At 2:00, the video is discussing gasses. Since the molecules in helium are lighter than the nitrogen and oxygen in air, helium has less resistance to vibration. So, when considering helium or air at the same temperature and pressure, sound travels faster in helium. However in liquids and solids, sound travels faster in denser materials.
In the video, at 2:00, it says that sound travels faster in less dense materials, but earlier in the course it said that sound travels faster in denser materials, like rock.
At 2:00, the video is discussing gasses. Since the molecules in helium are lighter than the nitrogen and oxygen in air, helium has less resistance to vibration. So, when considering helium or air at the same temperature and pressure, sound travels faster in helium. However in liquids and solids, sound travels faster in denser materials.
In the video, at 2:00, it says that sound travels faster in less dense materials, but earlier in the course it said that sound travels faster in denser materials, like rock.