22, 2020, 7:55pm UTC](https://forums.unrealengine.com/t/can-someone-explain-to-me-why-the-two-models-are-not-fully-matched/476025/7 "2020-12-22T19:55:52Z")
That’s unfortunate! Well, since they are simple figures I’ll make them by hand again. Thank you very much for your help, you’re very kind! ^^ \<3 \<3 So in case I wanted to import a custom model in future projects, is there a way to enable both options and make it work aswell? or is it impossible in this engine ---
**Author:** ![Kehel18](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Kehel18](https://forums.unrealengine.com/u/Kehel18)\ **Post date:** [December 23, 2020, 2:35am UTC](https://forums.unrealengine.com/t/can-someone-explain-to-me-why-the-two-models-are-not-fully-matched/476025/8 "2020-12-23T02:35:01Z")
It is, of course, possible, but as I said in prev comment - with nontrivial models it requires some go-arounds… It is just collision calc optimization VS collision flexibility. And Unreal give more to optimization.